Deck Materials Calculator (Decking, Joists, Posts and Screws)

Enter the deck size, the decking (width, gap, stock length, direction), the joists (spacing, stock length) and the posts (how the joists are carried, spacing). Thickness, joist width and prices can be left blank.

Units

Deck width

Deck depth

Board direction

Board width (mm)

Thickness (mm, optional)

Gap (mm)

Deck board stock length (mm)

Joist spacing (mm)

Joist width (mm, optional)

Joist and beam stock length (mm)

How the joists are carried

Post spacing (mm)

Waste factor (%)

Screws per crossing

Deck board price ($, optional)

Joist ($ each)

Post ($ each)

You can choose the unit (m, cm or mm) only for the deck size. Enter the decking, joists, stock lengths and spacings all in mm. A blank gap uses 5 mm, a blank waste factor 5%, and blank screws 2 per crossing.

Result and figure

Enter the deck size and the details of the decking, joists and posts on the left and press "Calculate". The result and a framing plan will appear here.

[ CLOSE ]

What you can do on this page
What is this calculation used for?
How to Use
Formulas and figures
Symbols and terms
- Symbols
- Terms
Good to know before you start
How to calculate it in Excel
How to calculate it in Google Sheets
How to calculate it in Python
How to write it in LaTeX and other math languages (copy and paste)
How to have ChatGPT do the calculation
DataChef Features
Related Features
NumberChef Calculators List

What you can do on this page

Enter the deck width × depth and the width, gap and stock length of the deck boards, and you get how many rows and boards you need (boards to buy with waste, and the total length) on the spot
Choose whether the boards run along the width or along the depth. The page also shows how wide to rip the last row (or whether the leftover can be taken up in the gaps)
From the joist spacing (such as 16 in on center) it finds the number of joists and their actual spacing, and from the post spacing (such as 6 ft) the number of posts. It covers both joists sitting directly on posts (or deck blocks) and joists on beams
It also estimates the deck screws from where the rows cross the joists. Enter prices (optional) for decking, joists, beams and posts to get the cost of each and the total
A plan view shows the rows of decking, joists, beams and posts, so you can see the layout as you count materials. A plain-language explanation of the formulas and copy-and-paste formulas for Excel, Google Sheets and Python are also on this page

This page counts materials from the layout seen from above. For anything about structure and safety, such as joist and post spacing, lumber sizes, footings, treated lumber and fastening, follow the local building code, the span tables and the decking maker's installation guide. Many US decks need a permit. To optimize how pieces are cut from stock lengths, saw kerf included, use the "Lumber Cut List Calculator".

What is this calculation used for?

A shopping list for a DIY backyard deck

For example, a 12 × 12 ft deck of 5/4×6 deck boards (5.5 in wide) with 1/8 in gaps takes $\lceil 144 \div 5.625 \rceil = 26$ rows, 28 boards of 12 ft with a 5% waste factor, 10 joists at 16 in on center, 30 deck blocks (or 9 posts with beams), and about 520 deck screws.
Working out the rows and boards before you go to the lumberyard cuts down on extra trips when you run short and on piles of leftovers. You can also compare stock lengths, for example when the length you can carry in your truck limits what you buy.

Checking the board count for composite decking

Composite deck boards come in set sizes, such as 5.5 in wide and 12, 16 or 20 ft long, and the maker sets the gap and joist spacing. For example, a 16 ft wide × 10 ft deep deck with 5.5 in boards and 3/16 in gaps running along the width takes $\lceil 120 \div 5.6875 \rceil = 22$ rows; with 16 ft boards (one per row) that is 22 net, or 24 with a 5% waste factor.
Enter the maker's gap and joist spacing as they are, and you can check the "boards per 100 sq ft" guides in a catalog against your own deck size (follow the maker's installation guide for the structure and fasteners).

A small platform or landing

For a 6 × 3 ft platform with the boards running along the 6 ft side, the width to cover is 36 in, so 5.5 in boards with 1/8 in gaps take $\lceil 36 \div 5.625 \rceil = 7$ rows (6.4 rounded up). Each board is 72 in, so a 12 ft board gives two rows, and $\lceil 7 \div 2 \rceil = 4$ boards are enough.
The smaller the deck, the more "rows per stock board" changes the count, so choosing the stock length goes straight into the material cost.

Reading a contractor's estimate

A contractor's estimate lists material quantities such as "decking ○ pcs", "joists ○ pcs" and "footings ○". Knowing these formulas, you can follow how the rows and pieces come from the deck size, and sort out what to ask in the meeting (board direction, joist spacing, beams or not, the waste allowance).
A real estimate also includes fascia, stairs, railings, footings, hardware, finishing, permits and labor, so the material counts alone cannot tell you whether the price is fair.

Deck tiles are a different calculation

Interlocking deck tiles for a balcony or patio are counted by dividing the area by the area of one tile, so the "Tile Calculator" approach fits them better than the rows-of-decking formula on this page.
Deciding whether you are building a framed deck with joists and posts or simply laying tiles, and picking the calculator that matches, is the first step in estimating materials.

Formulas and figures

Rows of decking

Figure

Standard notation (the usual math form)

$n$ $=$ $\lceil$ $B$ $\div$ $($ $b$ $+$ $s$ $)$ $\rceil$

In words (symbols replaced with words)

④ $n$: rows of decking $=$ $\lceil$ ① $B$: width to cover $\div$ $($ ② $b$: board width $+$ ③ $s$: gap $)$ $\rceil$

The formula in words

① Take the $B$: width to cover

② divide it by the $b$: board width plus the

③ $s$: gap (the width one row takes) to see how many rows it is, and round up any decimal (the symbol $\lceil\ \rceil$ means "round up")

④ to get the $n$: rows of decking

Quick example

For a 12 × 12 ft deck with 5/4×6 deck boards (5.5 in wide) and 1/8 in gaps, running along the width (so the width to cover is the 144 in depth), the rows of decking are

$n$: rows of decking $=$ $\lceil$ width to cover (144 in) $\div$ $($ board width (5.5 in) $+$ gap (0.125 in) $)$ $\rceil$

$144 \div (5.5 + 0.125) = 144 \div 5.625 = 25.6$

$\lceil 25.6 \rceil = 26$

Key idea

Deck boards are laid one row at a time with a gap, so the width one row takes is "board width + gap". The width to cover divided by this tells you how many rows it is, and since you cannot stop partway through a row, round up. The extra from rounding up is adjusted at the edge. In the example, 26 rows cover $5.5 \times 26 + 0.125 \times 25 = 146.125$ in, which is 2.125 in more than the 144 in to cover, so the last row is ripped to $5.5 - 2.125 = 3.375$ in. When the division comes out even or the leftover is tiny, a gap of up to one gap width (1/8 in here) is left at the edge instead; it is taken up by opening the gaps slightly. You may also see "(width to cover + gap) ÷ (board width + gap)". It gives nearly the same answer. The difference shows up only when the remainder is no wider than a gap: that formula adds one more row that would be almost nothing, while this calculator adjusts the gaps instead.

