Paint Thinning Calculator (Water or Thinner to Add, 2-Part Hardener)

Choose what to find, then enter the amount of paint (or base) and the thinning rate. Use the thinning rate from your paint can or the maker's data sheet.

Units

What to find Unit

Unthinned paint

Thinning rate (%, optional)

Thinning rate is given

The thinning rate is the amount of water or thinner added, as a percent of the unthinned paint. At 10%, add 1 L of water to 10 L of paint. The unit (L or kg) follows the unit of your amounts. Only if the maker gives the rate by weight and you measure by volume, set "Thinning rate is given" to "By weight" and enter the specific gravities.

Result and figure

Enter the amount of unthinned paint and the thinning rate (%) on the left and press "Calculate". The result 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 amount of unthinned paint (what is in the can) and the thinning rate (%), and see right away how much water or thinner to add and how much thinned paint you get
Work backward from the batch you want, such as "I want exactly 1 gallon of thinned paint", to the paint and the water or thinner you need. Handy when you do not want leftover mix, since thinned paint cannot go back into the can
For 2-part (2K) paints (base + hardener), find the hardener from the mix ratio (such as 4:1), then thin the mix. The thinning rate can be based on "base + hardener combined" or on "base only", whichever your product uses
If the maker gives the thinning rate by weight but you measure with a measuring cup (by volume), enter the weight per gallon of the paint and the thinner to get the amount by volume
A figure of a measuring cup filled with paint, hardener and water or thinner shows the proportions at a glance. A plain explanation of the formulas and copy-and-paste formulas for Excel, Google Sheets and Python are on this page too

The thinning rate and the mix ratio are set by the paint maker for each product (the can label or the product data sheet says things like "thin up to 10% with water" or "mix 4:1 with activator"). Always enter those values. To find how much paint you need for an area in the first place, use the paint calculator page to get the unthinned amount, then enter it here. Switch "Units" above the calculator to Metric to work in liters and kilograms.

What is this calculation used for?

DIY painting of walls and furniture (latex paint)

Some latex paint labels say something like "thin up to 10% with water", often for spraying. For a whole 1 gal can at 5%, add \(1 \times 5 \div 100 = 0.05\) gal (6.4 fl oz) of water, for 1.05 gal of thinned paint.
Too much water makes the film too thin and lowers hiding power (the ability to cover what is underneath), so you need more coats. Measure with a cup and start at the low end of the range. Check how many cans you need for the area first on the paint calculator page. Many modern latex paints are made to be used without thinning for brush and roller work, so follow your label.

Painting a small touch-up or one shelf (working backward from the batch)

Thinned paint cannot go back into the can, so any leftover mix is wasted. One shelf often needs only about 1 pint (0.125 gal) of mix. At 10%, mixing \(0.125 \div 1.1 \approx 0.114\) gal (about 14.5 fl oz) of paint with about 1.5 fl oz (about 3 tablespoons) of water is enough.
Working backward from the batch you need and thinning only that much saves both paint and money.

Mixing a 2-part urethane or epoxy coating

2-part urethane and epoxy coatings for garage floors, decks and boats come with data sheets like "mix 4:1 with activator, reduce 5–10%". For 1 gal of base, the activator is 0.25 gal (1 qt). At 5% of the 1.25 gal total, the reducer is 0.0625 gal (8 fl oz).
A 2-part coating must be applied within its pot life after mixing (often a few hours) or it sets, so mix only what you can apply in time. The "from the batch to make (2-part)" mode gives the base, hardener and thinner at once (always use the mix ratio and thinning rate from the data sheet).

Car and motorcycle refinishing (clear coat)

2-part automotive clear coats use a large share of hardener, such as "2:1" or "4:1", and are then thinned with reducer for spraying. Mixing 16 fl oz (0.125 gal) of clear at 2:1 takes 8 fl oz (0.0625 gal) of hardener. At 20% of the 24 fl oz total, the reducer is 4.8 fl oz.
Small amounts must be measured exactly. Mixing cups with printed ratio scales are common, and if the data sheet gives the ratio by weight, a scale with the amounts in lb (or oz ÷ 16) is the surest way.

When the maker gives weight % but you measure with a cup

Some data sheets give the thinning rate by weight, while on the job people usually measure with a cup. For 1 gal of paint at 10.8 lb/gal with 10% by weight of lacquer thinner (7.1 lb/gal), the thinner by volume is about 0.152 gal (about 19.5 fl oz, about 15.2% by volume). That is over 50% more than the 12.8 fl oz you would get by treating the weight % as a volume %.
The weight per gallon is on the product data sheet (or specific gravity × 8.34), so enter it when the rate is by weight.

