Ohm's Law Calculator (Voltage, Current, Resistance and Power)

Enter the two values you know out of voltage V, current I, resistance R and power P. The other two are calculated. Choose a unit for each field (different units are converted automatically).

Voltage V

Voltage unit

Current I

Current unit

Resistance R

Resistance unit

Power P

Power unit

Fill in exactly two fields (leave the other two blank). All values must be numbers greater than 0. The result also shows a graph of voltage against current (the V-I characteristic), whose slope is the resistance.

Result and graph

Fill in two of the fields on the left and press "Calculate". The result and a graph will appear here.

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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 any two of voltage \(V\), current \(I\), resistance \(R\) and power \(P\), and the other two are calculated together (for example, resistance and power from voltage and current)
Each field has its own unit (voltage: V, mV, kV; current: A, mA; resistance: Ω, kΩ, MΩ; power: W, kW, MW). Mixed units are converted automatically
Along with the result, a graph of voltage against current (the V-I characteristic) is shown, so you can see that the slope of the line through the origin is the resistance \(R\)
Use it for anything from school science (Ohm's law and electric power) to choosing resistors for electronics projects and practicing for electrician exams
A plain-language explanation of the formulas and copy-and-paste formulas for Excel, Google Sheets and Python are all on this page

This calculator is for purely resistive circuits where Ohm's law holds directly (DC circuits, or AC circuits with resistive loads such as heaters). For devices where power factor matters, such as motors and air conditioners, the rated power (W) may not equal voltage × current.

What is this calculation used for?

Checking power strips and extension cords for safety (home electricity)

A 1,500 W hair dryer on a 120 V outlet draws \(I = P \div V = 1500 \div 120 = 12.5\) A. A typical light-duty 16-gauge extension cord is rated for about 13 A, so the dryer alone nearly reaches the limit. Adding a space heater to the same cord would be dangerous.
Converting the watts on the label to amps and comparing them with the rating is the basic step that prevents cords from overheating and causing fires.

Choosing a resistor for an LED in electronics projects

Connect an LED directly to a battery or a 5 V supply and too much current flows and destroys it. So you put a resistor in series to limit the current. For example, with a 5 V supply, 2 V across the LED itself and a target current of 20 mA (0.02 A), the resistor gets \(5 - 2 = 3\) V, so the resistor you need is \(R = V \div I = 3 \div 0.02 = 150\) Ω.
This is the first practical calculation in electronics, and it appears in every beginner's book on Arduino and Raspberry Pi.

Current calculations for electricians (jobs)

Electricians calculate how much current will flow in outlet and lighting circuits to choose the wire size and the breaker rating. For example, a 4,500 W electric water heater on 240 V draws \(I = 4500 \div 240 = 18.75\) A, so it needs its own circuit with wire and a breaker sized for that current.
Ohm's law and the power formula are among the most important formulas on electrician licensing exams.

Why power lines use very high voltage (power grid)

Power lines have resistance too, and when current flows, \(P = I^2 \times R\) of power is lost as heat. Since the loss depends on the square of the current, sending the same power at a higher voltage and lower current cuts the loss sharply: double the voltage, the current halves, and the loss drops to one quarter.
This formula is why long-distance transmission lines run at very high voltages such as 345,000 V, 500,000 V or even 765,000 V.

Understanding what is inside an electric kettle or heater (how appliances work)

For a 1,500 W electric kettle on 120 V, you can work backward to the resistance of its heating element: \(R = V^2 \div P = 120 \times 120 \div 1500 = 9.6\) Ω. The current is \(1500 \div 120 = 12.5\) A.
The formula also reveals a surprising fact: at the same voltage, the more powerful the appliance, the lower its resistance.

Formulas and figures

Formula for voltage (Ohm's law)

Figure

Standard notation (the usual math form)

\(V\) \(=\) \(I\) \(\times\) \(R\)

In words (symbols replaced with words)

③ \(V\): voltage \(=\) ① \(I\): current \(\times\) ② \(R\): resistance

The formula in words

① Take the \(I\): current (A)

② multiply it by the \(R\): resistance (Ω)

③ and you get the \(V\): voltage (V)

Quick example

The voltage across a 50 Ω resistor with a current of 2 A flowing through it is

\(V\): voltage \(=\) current (2 A) \(\times\) resistance (50 Ω)

\(2 \times 50 = 100\)

Key idea

This is Ohm's law itself, a basic law of school science. If the resistance (how strongly the circuit resists current) stays the same, voltage and current are proportional: double the voltage and the current doubles too. Think of water in a pipe. Voltage is the push of the pump, current is the amount of water flowing, and resistance is how narrow the pipe is. The harder you push (the higher the voltage), the more flows; the narrower the pipe (the higher the resistance), the harder it is to flow.