Deck boards in stock lengths (net)

Standard notation (the usual math form)

$k$ $=$ $\lfloor$ $L$ $\div$ $A$ $\rfloor$

$M_{0}$ $=$ $\lceil$ $n$ $\div$ $k$ $\rceil$

In words (symbols replaced with words)

③ $k$: rows per stock board $=$ $\lfloor$ ① $L$: stock length of the decking $\div$ ② $A$: board length $\rfloor$

⑥ $M_{0}$: deck boards (net) $=$ $\lceil$ ④ $n$: rows of decking $\div$ ⑤ $k$: rows per stock board $\rceil$

The formula in words

① Take the $L$: stock length of the decking

② divide it by the $A$: board length and round down (the symbol $\lfloor\ \rfloor$ means "round down")

③ to get the $k$: rows per stock board . Next, take the

④ $n$: rows of decking

⑤ divide it by the $k$: rows per stock board and round up

⑥ to get the $M_{0}$: deck boards (net)

Quick example

For the example above (board length 144 in, 26 rows) with 12 ft (144 in) deck boards, the net number of boards is

$k$: rows per board $=$ $\lfloor$ stock (144 in) $\div$ board length (144 in) $\rfloor$

$M_{0}$: boards $=$ $\lceil$ rows (26) $\div$ rows per board (1) $\rceil$

$\lfloor 144 \div 144 \rfloor = 1$

$\lceil 26 \div 1 \rceil = 26$

Key idea

When the stock is well over twice the board length, one board gives two or more rows. For example, a 6 ft (72 in) wide landing built with 12 ft boards gives $\lfloor 144 \div 72 \rfloor = 2$ rows per board, so 7 rows take only $\lceil 7 \div 2 \rceil = 4$ boards. When the board length is longer than the stock (for example, a 20 ft wide deck with 16 ft boards), each row is two or more boards joined end to end (spliced). The net count is then $M_{0} = n \times \lceil A \div L \rceil$ (rows × boards per row), and this calculator switches to that formula automatically. Splices must land on a joist (often on doubled joists), and are staggered from row to row. This is a simple count that ignores saw kerf and reusing cut-offs. Also, if the boards overhang the frame at the ends, add the overhang to the board length (a 12 ft deck with a 1 in overhang on each end needs boards over 12 ft, so you would buy 14 ft boards). To find the layout with the fewest boards, kerf included, enter the rows and board length from here in the "Lumber Cut List Calculator".

Boards to buy with waste, and total length

Standard notation (the usual math form)

$M$ $=$ $\lceil$ $M_{0}$ $\times$ $\left(1 + \dfrac{r}{100}\right)$ $\rceil$

$\ell$ $=$ $n$ $\times$ $A$ $\times$ $\left(1 + \dfrac{r}{100}\right)$

In words (symbols replaced with words)

③ $M$: boards to buy with waste $=$ $\lceil$ ① $M_{0}$: deck boards (net) $\times$ ② waste multiplier $\left(1 + \dfrac{r}{100}\right)$ for waste factor $r$ (%) $\rceil$

⑥ $\ell$: total length with waste $=$ ④ $n$: rows of decking $\times$ ⑤ $A$: board length $\times$ waste multiplier $\left(1 + \dfrac{r}{100}\right)$ for waste factor $r$ (%)

The formula in words

① Take the $M_{0}$: deck boards (net)

② multiply it by the waste multiplier $\left(1 + \dfrac{r}{100}\right)$ and round up

③ to get the $M$: boards to buy with waste . Also, the

④ $n$: rows of decking times the

⑤ $A$: board length is the net total length, and the same multiplier gives the

⑥ $\ell$: total length with waste

Quick example

For 26 net boards (26 rows × 12 ft) and a 5% waste factor, the boards to buy and the total length with waste are

$M$: boards to buy $=$ $\lceil$ net (26) $\times$ multiplier (1.05) $\rceil$

$\lceil 26 \times 1.05 \rceil = \lceil 27.3 \rceil = 28$

$26 \times 12 \times 1.05 = 327.6\,\mathrm{ft}$

Key idea

The waste factor is the extra, as a share of the net amount, that you keep on hand. On a deck, the waste includes what is left after ripping the last row, cut-offs from trimming split ends, warped parts or knots, and spares for mistakes. 5 to 10% is common, and patterns with many splices or angled boards (such as a diagonal deck) need more. This calculator applies the waste factor only to the deck boards and their total length. There are fewer joists, beams and posts, and their cut-offs are already counted by the yield from each stock board, so no waste factor is applied to them. Add spares to those counts yourself if you want.

Number of joists and actual spacing

Figure

Standard notation (the usual math form)

$J$ $=$ $\lceil$ $A$ $\div$ $p$ $\rceil$ $+$ $1$

$p'$ $=$ $A$ $\div$ $($ $J$ $-$ $1$ $)$

In words (symbols replaced with words)

④ $J$: joists $=$ $\lceil$ ① $A$: board length $\div$ ② $p$: joist spacing $\rceil$ $+$ ③ 1 for both ends

⑤ $p'$: actual spacing $=$ $A$: board length $\div$ $($ $J$: joists $-$ $1$ $)$

The formula in words

① Take the $A$: board length

② divide it by the $p$: joist spacing and round up to get the number of spaces, then add

③ 1 for both ends

④ to get the $J$: joists . The board length divided by the number of spaces (joists − 1) gives the

⑤ $p'$: actual spacing

Quick example

For joists no more than 16 in apart under a 144 in board length (the width), the number of joists and the actual spacing are