Formulas and figures

Water or thinner to add and the thinned paint (from the paint amount)

Figure

Standard notation (the usual math form)

\(D\) \(=\) \(Q\) \(\times\) \(d\) \(\div\) \(100\)

\(M\) \(=\) \(Q\) \(\times\) \((\) \(1\) \(+\) \(d\) \(\div\) \(100\) \()\)

In words (symbols replaced with words)

③ \(D\): water or thinner to add \(=\) ① \(Q\): unthinned paint \(\times\) ② \(d\): thinning rate (%) \(\div\) \(100\)

④ \(M\): thinned paint \(=\) \(Q\): unthinned paint \(\times\) \((\) \(1\) \(+\) \(d\): thinning rate (%) \(\div\) \(100\) \()\)

The formula in words

① Multiply the \(Q\): unthinned paint

② by the \(d\): thinning rate (%) ÷ 100

③ and you get the \(D\): water or thinner to add Add it to the paint (\(1 + d \div 100\) times the paint) and you get the

④ \(M\): thinned paint

Quick example

To thin 10 gal of exterior latex paint (two 5-gallon buckets) by 10% for a sprayer (add water equal to 10% of the paint):

water to add \(D\) \(=\) paint (10 gal) \(\times\) thinning rate (10%) \(\div\) \(100\)

\(10 \times 10 \div 100 = 1\ \mathrm{gal}\)

\(10 + 1 = 10 \times 1.1 = 11\ \mathrm{gal}\)

Key idea

On this page, the thinning rate is the amount of water or thinner you add, as a percent of the unthinned paint (the same definition as on the paint calculator page). At 10%, you add 1 gal of water to 10 gal of paint and get 11 gal of thinned paint. Labels such as "thin up to 10% with water" or "reduce 10–15% with mineral spirits" are usually meant the same way, as a percent of the paint. "10% more" is "× 1.1". You add the water or thinner to the paint, so the thinned paint is always more than the paint you started with. Thinning balances how easily the paint flows against how thick the paint film is, so always stay within the range the maker gives. Too much makes the film too thin and lowers durability and sheen. Too little makes the paint hard to apply and leaves brush and roller marks.

Share of water or thinner in the whole mix

Standard notation (the usual math form)

\(s\) \(=\) \(\dfrac{d}{100 + d}\) \(\times\) \(100\)

In words (symbols replaced with words)

② \(s\): share of water or thinner in the mix (%) \(=\) ① \(\dfrac{\text{thinning rate } d}{100 + \text{thinning rate } d}\) \(\times\) \(100\)

The formula in words

① Divide the \(d\): thinning rate by "100 + \(d\)" (100 parts of paint plus \(d\) parts of water or thinner, the whole mix) and multiply by 100

② and you get the \(s\): share of water or thinner in the mix (%)

Quick example

In paint thinned at 10%, the share of water in the mix is

water share \(s\) \(=\) thinning rate (10%) ÷ (100 + 10) \(\times\) \(100\)

\(10 \div (100 + 10) \times 100 = 10 \div 110 \times 100 \approx 9.09\ \%\)

Key idea

"Thinned 10%" and "10% of the mix is water" are not the same. A 10% thinning rate adds 10 parts water to 100 parts paint, so 10 of the 110 parts of the mix are water, about 9.09% of the whole. If you wanted 10% of the whole mix to be water, you would add about 1.11 gal of water to 10 gal of paint, a different answer. Paint makers give the thinning rate as a percent of the paint, so this calculator uses that definition. Use this formula to check how thin your mix is as a share of the whole, or to translate when you see a rate written as "percent of the total".