Formula for current

Figure

Standard notation (the usual math form)

\(I\) \(=\) \(V\) \(\div\) \(R\)

In words (symbols replaced with words)

③ \(I\): current \(=\) ① \(V\): voltage \(\div\) ② \(R\): resistance

The formula in words

① Take the \(V\): voltage (V)

② divide it by the \(R\): resistance (Ω)

③ and you get the \(I\): current (A)

Quick example

The current that flows when a 20 Ω resistor is connected to a 100 V power supply is

\(I\): current \(=\) voltage (100 V) \(\div\) resistance (20 Ω)

\(100 \div 20 = 5\)

Key idea

This is formula 1 (\(V = I \times R\)) solved for current. At the same voltage, the higher the resistance, the smaller the current.

Formula for resistance

Graph

Standard notation (the usual math form)

\(R\) \(=\) \(V\) \(\div\) \(I\)

In words (symbols replaced with words)

③ \(R\): resistance \(=\) ① \(V\): voltage \(\div\) ② \(I\): current

The formula in words

① Take the \(V\): voltage (V)

② divide it by the \(I\): current (A)

③ and you get the \(R\): resistance (Ω)

Quick example

The resistance of a device that draws 0.2 A when 110 V is applied is

\(R\): resistance \(=\) voltage (110 V) \(\div\) current (0.2 A)

\(110 \div 0.2 = 550\)

Key idea

On a V-I graph (current on the horizontal axis, voltage on the vertical axis), this \(R = V \div I\) is the slope of the line through the origin. You can check it on this calculator's graph.

Formula for power

Graph

Standard notation (the usual math form)

\(P\) \(=\) \(V\) \(\times\) \(I\)

In words (symbols replaced with words)

③ \(P\): power \(=\) ① \(V\): voltage \(\times\) ② \(I\): current

The formula in words

① Take the \(V\): voltage (V)

② multiply it by the \(I\): current (A)

③ and you get the \(P\): power (W)

Quick example

The power used by a heater that draws 12 A from a 100 V supply is

\(P\): power \(=\) voltage (100 V) \(\times\) current (12 A)

\(100 \times 12 = 1200\)

Key idea

Power \(P\) (W, watts) is the amount of electrical energy used per second. It is the W in "1200 W" on an appliance's label. Combine Ohm's law (\(V = I \times R\)) with this formula (\(P = V \times I\)), and the four quantities voltage, current, resistance and power are linked so that knowing any two fixes the other two. This calculator detects which of the 6 possible pairs you entered and calculates the rest.

Formula for power (from voltage and resistance)

Standard notation (the usual math form)

\(P\) \(=\) \(V\) \(2\) \(\div\) \(R\)

In words (symbols replaced with words)

④ \(P\): power \(=\) ① \(V\): voltage ② squared \(\div\) ③ \(R\): resistance

The formula in words

① Take the \(V\): voltage (V)

② square it (multiply it by itself)

③ divide it by the \(R\): resistance (Ω)

④ and you get the \(P\): power (W)

Quick example

The power used by a 12 Ω heating element plugged into a 120 V outlet is

\(P\): power \(=\) voltage (120 V) squared \(\div\) resistance (12 Ω)

\(120 \times 120 \div 12 = 1200\)

Key idea

This is \(P = V \times I\) with \(I = V \div R\) (formula 2) substituted for \(I\). Even without knowing the current, you can find the power from the voltage and resistance alone.

Formula for power (from current and resistance)

Standard notation (the usual math form)

\(P\) \(=\) \(I\) \(2\) \(\times\) \(R\)

In words (symbols replaced with words)

④ \(P\): power \(=\) ① \(I\): current ② squared \(\times\) ③ \(R\): resistance

The formula in words

① Take the \(I\): current (A)

② square it (multiply it by itself)

③ multiply it by the \(R\): resistance (Ω)

④ and you get the \(P\): power (W)

Quick example

The power used by a 5 Ω resistor with a current of 2 A flowing through it is

\(P\): power \(=\) current (2 A) squared \(\times\) resistance (5 Ω)

\(2 \times 2 \times 5 = 20\)

Key idea

This is \(P = V \times I\) with \(V = I \times R\) (formula 1) substituted for \(V\). This power turns into heat in the resistor (Joule heating). Because of this formula, losses in power lines grow with the square of the current, which is why power plants send electricity at high voltage (see also "What is this calculation used for?").

Ohm's law, \(V = I \times R\), links voltage, current and resistance. Add the power formula \(P = V \times I\), and knowing any two of the four quantities fixes the other two. On a V-I graph, the slope of the line is the resistance.