$J$: joists $=$ $\lceil$ board length (144 in) $\div$ spacing (16 in) $\rceil$ $+$ 1 for both ends

$\lceil 144 \div 16 \rceil + 1 = 9 + 1 = 10$

$p' = 144 \div (10 - 1) = 16$

Key idea

Joists run at right angles to the decking and there is always one at each end of the deck, so the count is "number of spaces + 1" (the "fencepost problem": fence posts in a row with one at each end number one more than the spaces between them). The number of spaces is rounded up so the joists are no farther apart than the spacing you set. For example, for a board length of 146 in, $\lceil 146 \div 16 \rceil + 1 = 10 + 1 = 11$ joists, with an actual spacing of $146 \div 10 = 14.6$ in (rounding down to 10 joists would spread them to about 16.2 in, wider than 16 in). The joist spacing depends on the thickness, width and material of the deck boards, and makers publish a recommended spacing. Thicker boards can span more, while thin boards and composites need closer spacing (the IRC allows up to 24 in for both 2 in lumber and 5/4 boards laid straight across the joists; laid diagonally, 5/4 boards need joists spaced at 16 in or less). Always use the value from the installation guide for your decking. This formula puts the center of the end joists at the edges of the deck, so the real on-center spacing is a little narrower than set, by half an end joist width; it never goes over the spacing you set.

Number of posts (joists directly on posts)

Figure

Standard notation (the usual math form)

$K$ $=$ $J$ $\times$ $($ $\lceil$ $B$ $\div$ $q$ $\rceil$ $+$ $1$ $)$

In words (symbols replaced with words)

⑤ $K$: posts $=$ ① $J$: joists $\times$ $($ $\lceil$ ② $B$: joist length $\div$ ③ $q$: post spacing $\rceil$ $+$ ④ 1 for both ends $)$

The formula in words

① Take the $J$: joists and multiply by the posts per joist, which is the

② $B$: joist length divided by the

③ $q$: post spacing , rounded up, plus

④ 1 for both ends

⑤ to get the $K$: posts

Quick example

With posts (or deck blocks) no more than 72 in apart under each of 10 joists 144 in long, the number of posts is

$K$: posts $=$ joists (10) $\times$ $($ $\lceil$ joist length (144 in) $\div$ spacing (72 in) $\rceil$ $+$ 1 for both ends $)$

$\lceil 144 \div 72 \rceil + 1 = 2 + 1 = 3$

$10 \times 3 = 30$

Key idea

When the joists sit directly on posts, each joist has a row of posts right under it, so the posts form a grid: "joists × posts per joist". The posts per joist follow the same fencepost rule (spaces + 1) as the joists. This layout is typical of low "floating" decks set on precast deck blocks. The post spacing depends on the size and material of the joists (or beams), and 4 to 8 ft is common, but this calculator only counts the posts needed to stay within the spacing you enter. The footings under the posts, setting the post heights, fastening them to the joists, frost depth and treated lumber are safety matters; follow the local building code and the maker's guide.

Beams and posts when beams are used

Standard notation (the usual math form)

$G$ $=$ $\lceil$ $B$ $\div$ $q$ $\rceil$ $+$ $1$

$K$ $=$ $G$ $\times$ $($ $\lceil$ $A$ $\div$ $q$ $\rceil$ $+$ $1$ $)$

In words (symbols replaced with words)

③ $G$: beams $=$ $\lceil$ ① $B$: width to cover $\div$ ② $q$: post (beam) spacing $\rceil$ $+$ $1$

⑤ $K$: posts $=$ $G$: beams $\times$ $($ $\lceil$ ④ $A$: beam length $\div$ $q$: post spacing $\rceil$ $+$ $1$ $)$

The formula in words

① Take the $B$: width to cover

② divide it by the $q$: post (beam) spacing , round up and add 1 for both ends

③ to get the $G$: beams . Next, the posts per beam are the

④ $A$: beam length divided by the same spacing, rounded up, plus 1; multiply that by the beams

⑤ to get the $K$: posts

Quick example

For the same deck (board length 144 in, width to cover 144 in) with beams under the joists and beams and posts no more than 72 in apart, the beams and posts are

$G = \lceil 144 \div 72 \rceil + 1 = 2 + 1 = 3$

$K = 3 \times (\lceil 144 \div 72 \rceil + 1) = 3 \times 3 = 9$

Key idea

A beam is a heavier member running at right angles to the joists (the same direction as the decking) that carries the joists, with posts only under the beams. Compared with posts under every joist (30 in the previous example), far fewer posts are needed (9 here), so there are fewer footings to dig. In exchange, the beams are larger than the joists (often doubled 2×8s or 2×10s), which adds material. The beam is as long as the board length $A$, so its stock boards are counted the same way as the decking (how many per stock board, or spliced if too short). Many US decks attached to a house carry the house side on a ledger board fastened to the house instead of a beam and posts; in that case, subtract one beam and its posts from the count. Choose the structure to match the size and height of the deck, the local building code and the standard design for your decking.

Deck screws (rough)

Standard notation (the usual math form)

$V$ $=$ $n$ $\times$ $J$ $\times$ $v$

In words (symbols replaced with words)

④ $V$: deck screws $=$ ① $n$: rows of decking $\times$ ② $J$: joists $\times$ ③ $v$: screws per crossing

The formula in words

① The $n$: rows of decking times the

② $J$: joists is the number of places where the decking crosses a joist. Multiply that by the

③ $v$: screws per crossing

④ to get the $V$: deck screws

Quick example

With 26 rows of decking, 10 joists and 2 screws at each crossing, the deck screws are

$V$: deck screws $=$ rows (26) $\times$ joists (10) $\times$ per crossing (2)

$26 \times 10 \times 2 = 520$

Key idea

Each deck board is fastened where it crosses a joist, so the screws can be estimated as "crossings × screws per crossing". One screw near each edge of the board is usual, so 2 per crossing is the default (the rule of thumb of about 350 screws per 100 ft² of decking gives a similar count). This is only the screws for fastening the decking to the joists. Extra screws at splices, the hardware and fasteners that connect joists, beams and posts, and the screws for fascia are not included. Screws are sold by the box or by the pound, so buying 10 to 20% extra is a safe choice. For screw length (about 2 to 3 times the board thickness is a common guide, such as 2-1/2 in for 5/4 boards), material (coated or stainless for outdoor use, and compatible with treated lumber), and whether composites need special screws or hidden fasteners, follow the decking maker's installation guide.