Paint needed (from the batch you want to make)

Standard notation (the usual math form)

\(Q\) \(=\) \(M\) \(\div\) \((\) \(1\) \(+\) \(d\) \(\div\) \(100\) \()\)

\(D\) \(=\) \(M\) \(-\) \(Q\)

In words (symbols replaced with words)

③ \(Q\): paint needed \(=\) ① \(M\): batch to make \(\div\) \((\) \(1\) \(+\) ② \(d\): thinning rate (%) \(\div\) \(100\) \()\)

④ \(D\): water or thinner \(=\) \(M\): batch to make \(-\) \(Q\): paint needed

The formula in words

① Divide the \(M\): thinned paint you want to make

② by "1 + \(d\): thinning rate (%) ÷ 100" (the thinning multiplier)

③ and you get the \(Q\): paint needed What is left of the batch after that is the

④ \(D\): water or thinner to add

Quick example

To make exactly 1 gal of oil-based paint thinned 15%, the paint and thinner you need are

paint needed \(Q\) \(=\) batch (1 gal) \(\div\) \((\) \(1\) \(+\) thinning rate (15%) \(\div\) \(100\) \()\)

\(1 \div (1 + 15 \div 100) = 1 \div 1.15 \approx 0.870\ \mathrm{gal}\)

\(1 - 0.870 = 0.130\ \mathrm{gal} \approx 16.7\ \mathrm{fl\ oz}\)

Key idea

This is the first formula run backward. The thinning "multiplied" the paint up, so working backward you "divide" by the multiplier \(1 + d \div 100\). The easiest way to get the water or thinner is the batch minus the paint (\(Q \times d \div 100\) gives the same value). Thinned paint cannot go back into the can, so for small jobs like a piece of furniture or a touch-up, work backward from the batch you need to avoid leftovers. To find the unthinned amount from the area to paint, use the value from the paint calculator page.

Hardener for a 2-part paint (from the mix ratio)

Figure

Standard notation (the usual math form)

\(H\) \(=\) \(A\) \(\times\) \(\dfrac{b}{a}\)

In words (symbols replaced with words)

③ \(H\): hardener \(=\) ① \(A\): base \(\times\) ② \(\dfrac{\text{hardener part } b}{\text{base part } a}\)

The formula in words

① Multiply the \(A\): base

② by the mix ratio "base \(a\) : hardener \(b\)" as \(b\) ÷ \(a\) (hardener part ÷ base part)

③ and you get the \(H\): hardener

Quick example

For a 2-part paint with a mix ratio of base : hardener = 10 : 1, using 1 gal of base, the hardener is

hardener \(H\) \(=\) base (1 gal) \(\times\) hardener part (1) ÷ base part (10)

\(1 \times 1 \div 10 = 0.1\ \mathrm{gal} = 12.8\ \mathrm{fl\ oz}\)

Key idea

A 2-part (2K) paint cures by a chemical reaction when you mix the base with the hardener (also called activator). They are common in urethane and epoxy coatings for garage floors, decks, boats and car refinishing, and they usually give a harder, tougher film than 1-part paints. The maker sets the mix ratio, such as "10:1" or "4:1", which is a ratio of amounts: 10 parts base to 1 part hardener. Once you know the amount of base, the hardener is \(b \div a\) times that (for 10:1, \(1 \div 10 = 0.1\) times). Makers give the mix ratio either by weight (measured with a scale) or by volume (measured with a mixing cup). Enter the amounts in lb for a ratio by weight, or in gal for a ratio by volume, and the ratio works as is. Too much or too little hardener keeps the paint from curing properly (it can stay soft or crack), so always measure exactly to the ratio. Once mixed, the paint must be used within its pot life, so mix only as much as you can apply in time.

Thinning a 2-part paint (based on base + hardener combined)

Standard notation (the usual math form)

\(D\) \(=\) \((\) \(A\) \(+\) \(H\) \()\) \(\times\) \(d\) \(\div\) \(100\)

\(M\) \(=\) \(A\) \(+\) \(H\) \(+\) \(D\)

In words (symbols replaced with words)

④ \(D\): water or thinner \(=\) \((\) ① \(A\): base \(+\) ② \(H\): hardener \()\) \(\times\) ③ \(d\): thinning rate (%) \(\div\) \(100\)

⑤ \(M\): thinned paint \(=\) \(A\): base \(+\) \(H\): hardener \(+\) \(D\): water or thinner

The formula in words

① Add the \(A\): base

② and the \(H\): hardener to get the combined amount after mixing,

③ multiply it by the \(d\): thinning rate (%) ÷ 100

④ and you get the \(D\): water or thinner to add Add up the base, the hardener and the water or thinner, and you get the

⑤ \(M\): thinned paint

Quick example

To thin 1 gal of base and 0.1 gal of hardener (10 : 1) with 10% thinner based on the combined amount:

thinner \(D\) \(=\) \((\) base (1 gal) \(+\) hardener (0.1 gal) \()\) \(\times\) thinning rate (10%) \(\div\) \(100\)