Symbols and terms

Symbols

\(V\)	vee	Voltage, how strongly the circuit is pushed to make current flow. It comes from the first letter of "voltage". Its unit is also V (volts), so the quantity symbol \(V\) and the unit V are the same letter (example - \(V = 120\) V).
\(I\)	eye	Current, the amount of electricity flowing through the circuit. The letter comes from the French "intensité de courant" (intensity of current), and current is written \(I\) in English too. The unit is A (amperes, or amps).
\(R\)	are	Resistance (electrical resistance), how strongly the circuit resists current. It comes from the first letter of "resistance". The unit is Ω (ohms).
\(P\)	pee	Power, the electrical energy used per second. It comes from the first letter of "power". The unit is W (watts).
V	volt	The unit of voltage. An AA battery is 1.5 V and a standard US household outlet is 120 V. mV (millivolts, one thousandth) and kV (kilovolts, a thousand times) are also common.
A	ampere (amp)	The unit of current. The "15 A" or "20 A" on a household circuit breaker is this unit. In electronics projects, mA (milliamps, one thousandth) is common.
Ω	ohm	The unit of resistance. It is the capital Greek letter omega, named after Georg Ohm, the German physicist who discovered Ohm's law. kΩ (kilohms, a thousand times) and MΩ (megohms, a million times) are also common.
W	watt	The unit of power. Using energy at 1 joule per second is 1 W. It is familiar from appliance labels (for example, a 1,500 W hair dryer). There are also kW (kilowatts, a thousand times) and MW (megawatts, a million times).

Terms

voltage	How strongly current is pushed through a circuit. In the water comparison, it is the push of the pump. Two batteries in series double the voltage and push current more strongly.
current	The amount of electricity flowing through a circuit. In the water comparison, it is the amount of water flowing per second. What actually flows is electrons.
resistance (electrical resistance)	How strongly something resists the flow of current. In the water comparison, it is how narrow the pipe is. At the same voltage, the higher the resistance, the smaller the current. Current through a resistance makes heat, as in the heating element of a space heater.
power	The amount of electrical energy used (or supplied) per second, in W (watts). It is the wattage on an appliance's label. Power × time of use is energy (in kWh and similar units), the quantity your electric bill is based on.
Ohm's law	The law "voltage = current × resistance" (\(V = I \times R\)), which says voltage and current are proportional. The German physicist Georg Ohm published it in 1827. It is the most basic law of electric circuits taught in school science.
Joule heating	The heat produced when current flows through a resistance. The heat produced per second is \(P = I^2 \times R\) (W), known as Joule's law. Space heaters, hair dryers and electric kettles are appliances that use this heat.
V-I characteristic	A graph of the relation between voltage and current, with current on the horizontal axis and voltage on the vertical axis (or the other way around; then it is often called an I-V curve). For a resistor that follows Ohm's law, it is a straight line through the origin, and its slope is the resistance. This calculator shows this graph with the result.
power factor	In an AC circuit, the share of the apparent power (voltage × current) that is actually used. For motors, air conditioners and similar devices it is less than 1, so power (W) = voltage × current no longer holds. For purely resistive devices such as heaters and incandescent bulbs it is almost 1, and this calculator's formulas work as is.

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.

Electric circuits, current and voltage (middle school science)	Knowing that the current in a circuit is measured in A (amps) and the voltage across it in V (volts) Knowing where current and voltage stay the same and where they split in series and parallel circuits
Ohm's law (middle and high school science)	Knowing that voltage and current are proportional when the resistance is constant (\(V = I \times R\)) Being able to rearrange the formula to find the current or the resistance
Electric power (middle and high school science)	Knowing that power (W) is the electrical energy used per second and is found with \(P = V \times I\)
Proportional relationships and graphs (Grades 7–8)	Knowing that for a proportional relationship such as \(y = ax\), the graph is a straight line through the origin and \(a\) is its slope
Multiplying and dividing decimals (Grades 5–6)	Being able to divide decimals such as \(110 \div 0.2\) Being able to convert units by a factor of 1,000, such as 1 kΩ = 1000 Ω and 1 mA = 0.001 A

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 voltage (V = I × R)

Current I (A)	0.2
Resistance R (Ω)	550
Voltage V (V)	=B1*B2

Table to find the current (I = V ÷ R)

Voltage V (V)	110
Resistance R (Ω)	550
Current I (A)	=B1/B2

Table to find the resistance (R = V ÷ I)

Voltage V (V)	110
Current I (A)	0.2
Resistance R (Ω)	=B1/B2

Table to find the power (P = V × I)

Voltage V (V)	110
Current I (A)	0.2
Power P (W)	=B1*B2

Table to find the power (P = V² ÷ R)

Voltage V (V)	110
Resistance R (Ω)	550
Power P (W)	=B1^2/B2

Table to find the power (P = I² × R)

Current I (A)	0.2
Resistance R (Ω)	550
Power P (W)	=B1^2*B2

After pasting, B1 and B2 are your inputs and B3 is calculated automatically.
"*" is multiplication, "/" is division and "^" is a power (exponent). "=B1^2/B2" means "square B1 and divide by B2".
For example, B3 shows 110 (V) in the first table, 0.2 (A) in the second, 550 (Ω) in the third, and 22 (W) in each of the fourth to sixth tables. Just replace B1 and B2 with your own numbers. Enter the values in V, A, Ω and W (convert values in mA or kΩ first, using 1 A = 1,000 mA and 1 kΩ = 1,000 Ω).