Estimated cost

Standard notation (the usual math form)

$C$ $=$ $u$ $\times$ $Q$

$T$ $=$ $C_{1}$ $+$ $C_{2}$ $+$ $C_{3}$ $+$ $C_{4}$

In words (symbols replaced with words)

③ $C$: cost of each material $=$ ① $u$: unit price $\times$ ② $Q$: quantity

④ $T$: estimated total $=$ $C_{1}$: decking cost $+$ $C_{2}$: joist cost $+$ $C_{3}$: beam cost $+$ $C_{4}$: post cost

The formula in words

① Take the $u$: unit price (decking per board or per foot; joists, beams and posts per piece)

② multiply it by the $Q$: quantity (pieces or total length, to match the price)

③ to get the $C$: cost of each material , and add the decking, joists, beams and posts

④ to get the $T$: estimated total

Quick example

If you buy 28 deck boards at $20 each, 10 joists at $20 each and 30 posts (deck blocks) at $12 each, the cost is

$20 \times 28 = 560$

$20 \times 10 = 200$

$12 \times 30 = 360$

$560 + 200 + 360 = 1120$

Key idea

The key is to match the unit of the price and the quantity. If you buy decking per board, multiply by the boards to buy with waste $M$; if the price is per linear foot (as for many composite boards), multiply by the total length with waste $\ell$. Joists and beams are counted in stock boards, and posts by the piece. This covers only the materials for decking, joists, beams and posts. Footings or deck blocks, screws, hangers and hardware, stain or sealer, fascia, stairs and railings, tool rental, permits and labor if you hire it out are extra. You can find the amount of stain with the "Paint Calculator".

The rows of decking are "width to cover ÷ (board width + gap)", rounded up, and the joists and posts are "length ÷ spacing", rounded up, plus 1 for both ends (the fencepost problem). Add a waste factor (about 5 to 10%) to the deck boards and total length, and follow the building code and the maker's installation guide for spacing and fastening.

Symbols and terms

Symbols

$W$	double-u	The deck width, from the first letter of "width". You can enter it in ft or in, and it is turned into inches for the calculation.
$D$	dee	The deck depth (how far it sticks out from the house), from the first letter of "depth".
$A$	ay	The deck size along the boards (the board length). $A = W$ if the boards run along the width, and $A = D$ if they run along the depth. It is also the length along which the joists are spaced.
$B$	bee	The deck size across the boards (the width to cover with rows of decking). $B = D$ if the boards run along the width, and $B = W$ if they run along the depth. It is also the joist length.
$b$	small bee	The width of one deck board (in), from the first letter of "board". 5.5 in for a 2×6 or a 5/4×6 deck board.
$s$	ess	The gap between boards (in), from the first letter of "space". The default is 1/8 in.
$L$	el	The stock length of the deck boards (in), from the first letter of "length".
$n$	en	The rows of decking, found with $n = \lceil B \div (b + s) \rceil$. From the first letter of "number".
$k$	kay	The rows you get from one stock board, found with $k = \lfloor L \div A \rfloor$.
$M_{0}$	M sub zero	The deck boards in stock lengths (net, without waste), found with $M_{0} = \lceil n \div k \rceil$ (or $n \times \lceil A \div L \rceil$ when spliced).
$r$	ar	The waste factor (%), from the first letter of "rate". The default is 5%.
$M$	em	The boards to buy with waste, found with $M = \lceil M_{0} \times (1 + r \div 100) \rceil$. From the first letter of "material".
$\ell$	script el	The total length of decking with waste (ft), found with $\ell = n \times A \times (1 + r \div 100)$. It is written as a script l to keep it apart from the stock length $L$.
$p$	pee	The joist spacing (on center, in), from the first letter of "pitch". The example uses 16 in, but use the decking maker's recommended value.
$p'$	p prime	The actual joist spacing (in), after rounding the count up so the spacing is no more than $p$. It is found with $A \div (J - 1)$.
$J$	jay	The number of joists, found with $J = \lceil A \div p \rceil + 1$. From the first letter of "joist".
$q$	cue	The post spacing (on center, in), also used for the beam spacing when beams are used. The example uses 72 in, but use the value from the span tables and the building code for your joists and beams.
$K$	capital kay	The number of posts: $K = J \times (\lceil B \div q \rceil + 1)$ with joists directly on posts, or $K = G \times (\lceil A \div q \rceil + 1)$ with beams.
$G$	gee	The number of beams, found with $G = \lceil B \div q \rceil + 1$. From the first letter of "girder" (another word for a beam).
$v$	vee	The screws at each place where a row of decking crosses a joist. The default is 2.
$V$	capital vee	The rough number of deck screws, found with $V = n \times J \times v$.
$u$	you	The unit price of a material ($), from the first letter of "unit price". Decking per board or per foot; joists, beams and posts per piece.
$Q$	capital cue	The quantity to match the price (pieces or total length), from the first letter of "quantity".
$C$	see	The cost of each material ($), found with $C = u \times Q$. From the first letter of "cost".
$T$	tee	The estimated total ($), from the first letter of "total". It is the materials only, for decking, joists, beams and posts.
$\lceil x \rceil$	ceiling of x	The symbol for rounding up to a whole number, called the ceiling function. (Example - $\lceil 25.6 \rceil = 26$, $\lceil 9 \rceil = 9$)
$\lfloor x \rfloor$	floor of x	The symbol for rounding down to a whole number, called the floor function. (Example - $\lfloor 1.01 \rfloor = 1$, $\lfloor 2.22 \rfloor = 2$)