\((1 + 0.1) \times 10 \div 100 = 1.1 \times 0.1 = 0.11\ \mathrm{gal} \approx 14.1\ \mathrm{fl\ oz}\)

\(1 + 0.1 + 0.11 = 1.21\ \mathrm{gal}\)

Key idea

There are two ways to state the thinning rate of a 2-part paint. The more common one treats the mixed base and hardener as the paint and adds \(d\)% thinner to that total (this formula). Some makers instead write "\(d\)% of the base", and then \(D = A \times d \div 100\), so you add a little less thinner, leaving out the hardener's share (in the example, \(1 \times 0.1 = 0.1\) gal). Tell which one applies from the data sheet wording ("of the mixed material" or "after mixing" points to the combined amount; "of the base" points to base only). This calculator switches with "Thinning rate based on (2-part)". When you work backward from the batch you want, the combined basis first finds \(A + H = M \div (1 + d \div 100)\) and splits it by the mix ratio, and the base-only basis finds \(A = M \div (1 + b \div a + d \div 100)\) (the calculator does this for you).

Converting a thinning rate by weight to volume (with weight per gallon)

Standard notation (the usual math form)

\(D\) \(=\) \(Q\) \(\times\) \(\dfrac{\rho_p}{\rho_s}\) \(\times\) \(d\) \(\div\) \(100\)

In words (symbols replaced with words)

④ \(D\): water or thinner (gal) \(=\) ① \(Q\): paint by volume (gal) \(\times\) ② \(\dfrac{\text{paint weight per gallon } \rho_p}{\text{thinner weight per gallon } \rho_s}\) \(\times\) ③ \(d\): thinning rate (weight %) \(\div\) \(100\)

The formula in words

① Multiply the \(Q\): paint by volume (gal)

② by the paint weight per gallon \(\rho_p\) ÷ thinner weight per gallon \(\rho_s\) to find how many gallons of thinner weigh the same as the paint,

③ then multiply by the \(d\): thinning rate (weight %) ÷ 100

④ and you get the \(D\): water or thinner to add by volume (gal)

Quick example

To thin 1 gal of paint that weighs 10.8 lb/gal by 10% by weight with lacquer thinner (7.1 lb/gal), the thinner to measure with a cup is

thinner \(D\) (gal) \(=\) paint by volume (1 gal) \(\times\) weight ratio (10.8 ÷ 7.1) \(\times\) thinning rate (10%) \(\div\) \(100\)

\(1 \times 10.8 = 10.8\ \mathrm{lb}\)

\(10.8 \times 10 \div 100 = 1.08\ \mathrm{lb}\)

\(1.08 \div 7.1 \approx 0.152\ \mathrm{gal} \approx 19.5\ \mathrm{fl\ oz}\)

Key idea

Industrial and automotive data sheets often give the thinning rate by weight (weight %), assuming you use a scale. In DIY, though, people usually measure with a cup (by volume). Paint is heavier than water (about 10 to 12 lb/gal), and solvent thinners are lighter than water (about 6.5 to 7 lb/gal), so if you use a weight % as a volume %, you add too little thinner. The idea has three steps. First multiply the paint volume by its weight per gallon to get the paint weight (lb). Multiply that by the thinning rate to get the thinner weight (lb). Finally divide by the thinner's weight per gallon to get the thinner volume (gal). In the example, 10% by weight becomes about 15.2% by volume. When you thin latex paint with water (8.34 lb/gal), use 8.34 as \(\rho_s\). The other way around, if you enter amounts in lb and the rate is by volume, flip the ratio (\(\rho_s \div \rho_p\)) (the calculator handles this from "Thinning rate is given"). The weight per gallon is on the product data sheet. If you only have the specific gravity (kg/L), both ratios give the same result, because only the ratio of the two values matters.

The water or thinner to add is paint × thinning rate ÷ 100, and the thinned paint is (1 + thinning rate ÷ 100) times the paint. To work backward from the batch you want, divide by that multiplier. For a 2-part paint, first find the hardener from the mix ratio, then thin. Always follow the maker's thinning rate and mix ratio.