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 voltage (V = I × R)

Current I (A)	0.2
Resistance R (Ω)	550
Voltage V (V)	=B1*B2

Table to find the current (I = V ÷ R)

Voltage V (V)	110
Resistance R (Ω)	550
Current I (A)	=B1/B2

Table to find the resistance (R = V ÷ I)

Voltage V (V)	110
Current I (A)	0.2
Resistance R (Ω)	=B1/B2

Table to find the power (P = V × I)

Voltage V (V)	110
Current I (A)	0.2
Power P (W)	=B1*B2

Table to find the power (P = V² ÷ R)

Voltage V (V)	110
Resistance R (Ω)	550
Power P (W)	=B1^2/B2

Table to find the power (P = I² × R)

Current I (A)	0.2
Resistance R (Ω)	550
Power P (W)	=B1^2*B2

The same formulas as in Excel work as is. Copy the whole table, paste it into cell A1, and replace B1 and B2 with your own numbers.

How to calculate it in Python

voltage = 110.0   # voltage V (V)
current = 0.2     # current I (A)

resistance = voltage / current   # resistance R = V ÷ I (Ω)
power = voltage * current        # power P = V × I (W)

print(f"Resistance: {resistance} Ω")
print(f"Power: {power} W")

# Check: V = I × R, P = V² ÷ R, P = I² × R (all three give the same result)
voltage_check = current * resistance
power_from_v_r = voltage ** 2 / resistance
power_from_i_r = current ** 2 * resistance
print(f"Check (voltage): {voltage_check} V")
print(f"Check (power, V²÷R): {power_from_v_r} W")
print(f"Check (power, I²×R): {power_from_i_r} W")

Runs with the standard library only. In this example the resistance is 550.0 Ω and the power 22.0 W, and the checks give a voltage of 110.0 V and 22 W for both power formulas. (Only the I²×R check shows a tiny error, such as "22.000000000000004", because of how computers handle decimals; it means the same as 22 W.) "**" is a power (exponent). Change the voltage and current at the top and run it again. Enter the values in V, A, Ω and W.

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

Formula for voltage (Ohm's law)

V = I × R

V = I R

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>V</mi>
    <mo>=</mo>
    <mi>I</mi>
    <mo>&#x2062;</mo>
    <mi>R</mi>
  </mrow>
</math>

V = I R

i*r

v := i*r;

V = I*R;

V = IR

Formula for current

I = V ÷ R

I = \dfrac{V}{R}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>I</mi>
    <mo>=</mo>
    <mfrac><mi>V</mi><mi>R</mi></mfrac>
  </mrow>
</math>

I = V/R

v/r

i := v/r;

I = V/R;

I = V/R

Formula for resistance

R = V ÷ I

R = \dfrac{V}{I}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>R</mi>
    <mo>=</mo>
    <mfrac><mi>V</mi><mi>I</mi></mfrac>
  </mrow>
</math>

R = V/I

v/i

r := v/i;

R = V/I;

R = V/I

Formula for power

P = V × I

P = V I

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>P</mi>
    <mo>=</mo>
    <mi>V</mi>
    <mo>&#x2062;</mo>
    <mi>I</mi>
  </mrow>
</math>

P = V I

v*i

p := v*i;

P = V*I;

P = VI

Formula for power (from voltage and resistance)

P = V² ÷ R

P = \dfrac{V^{2}}{R}

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>P</mi>
    <mo>=</mo>
    <mfrac>
      <msup><mi>V</mi><mn>2</mn></msup>
      <mi>R</mi>
    </mfrac>
  </mrow>
</math>

P = V^2/R

v^2/r

p := v^2/r;

P = V^2/R;

P = V^2/R

Formula for power (from current and resistance)

P = I² × R

P = I^{2} R

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mrow>
    <mi>P</mi>
    <mo>=</mo>
    <msup><mi>I</mi><mn>2</mn></msup>
    <mo>&#x2062;</mo>
    <mi>R</mi>
  </mrow>
</math>

P = I^2 R

i^2*r

p := i^2*r;

P = I^2*R;

P = I^2R

How to have ChatGPT do the calculation

You are an Ohm's law calculation assistant. 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).

When 110 V is applied to a device, a current of 0.2 A flows.
Using Ohm's law V = I × R and the power formula P = V × I, find each of the following:
1. The resistance of this device (Ω)
2. The power used by this device (W)
3. The current (A) and power (W) when 120 V is applied to the same resistance

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