Terms

decking	The boards laid side by side on top of the deck, also called deck boards. They come in wood (pressure-treated pine, cedar, hardwoods) and composite. 5/4×6 (1 × 5-1/2 in actual) and 2×6 (1-1/2 × 5-1/2 in actual) are the common sizes.
joist	A framing board set on edge just under the decking, at right angles to it and evenly spaced, that carries the deck boards. The boards are screwed to the joists. 16 in on center is standard for decks, and the decking maker sets the recommended spacing for the board thickness and material.
beam	A heavier member (often doubled 2×8s or 2×10s) under the joists that carries them. With beams, the posts go only under the beams, so fewer posts are needed.
post	A short column standing on a footing that holds up the joists or beams. Besides wood posts (often 4×4 or 6×6), adjustable steel posts are used.
footing	The concrete base a post stands on, such as a poured concrete footing or a precast deck block. You need one per post, set level and at the right height. Footings for attached decks usually must go below the frost line; follow the building code.
stock length	The standard lengths lumber and deck boards are sold in. US lumber comes in even-foot lengths such as 8, 10, 12, 14 and 16 ft, and composite deck boards often in 12, 16 and 20 ft. (Metric timber is often sold in 300 mm steps, such as 2.4, 3.6 and 4.8 m.)
splice	Joining two or more boards end to end to make one row when one stock length is too short (a butt joint). The joint must land on a joist (or beam), and the joints are usually staggered from row to row.
yield	How many pieces of the length you need can be cut from one stock board, found by dividing the stock length by the piece length and rounding down. To include saw kerf, use the "Lumber Cut List Calculator".
waste factor	The extra, as a share of the net amount, for ripping the last row, trimming split ends, cutting around knots or warps, and spares for mistakes. 5 to 10% is common for deck boards.
deck width	The length of the deck along the house (along the patio door or wall). It is often the longer side of the deck.
deck depth	How far the deck sticks out from the house wall into the yard (also called the projection).
on center (o.c.)	The distance from the center of one member to the center of the next. Joist and post spacings are given this way in building, such as "16 in o.c." (it is different from the clear space between members).
fencepost problem	A classic math puzzle about how the number of items in a row relates to the number of spaces. With one at each end, "items = spaces + 1". The "+1" in the joist and post formulas comes from this idea.
rounding up	Changing a number with a decimal part to the next whole number. Rows and pieces cannot stop partway, so they are always rounded up.
rounding down	Dropping the decimal part to get a whole number. The pieces you get from one stock board are rounded down, since you cannot cut a piece that is too short.
composite decking	Deck boards made of wood fibers and plastic (PVC decking is similar). They do not rot and need little upkeep, but they expand and shrink with temperature, so the maker sets the gap and joist spacing.
hardwood decking	Deck boards of hard, rot-resistant tropical woods such as ipe, cumaru and garapa. They are so hard that pilot holes are needed, and their sizes often differ from softwood lumber.
fascia	A board fastened to the side of the deck to hide the rim joist, board ends and framing. It is not in the counts on this page, so get it separately if needed.
end grain	The cut end of a board, across its length. The exposed fibers split and chip easily and soak up rain. Trimming split ends is part of the waste on deck boards.
pressure-treated lumber	Softwood lumber (often southern yellow pine) with preservative forced in under pressure to resist rot and insects. It is the usual framing lumber for US decks and a low-cost decking choice. Use fasteners rated for treated lumber, and choose "ground contact" lumber for posts and parts near the ground.
adjustable post	A steel post used instead of a wood post, with a screw to fine-tune the height. It sits on a footing and carries the joists or beams. Posts are counted the same way as wood posts.

Good to know before you start

Here is what helps you use the calculation on this page with real understanding, not just by pressing the button.

Converting units of length (Grades 4–5)	Knowing that 1 ft = 12 in, and turning 12 ft into 144 in
Division with remainders (Grades 3–4)	Using a division such as $144 \div 5.625$ to find "how many fit" and what is left over
Rounding (Grades 3–4)	Knowing the difference between rounding up, rounding down and rounding to the nearest Being able to explain in your own words why material counts are rounded up, while the pieces from one stock board are rounded down
The fencepost problem (Grades 3–5)	Being able to show with a drawing that posts in a row with one at each end number "spaces + 1"
Percents (Grade 6)	Knowing that "5% more" is the same as "× 1.05"
Multiplying and dividing decimals (Grade 5)	Understanding decimal calculations such as $26 \times 1.05$ and $146 \div 10$ (a calculator can do the arithmetic)

How to calculate it in Excel

Copy the whole table below and paste it into cell A1 in Excel. It works as is.

Table to find the rows of decking

Width to cover B (in)	144
Board width b (in)	5.5
Gap s (in)	0.125
Rows of decking n	=ROUNDUP(B1/(B2+B3),0)

Table to find the deck boards in stock lengths (net)

Rows of decking n	26
Stock length L (in)	144
Board length A (in)	144
Rows per stock board k	=ROUNDDOWN(B2/B3,0)
Deck boards (net)	=ROUNDUP(B1/B4,0)

Table to find the boards to buy with waste and the total length

Deck boards (net)	26
Waste factor r (%)	5
Rows of decking n	26
Board length A (in)	144
Boards to buy with waste M	=ROUNDUP(B1*(1+B2/100),0)
Total length with waste (ft)	=B3B4/12(1+B2/100)

Table to find the number of joists and the actual spacing

Board length A (in)	144
Joist spacing p (in)	16
Joists J	=ROUNDUP(B1/B2,0)+1
Actual spacing (in)	=B1/(B3-1)

Table to find the number of posts (joists directly on posts)

Joists J	10
Joist length B (in)	144
Post spacing q (in)	72
Posts K	=B1*(ROUNDUP(B2/B3,0)+1)

Table to find the beams and posts when beams are used

Width to cover B (in)	144
Post spacing q (in)	72
Beam length A (in)	144
Beams G	=ROUNDUP(B1/B2,0)+1
Posts K	=B4*(ROUNDUP(B3/B2,0)+1)

Table to find the deck screws (rough)

Rows of decking n	26
Joists J	10
Screws per crossing v	2
Deck screws V	=B1B2B3

Table to find the estimated cost

Deck board price ($ each)	20
Deck boards to buy	28
Joist price ($ each)	20
Joist boards	10
Post price ($ each)	12
Posts	30
Estimated total ($)	=B1B2+B3B4+B5*B6

After pasting, the upper rows of column B are your inputs and the last rows (and the rows in between that calculate) are worked out automatically.
"ROUNDUP(value, 0)" rounds up to a whole number (the ⌈ ⌉ in the formulas), and "ROUNDDOWN(value, 0)" rounds down (the ⌊ ⌋).
B4 in the first table is 26 rows. In the second, B4 is 1 row and B5 is 26 boards. In the third, B5 is 28 boards and B6 is 327.6 ft. In the fourth, B3 is 10 joists and B4 is 16 in. B4 in the fifth is 30 posts. In the sixth, B4 is 3 beams and B5 is 9 posts. B4 in the seventh is 520 screws, and B7 in the eighth is $1,120. Just change column B to your own numbers.