Symbols and terms

Symbols

\(Q\)	cue	The amount of unthinned paint (gal or lb) - the paint taken from the can before thinning. It comes from "quantity". It is the same symbol as the "paint needed, with waste" \(Q\) on the paint calculator page, so you can enter that value as is.
\(d\)	dee	The thinning rate (%) - the water or thinner added as a percent of the paint. It comes from "dilution".
\(D\)	capital dee	The water or thinner to add. It is found with \(D = Q \times d \div 100\). It comes from "diluent" (a liquid used for thinning).
\(M\)	em	The thinned paint. It is found with \(M = Q + D = Q \times (1 + d \div 100)\). When you work backward, you set this value first as the batch you want and then find the paint \(Q\). It comes from "mixture".
\(s\)	ess	The share of water or thinner in the whole mix (%). It is found with \(s = d \div (100 + d) \times 100\). It comes from "share".
\(A\)	ay	The amount of base in a 2-part paint. The base is often called part A, and that is the A here.
\(H\)	aitch	The amount of hardener in a 2-part paint. It is found with \(H = A \times b \div a\). It comes from "hardener".
\(a,\ b\)	ay, bee	The two numbers of the mix ratio "base \(a\) : hardener \(b\)". For "4:1", \(a = 4\) and \(b = 1\).
\(\rho_p\)	rho sub pee	The paint weight per gallon (lb/gal), or its specific gravity (kg/L). \(\rho\) (rho) is the usual physics symbol for density, and the subscript p stands for "paint".
\(\rho_s\)	rho sub ess	The weight per gallon (lb/gal) of the water or thinner, or its specific gravity. Water is 8.34 lb/gal and mineral spirits about 6.5 lb/gal. The subscript s stands for "solvent".

Terms

Thinning	Adding water or thinner to paint. It makes the paint flow easily and helps avoid brush and roller marks. Too much makes the film too thin and lowers performance, so stay within the range the maker gives.
Thinning rate	The water or thinner added as a percent of the unthinned paint. This page uses the definition "10% of 1 gal of paint is 0.1 gal of water". It is not the same as "what percent of the whole mix is water".
Unthinned paint	The paint as it is in the can, before thinning. For a 2-part paint, the base and hardener mixed together count as the unthinned paint.
Thinner (solvent)	A liquid for thinning paint. Latex (water-based) paint uses water, and oil-based (solvent-based) paint uses a thinner such as mineral spirits or lacquer thinner. The wrong one can make the paint separate or fail to cure.
Thinned paint	Paint with water or thinner added, ready to apply with a brush, roller or sprayer. It cannot go back into the can, so make only as much as you can use.
Latex paint (water-based)	Paint thinned with water. It has little odor and tools clean up with water, so it is the main choice for indoor DIY. Labels often say something like "thin up to 10% with water", and some brands say not to thin for brush and roller use.
Oil-based paint (alkyd)	Paint thinned with a solvent thinner, also called solvent-based paint. It sticks well and lasts, and is used for trim, doors and metal. Always thin it with the thinner the label names.
Mineral spirits (paint thinner)	The usual thinner for oil-based (alkyd) paint, sold as "paint thinner". Do not use it with lacquer.
Lacquer thinner	A thinner made for lacquer (fast-drying finishes for furniture and spraying). It is a strong solvent and can damage oil-based paint films, so check what it is meant for.
2-part paint (2K)	Paint that cures by a chemical reaction when you mix two parts, the base and the hardener. Common in urethane and epoxy coatings for floors, boats and car refinishing, it usually gives a tougher film than 1-part paint. Use it up within its pot life.
Base (part A)	The main liquid of a 2-part paint, with the color and resin. It only cures after it is mixed with the hardener.
Hardener (activator, part B)	The liquid that reacts with the base of a 2-part paint to cure it. Measure it exactly to the mix ratio; too much or too little causes curing problems.
Mix ratio	The ratio of base to hardener, set by the maker, such as "base : hardener = 4 : 1". Whether it is by weight or by volume differs by product, so check the data sheet.
By weight	A ratio or percent measured by weight (lb, oz) with a scale. Industrial and automotive data sheets often give thinning rates and mix ratios by weight.
By volume	A ratio or percent measured by volume (gal, qt, fl oz) with a measuring cup. Paint and thinner weigh different amounts per gallon, so the value differs from the ratio by weight.
Specific gravity and weight per gallon	Specific gravity is how heavy a liquid is compared with water (kg per liter). US data sheets usually give the weight per gallon instead. Water is 1 (8.34 lb/gal), many paints are 1.2 to 1.4 (10 to 12 lb/gal), and solvent thinners about 0.8 (about 6.5 to 7 lb/gal). Use either to convert between weight and volume.
Pot life	The time after mixing a 2-part paint until it starts to set and can no longer be applied. It gets shorter when it is hot, so mix only what you can apply within the pot life.
Clean water	The water used to thin latex paint. Clean tap water is fine. "Thin up to 10% with water" is the same as "add up to 10% water, as a percent of the paint".
SDS (safety data sheet)	A document from the maker listing a product's ingredients, specific gravity and safe handling. It is on the maker's website. You can check the specific gravity there, and the product data sheet (TDS) often lists the weight per gallon.
Paint film	The thin layer of dried paint on the surface. Durability, water resistance and sheen depend on its thickness, so too much thinning makes a weaker film.
Thinning multiplier	The number you multiply by to add a percent to an amount, such as "1 + thinning rate ÷ 100". At 10% it is 1.1. Paint × 1.1 gives the thinned paint, and thinned paint ÷ 1.1 gives the paint.