How to calculate it in Google Sheets

Copy the whole table below and paste it into cell A1 in Google Sheets. It works as is.

Table to find the rows of decking

Width to cover B (in)	144
Board width b (in)	5.5
Gap s (in)	0.125
Rows of decking n	=ROUNDUP(B1/(B2+B3),0)

Table to find the deck boards in stock lengths (net)

Rows of decking n	26
Stock length L (in)	144
Board length A (in)	144
Rows per stock board k	=ROUNDDOWN(B2/B3,0)
Deck boards (net)	=ROUNDUP(B1/B4,0)

Table to find the boards to buy with waste and the total length

Deck boards (net)	26
Waste factor r (%)	5
Rows of decking n	26
Board length A (in)	144
Boards to buy with waste M	=ROUNDUP(B1*(1+B2/100),0)
Total length with waste (ft)	=B3B4/12(1+B2/100)

Table to find the number of joists and the actual spacing

Board length A (in)	144
Joist spacing p (in)	16
Joists J	=ROUNDUP(B1/B2,0)+1
Actual spacing (in)	=B1/(B3-1)

Table to find the number of posts (joists directly on posts)

Joists J	10
Joist length B (in)	144
Post spacing q (in)	72
Posts K	=B1*(ROUNDUP(B2/B3,0)+1)

Table to find the beams and posts when beams are used

Width to cover B (in)	144
Post spacing q (in)	72
Beam length A (in)	144
Beams G	=ROUNDUP(B1/B2,0)+1
Posts K	=B4*(ROUNDUP(B3/B2,0)+1)

Table to find the deck screws (rough)

Rows of decking n	26
Joists J	10
Screws per crossing v	2
Deck screws V	=B1B2B3

Table to find the estimated cost

Deck board price ($ each)	20
Deck boards to buy	28
Joist price ($ each)	20
Joist boards	10
Post price ($ each)	12
Posts	30
Estimated total ($)	=B1B2+B3B4+B5*B6

The same formulas as in Excel work as is (ROUNDUP and ROUNDDOWN have the same names). Copy the whole table, paste it into cell A1, and replace the numbers in column B with your own.

How to calculate it in Python

import math

deck_w_in = 144         # deck width (in), 12 ft
deck_d_in = 144         # deck depth (in), 12 ft
board_w_in = 5.5        # board width (in), 5/4x6 or 2x6 actual
gap_in = 0.125          # gap between boards (in), 1/8
board_len_in = 144      # stock length of the deck boards (in), 12 ft
joist_pitch_in = 16     # joist spacing (in, on center)
joist_len_in = 144      # stock length of the joists (in), 12 ft
post_pitch_in = 72      # post spacing (in), 6 ft
loss_rate = 5           # waste factor (%)
screws_per_joint = 2    # screws per crossing

# Boards run along the width: board length A = width, width to cover B = depth (swap them to run along the depth)
board_len_dir = deck_w_in
across = deck_d_in

rows = math.ceil(across / (board_w_in + gap_in))                    # rows of decking (rounded up)
if board_len_dir <= board_len_in:
    per_stock = math.floor(board_len_in / board_len_dir)             # rows per stock board (rounded down)
    boards_net = math.ceil(rows / per_stock)                         # net stock boards
else:
    boards_net = rows * math.ceil(board_len_dir / board_len_in)      # when spliced
boards_buy = math.ceil(boards_net * (1 + loss_rate / 100))           # boards to buy with waste
board_length_ft = rows * board_len_dir / 12 * (1 + loss_rate / 100)  # total length with waste (ft)

spans = math.ceil(board_len_dir / joist_pitch_in)                    # number of joist spaces
joists = spans + 1                                                   # joists (both ends included)
joist_pitch_actual = board_len_dir / spans                           # actual spacing
joist_stocks = math.ceil(joists / math.floor(joist_len_in / across)) # joist stock boards (when the stock is longer than a joist)

posts_per_joist = math.ceil(across / post_pitch_in) + 1              # posts per joist
posts = joists * posts_per_joist                                     # posts (joists directly on posts)
screws = rows * joists * screws_per_joint                            # rough deck screws

print(f"Rows of decking: {rows}")
print(f"Deck boards: {boards_net} net -> {boards_buy} with waste (total length {board_length_ft:.2f} ft)")
print(f"Joists: {joists} (actual spacing {joist_pitch_actual:.1f} in, {joist_stocks} stock boards)")
print(f"Posts: {posts}")
print(f"Deck screws: {screws}")

Runs with the standard library only. math.ceil() rounds up (the ⌈ ⌉ in the formulas) and math.floor() rounds down (the ⌊ ⌋). Replace the sizes, spacings and waste factor at the top with your own numbers and run it. To run the boards along the depth, swap the values of board_len_dir and across.

How to write it in LaTeX and other math languages (copy and paste)

Rows of decking

n = ⌈B ÷ (b + s)⌉

n = \left\lceil \frac{B}{b + s} \right\rceil

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>n</mi>
    <mo>=</mo>
    <mo>&#x2308;</mo>
    <mfrac><mi>B</mi><mrow><mi>b</mi><mo>+</mo><mi>s</mi></mrow></mfrac>
    <mo>&#x2309;</mo>
  </mrow>
</math>

n = |~ B / (b + s) ~|

Ceiling[B/(b + s)]

n := ceil(B/(b + s));

n = ceil(B/(b + s));

n = ⌈B/(b + s)⌉

Deck boards in stock lengths (net)

k = ⌊L ÷ A⌋,  M₀ = ⌈n ÷ k⌉

k = \left\lfloor \frac{L}{A} \right\rfloor,\quad M_{0} = \left\lceil \frac{n}{k} \right\rceil

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>k</mi>
    <mo>=</mo>
    <mo>&#x230A;</mo>
    <mfrac><mi>L</mi><mi>A</mi></mfrac>
    <mo>&#x230B;</mo>
    <mo>,</mo>
    <msub><mi>M</mi><mn>0</mn></msub>
    <mo>=</mo>
    <mo>&#x2308;</mo>
    <mfrac><mi>n</mi><mi>k</mi></mfrac>
    <mo>&#x2309;</mo>
  </mrow>
</math>

k = |__ L / A __|,  M_0 = |~ n / k ~|

k = Floor[L/A]; m0 = Ceiling[n/k]

k := floor(L/A);  M0 := ceil(n/k);

k = floor(L/A); M0 = ceil(n/k);

k = ⌊L/A⌋, M_0 = ⌈n/k⌉

Boards to buy with waste, and total length

M = ⌈M₀ × (1 + r/100)⌉,  ℓ = n × A × (1 + r/100)

M = \left\lceil M_{0} \left(1 + \frac{r}{100}\right) \right\rceil,\quad \ell = n A \left(1 + \frac{r}{100}\right)

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>M</mi>
    <mo>=</mo>
    <mo>&#x2308;</mo>
    <msub><mi>M</mi><mn>0</mn></msub>
    <mrow><mo>(</mo><mn>1</mn><mo>+</mo><mfrac><mi>r</mi><mn>100</mn></mfrac><mo>)</mo></mrow>
    <mo>&#x2309;</mo>
    <mo>,</mo>
    <mi>&#x2113;</mi>
    <mo>=</mo>
    <mi>n</mi><mo>&#x2062;</mo><mi>A</mi>
    <mrow><mo>(</mo><mn>1</mn><mo>+</mo><mfrac><mi>r</mi><mn>100</mn></mfrac><mo>)</mo></mrow>
  </mrow>
</math>

M = |~ M_0 (1 + r/100) ~|,  l = n A (1 + r/100)

m = Ceiling[m0 (1 + r/100)]; l = n a (1 + r/100)

M := ceil(M0*(1 + r/100));  l := n*A*(1 + r/100);

M = ceil(M0*(1 + r/100)); l = n*A*(1 + r/100);

M = ⌈M_0 (1 + r/100)⌉, ℓ = nA(1 + r/100)

Number of joists and actual spacing

J = ⌈A ÷ p⌉ + 1,  p' = A ÷ (J − 1)

J = \left\lceil \frac{A}{p} \right\rceil + 1,\quad p' = \frac{A}{J - 1}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>J</mi>
    <mo>=</mo>
    <mo>&#x2308;</mo>
    <mfrac><mi>A</mi><mi>p</mi></mfrac>
    <mo>&#x2309;</mo>
    <mo>+</mo>
    <mn>1</mn>
    <mo>,</mo>
    <msup><mi>p</mi><mo>&#x2032;</mo></msup>
    <mo>=</mo>
    <mfrac><mi>A</mi><mrow><mi>J</mi><mo>&#x2212;</mo><mn>1</mn></mrow></mfrac>
  </mrow>
</math>

J = |~ A / p ~| + 1,  p' = A / (J - 1)

j = Ceiling[a/p] + 1; pActual = a/(j - 1)

J := ceil(A/p) + 1;  pActual := A/(J - 1);

J = ceil(A/p) + 1; pActual = A/(J - 1);

J = ⌈A/p⌉ + 1, p' = A/(J − 1)

Number of posts (joists directly on posts)

K = J × (⌈B ÷ q⌉ + 1)

K = J \left( \left\lceil \frac{B}{q} \right\rceil + 1 \right)

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>K</mi>
    <mo>=</mo>
    <mi>J</mi>
    <mo>&#x2062;</mo>
    <mrow>
      <mo>(</mo>
      <mo>&#x2308;</mo>
      <mfrac><mi>B</mi><mi>q</mi></mfrac>
      <mo>&#x2309;</mo>
      <mo>+</mo>
      <mn>1</mn>
      <mo>)</mo>
    </mrow>
  </mrow>
</math>

K = J (|~ B / q ~| + 1)

k = j (Ceiling[b/q] + 1)

K := J*(ceil(B/q) + 1);

K = J*(ceil(B/q) + 1);

K = J(⌈B/q⌉ + 1)

Beams and posts when beams are used

G = ⌈B ÷ q⌉ + 1,  K = G × (⌈A ÷ q⌉ + 1)

G = \left\lceil \frac{B}{q} \right\rceil + 1,\quad K = G \left( \left\lceil \frac{A}{q} \right\rceil + 1 \right)

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>G</mi>
    <mo>=</mo>
    <mo>&#x2308;</mo>
    <mfrac><mi>B</mi><mi>q</mi></mfrac>
    <mo>&#x2309;</mo>
    <mo>+</mo>
    <mn>1</mn>
    <mo>,</mo>
    <mi>K</mi>
    <mo>=</mo>
    <mi>G</mi>
    <mo>&#x2062;</mo>
    <mrow>
      <mo>(</mo>
      <mo>&#x2308;</mo>
      <mfrac><mi>A</mi><mi>q</mi></mfrac>
      <mo>&#x2309;</mo>
      <mo>+</mo>
      <mn>1</mn>
      <mo>)</mo>
    </mrow>
  </mrow>
</math>

G = |~ B / q ~| + 1,  K = G (|~ A / q ~| + 1)

g = Ceiling[b/q] + 1; k = g (Ceiling[a/q] + 1)

G := ceil(B/q) + 1;  K := G*(ceil(A/q) + 1);

G = ceil(B/q) + 1; K = G*(ceil(A/q) + 1);

G = ⌈B/q⌉ + 1, K = G(⌈A/q⌉ + 1)

Deck screws (rough)

V = n × J × v

V = n J v

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>V</mi>
    <mo>=</mo>
    <mi>n</mi>
    <mo>&#xD7;</mo>
    <mi>J</mi>
    <mo>&#xD7;</mo>
    <mi>v</mi>
  </mrow>
</math>

V = n * J * v

n j v

V := n*J*v;

V = n*J*v;

V = n × J × v

Estimated cost

C = u × Q,  T = C₁ + C₂ + C₃ + C₄

C = u Q,\quad T = C_{1} + C_{2} + C_{3} + C_{4}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>C</mi>
    <mo>=</mo>
    <mi>u</mi>
    <mo>&#xD7;</mo>
    <mi>Q</mi>
    <mo>,</mo>
    <mi>T</mi>
    <mo>=</mo>
    <msub><mi>C</mi><mn>1</mn></msub>
    <mo>+</mo>
    <msub><mi>C</mi><mn>2</mn></msub>
    <mo>+</mo>
    <msub><mi>C</mi><mn>3</mn></msub>
    <mo>+</mo>
    <msub><mi>C</mi><mn>4</mn></msub>
  </mrow>
</math>

C = u * Q,  T = C_1 + C_2 + C_3 + C_4

c = u q; t = c1 + c2 + c3 + c4

C := u*Q;  T := C1 + C2 + C3 + C4;

C = u*Q; T = C1 + C2 + C3 + C4;

C = u × Q, T = C_1 + C_2 + C_3 + C_4

How to have ChatGPT do the calculation

You are a quantity calculation assistant for deck materials. Do the following calculation by actually running Python code, and base your answer only on the numbers from the execution result (do not answer by mental math or guessing).