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.

Percents (Grade 6)	"10% of the paint" is the paint amount × 0.1 "10% more" can be calculated as "× 1.1"
Ratios (Grade 6)	"10:1" shows the relationship "10 parts base to 1 part hardener" You can use a ratio to find one amount from the other (for example, with 2 gal of base, the hardener is \(2 \times \dfrac{1}{10}\) gal)
Multiplying and dividing decimals (Grades 5–6)	What calculations like \(1 \div 1.15\) and \(1.1 \times 0.1\) stand for (a calculator is fine for the arithmetic)
Unit rates and converting units (Grades 5–6)	Using the weight per gallon to go between volume (gal) and weight (lb) Converting gallons, quarts and fluid ounces (\(1\,\text{gal} = 4\,\text{qt} = 128\,\text{fl oz}\)) and pounds and ounces (\(1\,\text{lb} = 16\,\text{oz}\))
Working backward with equations (Grades 6–7)	From "paint × 1.1 = thinned paint" you can go backward to "paint = thinned paint ÷ 1.1"

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 water or thinner to add and the thinned paint

Unthinned paint (gal)	10
Thinning rate (%)	10
Water or thinner to add (gal)	=B1*B2/100
Thinned paint (gal)	=B1*(1+B2/100)

Table to find the share of water or thinner in the mix

Thinning rate (%)	10
Share of the mix (%)	=B1/(100+B1)*100

Table to find the paint needed

Thinned paint to make (gal)	1
Thinning rate (%)	15
Paint needed (gal)	=B1/(1+B2/100)
Water or thinner to add (gal)	=B1-B3

Table to find the hardener for a 2-part paint

Base (gal)	1
Mix ratio, base part	10
Mix ratio, hardener part	1
Hardener (gal)	=B1*B3/B2

Table to thin a 2-part paint

Base (gal)	1
Hardener (gal)	0.1
Thinning rate (%)	10
Thinner to add (gal)	=(B1+B2)*B3/100
Thinned paint (gal)	=B1+B2+B4

Table to convert a thinning rate by weight to volume

Paint by volume (gal)	1
Paint weight (lb/gal)	10.8
Thinner weight (lb/gal)	7.1
Thinning rate (weight %)	10
Thinner to add by volume (gal)	=B1B2/B3B4/100

After pasting, the upper rows of column B are the inputs and the last rows are calculated automatically.
In the first table B3 is 1 and B4 is 11. In the second table B2 is about 9.09. In the third table B3 is about 0.870 and B4 about 0.130. In the fourth table B4 is 0.1, in the fifth B4 is 0.11 and B5 is 1.21, and in the sixth B5 is about 0.152.
The same formulas work with L, kg or lb as well (just change column B to your own numbers). To get fl oz, multiply gallons by 128.

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 water or thinner to add and the thinned paint

Unthinned paint (gal)	10
Thinning rate (%)	10
Water or thinner to add (gal)	=B1*B2/100
Thinned paint (gal)	=B1*(1+B2/100)

Table to find the share of water or thinner in the mix

Thinning rate (%)	10
Share of the mix (%)	=B1/(100+B1)*100

Table to find the paint needed

Thinned paint to make (gal)	1
Thinning rate (%)	15
Paint needed (gal)	=B1/(1+B2/100)
Water or thinner to add (gal)	=B1-B3

Table to find the hardener for a 2-part paint

Base (gal)	1
Mix ratio, base part	10
Mix ratio, hardener part	1
Hardener (gal)	=B1*B3/B2

Table to thin a 2-part paint

Base (gal)	1
Hardener (gal)	0.1
Thinning rate (%)	10
Thinner to add (gal)	=(B1+B2)*B3/100
Thinned paint (gal)	=B1+B2+B4

Table to convert a thinning rate by weight to volume

Paint by volume (gal)	1
Paint weight (lb/gal)	10.8
Thinner weight (lb/gal)	7.1
Thinning rate (weight %)	10
Thinner to add by volume (gal)	=B1B2/B3B4/100

The same formulas as Excel (basic arithmetic only) work as is. Copy the whole table, paste it into cell A1 and change column B to your own numbers.