I am building a 12 ft wide × 12 ft deep deck with 5/4×6 deck boards (5.5 in wide, 12 ft long) and 1/8 in gaps, running along the width (board length 144 in). Joists go no more than 16 in apart, with one at each end, and each joist (144 in long) is cut from a 12 ft board. Posts go under every joist, no more than 72 in apart, with one at each end.
Find each of the following:
1. The rows of decking (width to cover 144 in ÷ (5.5 + 0.125), rounded up) and how wide to rip the last row
2. The net number of deck boards, and the boards to buy with a 5% waste factor (rounded up)
3. The number of joists (144 ÷ 16, rounded up, + 1), the actual spacing, and the joist boards
4. The number of posts (joists × (144 ÷ 72, rounded up, + 1))
5. The rough number of deck screws (rows × joists × 2)

Show the formulas you used and the numbers from the execution result.

How to Use

1

Enter your numbers

Type the numbers you want to calculate with into the input fields
2

Calculate

Press the "Calculate" button
3

Check the result

The result appears on the spot. The same page also explains the idea behind the calculation and the formula

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\(W\)	double-u	The deck width, from the first letter of "width". You can enter it in ft or in, and it is turned into inches for the calculation.
\(D\)	dee	The deck depth (how far it sticks out from the house), from the first letter of "depth".
\(A\)	ay	The deck size along the boards (the board length). \(A = W\) if the boards run along the width, and \(A = D\) if they run along the depth. It is also the length along which the joists are spaced.
\(B\)	bee	The deck size across the boards (the width to cover with rows of decking). \(B = D\) if the boards run along the width, and \(B = W\) if they run along the depth. It is also the joist length.
\(b\)	small bee	The width of one deck board (in), from the first letter of "board". 5.5 in for a 2×6 or a 5/4×6 deck board.
\(s\)	ess	The gap between boards (in), from the first letter of "space". The default is 1/8 in.
\(L\)	el	The stock length of the deck boards (in), from the first letter of "length".
\(n\)	en	The rows of decking, found with \(n = \lceil B \div (b + s) \rceil\). From the first letter of "number".
\(k\)	kay	The rows you get from one stock board, found with \(k = \lfloor L \div A \rfloor\).
\(M_{0}\)	M sub zero	The deck boards in stock lengths (net, without waste), found with \(M_{0} = \lceil n \div k \rceil\) (or \(n \times \lceil A \div L \rceil\) when spliced).
\(r\)	ar	The waste factor (%), from the first letter of "rate". The default is 5%.
\(M\)	em	The boards to buy with waste, found with \(M = \lceil M_{0} \times (1 + r \div 100) \rceil\). From the first letter of "material".
\(\ell\)	script el	The total length of decking with waste (ft), found with \(\ell = n \times A \times (1 + r \div 100)\). It is written as a script l to keep it apart from the stock length \(L\).
\(p\)	pee	The joist spacing (on center, in), from the first letter of "pitch". The example uses 16 in, but use the decking maker's recommended value.
\(p'\)	p prime	The actual joist spacing (in), after rounding the count up so the spacing is no more than \(p\). It is found with \(A \div (J - 1)\).
\(J\)	jay	The number of joists, found with \(J = \lceil A \div p \rceil + 1\). From the first letter of "joist".
\(q\)	cue	The post spacing (on center, in), also used for the beam spacing when beams are used. The example uses 72 in, but use the value from the span tables and the building code for your joists and beams.
\(K\)	capital kay	The number of posts: \(K = J \times (\lceil B \div q \rceil + 1)\) with joists directly on posts, or \(K = G \times (\lceil A \div q \rceil + 1)\) with beams.
\(G\)	gee	The number of beams, found with \(G = \lceil B \div q \rceil + 1\). From the first letter of "girder" (another word for a beam).
\(v\)	vee	The screws at each place where a row of decking crosses a joist. The default is 2.
\(V\)	capital vee	The rough number of deck screws, found with \(V = n \times J \times v\).
\(u\)	you	The unit price of a material ($), from the first letter of "unit price". Decking per board or per foot; joists, beams and posts per piece.
\(Q\)	capital cue	The quantity to match the price (pieces or total length), from the first letter of "quantity".
\(C\)	see	The cost of each material ($), found with \(C = u \times Q\). From the first letter of "cost".
\(T\)	tee	The estimated total ($), from the first letter of "total". It is the materials only, for decking, joists, beams and posts.
\(\lceil x \rceil\)	ceiling of x	The symbol for rounding up to a whole number, called the ceiling function. (Example - \(\lceil 25.6 \rceil = 26\), \(\lceil 9 \rceil = 9\))
\(\lfloor x \rfloor\)	floor of x	The symbol for rounding down to a whole number, called the floor function. (Example - \(\lfloor 1.01 \rfloor = 1\), \(\lfloor 2.22 \rfloor = 2\))

Deck Materials Calculator (Decking, Joists, Posts and Screws)

Table of Contents

What you can do on this page

What is this calculation used for?

Formulas and figures

Symbols and terms

Symbols

Terms

Good to know before you start

How to calculate it in Excel

How to calculate it in Google Sheets

How to calculate it in Python

How to write it in LaTeX and other math languages (copy and paste)

How to have ChatGPT do the calculation

How to Use

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Find how much a deck board changes in width as it dries or takes on water, to set the gap

Shelf Sag and Load Calculator (Deflection from Material, Thickness, Span and Load; Maximum Span, Load and Thickness)

Use the joist spacing as the span and check with the beam formula how much a deck board sags under a person

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