How to calculate it in Python

neat = 10                 # unthinned paint (gal, lb, L or kg)
dilution_rate = 10        # thinning rate (%, added as a percent of the paint)

diluent = neat * dilution_rate / 100             # water or thinner to add
mixed = neat + diluent                           # thinned paint
diluent_share = diluent / mixed * 100            # share of water or thinner in the mix (%)

# Working backward from the batch you want
target = 1                                       # thinned paint to make
neat_needed = target / (1 + dilution_rate / 100) # paint needed
diluent_needed = target - neat_needed            # water or thinner to add

# 2-part: hardener from the mix ratio (base a : hardener b), thinned on the combined amount
main = 1                                         # base (gal)
ratio_main, ratio_hardener = 10, 1               # mix ratio 10:1
hardener = main * ratio_hardener / ratio_main    # hardener
diluent_two_part = (main + hardener) * dilution_rate / 100   # thinning on base + hardener
mixed_two_part = main + hardener + diluent_two_part

# Converting a thinning rate by weight to volume (multiply by paint weight ÷ thinner weight)
paint_lb_per_gal, solvent_lb_per_gal = 10.8, 7.1
paint_gal = 1
diluent_volume = paint_gal * paint_lb_per_gal / solvent_lb_per_gal * dilution_rate / 100

print(f"Water or thinner to add: {diluent:.3f}, thinned paint: {mixed:.3f}")
print(f"Share of the mix: {diluent_share:.2f} %")
print(f"Paint needed for {target}: {neat_needed:.3f}, water or thinner: {diluent_needed:.3f}")
print(f"Hardener: {hardener:.3f} gal, thinner: {diluent_two_part:.3f} gal, thinned paint: {mixed_two_part:.3f} gal")
print(f"Thinner for {dilution_rate}% by weight, as volume: {diluent_volume:.3f} gal ({diluent_volume * 128:.1f} fl oz)")

It runs with the standard library only. Change the paint amount and thinning rate, the batch to make, the base and mix ratio, and the weights per gallon at the top to your own numbers and run it. The amounts work in gal, lb, L or kg as is (only the weight-to-volume conversion assumes the paint is measured by volume and the rate is by weight). Specific gravities (kg/L) work in place of lb/gal too, since only their ratio is used.

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

Water or thinner to add and the thinned paint (from the paint amount)

D = Q × d ÷ 100,  M = Q × (1 + d ÷ 100)

D = Q \times \frac{d}{100},\quad M = Q \left(1 + \frac{d}{100}\right)

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>D</mi>
    <mo>=</mo>
    <mi>Q</mi><mo>&#xD7;</mo><mfrac><mi>d</mi><mn>100</mn></mfrac>
    <mo>,</mo>
    <mi>M</mi>
    <mo>=</mo>
    <mi>Q</mi>
    <mo>&#x2062;</mo>
    <mrow><mo>(</mo><mn>1</mn><mo>+</mo><mfrac><mi>d</mi><mn>100</mn></mfrac><mo>)</mo></mrow>
  </mrow>
</math>

D = Q xx d / 100,  M = Q (1 + d / 100)

{q*d/100, q*(1 + d/100)}

Dw := Q*d/100;  M := Q*(1 + d/100);

D = Q*d/100; M = Q*(1 + d/100);

D = Q × d/100, M = Q (1 + d/100)

Share of water or thinner in the whole mix

s = d ÷ (100 + d) × 100

s = \frac{d}{100 + d} \times 100

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>s</mi>
    <mo>=</mo>
    <mfrac><mi>d</mi><mrow><mn>100</mn><mo>+</mo><mi>d</mi></mrow></mfrac>
    <mo>&#xD7;</mo>
    <mn>100</mn>
  </mrow>
</math>

s = d / (100 + d) xx 100

d/(100 + d)*100

s := d/(100 + d)*100;

s = d/(100 + d)*100;

s = d/(100 + d) × 100

Paint needed (from the batch you want to make)

Q = M ÷ (1 + d ÷ 100),  D = M − Q

Q = \frac{M}{1 + \dfrac{d}{100}},\quad D = M - Q

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>Q</mi>
    <mo>=</mo>
    <mfrac><mi>M</mi><mrow><mn>1</mn><mo>+</mo><mfrac><mi>d</mi><mn>100</mn></mfrac></mrow></mfrac>
    <mo>,</mo>
    <mi>D</mi>
    <mo>=</mo>
    <mi>M</mi><mo>&#x2212;</mo><mi>Q</mi>
  </mrow>
</math>

Q = M / (1 + d / 100),  D = M - Q

{m/(1 + d/100), m - m/(1 + d/100)}

Q := M/(1 + d/100);  Dw := M - Q;

Q = M/(1 + d/100); D = M - Q;

Q = M/(1 + d/100), D = M − Q

Hardener for a 2-part paint (from the mix ratio)

H = A × b ÷ a

H = A \times \frac{b}{a}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>H</mi>
    <mo>=</mo>
    <mi>A</mi><mo>&#xD7;</mo><mfrac><mi>b</mi><mi>a</mi></mfrac>
  </mrow>
</math>

H = A xx b / a

a1*b/a

H := A*b/a;

H = A*b/a;

H = A × b/a

Thinning a 2-part paint (based on base + hardener combined)

D = (A + H) × d ÷ 100,  M = A + H + D

D = (A + H) \times \frac{d}{100},\quad M = A + H + D

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>D</mi>
    <mo>=</mo>
    <mrow><mo>(</mo><mi>A</mi><mo>+</mo><mi>H</mi><mo>)</mo></mrow>
    <mo>&#xD7;</mo>
    <mfrac><mi>d</mi><mn>100</mn></mfrac>
    <mo>,</mo>
    <mi>M</mi>
    <mo>=</mo>
    <mi>A</mi><mo>+</mo><mi>H</mi><mo>+</mo><mi>D</mi>
  </mrow>
</math>

D = (A + H) xx d / 100,  M = A + H + D

{(a1 + h)*d/100, a1 + h + (a1 + h)*d/100}

Dw := (A + H)*d/100;  M := A + H + Dw;

D = (A + H)*d/100; M = A + H + D;

D = (A + H) × d/100, M = A + H + D

Converting a thinning rate by weight to volume (with weight per gallon)

D = Q × (ρₚ ÷ ρₛ) × d ÷ 100

D = Q \times \frac{\rho_p}{\rho_s} \times \frac{d}{100}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>D</mi>
    <mo>=</mo>
    <mi>Q</mi>
    <mo>&#xD7;</mo>
    <mfrac><msub><mi>&#x3C1;</mi><mi>p</mi></msub><msub><mi>&#x3C1;</mi><mi>s</mi></msub></mfrac>
    <mo>&#xD7;</mo>
    <mfrac><mi>d</mi><mn>100</mn></mfrac>
  </mrow>
</math>

D = Q xx rho_p / rho_s xx d / 100

q*(rhop/rhos)*d/100

Dw := Q*(rho_p/rho_s)*d/100;

D = Q*(rho_p/rho_s)*d/100;

D = Q × (ρ_p/ρ_s) × d/100

How to have ChatGPT do the calculation

You are an assistant for paint mixing calculations. Do the calculations below by actually running Python code, and base your answer only on the numbers from the output (do not calculate in your head or guess).
Use this definition of the thinning rate: the water or thinner added, as a percent of the unthinned paint.

1. Thin 10 gallons of latex paint at a 10% thinning rate. Find the water to add (paint × rate / 100) and the thinned paint. Also find the share of water in the whole mix (%).
2. Make exactly 1 gallon of oil-based paint thinned 15%. Find the paint needed (1 ÷ (1 + 15/100)) and the thinner to add, in gallons and fl oz.
3. A 2-part paint has a mix ratio of base : hardener = 10 : 1, and you use 1 gallon of base. Find the hardener, the thinner to add at 10% of the base + hardener total, and the thinned paint.
4. Thin 1 gallon of paint that weighs 10.8 lb/gal by 10% by weight with lacquer thinner that weighs 7.1 lb/gal. Find the thinner volume to measure with a cup (gal and fl oz).

Show the formulas you used and the numbers from the output.

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