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Electrical power (P = I²R) calculator

GCSEA-level Electricity

The power dissipated in a resistor, found from the current and the resistance.

P = I² R

This free calculator is for GCSE and A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

PPower to find 72.0 W

ICurrent

RResistance

Significant figures

Working

Write the equation
P = I² R
Substitute the values
P = 3² × 8
Calculate
P = 72 W
Give the answer to 3 significant figures
P = 72.0 W

Worked example

Find P, the power. Given: current I = 3.00 A; resistance R = 8.00 Ω.

Write the equation
P = I² R
Substitute the values
P = 3² × 8
Calculate
P = 72 W
Give the answer to 3 significant figures
P = 72.0 W

Symbols and units

Symbol	Quantity	SI unit
P	Power	W
I	Current	A
R	Resistance	Ω

Electrical power (P = I²R): rearranged

To find power: P = I² R
To find current: I = √(P / R)
To find resistance: R = P / I²

Exam tip

Square the current only, not the resistance. Doubling the current makes the power four times bigger.

Common questions

What is the formula for electrical power?

The equation is P = I² R, where P is the power (W); I is the current (A); R is the resistance (Ω).

What units should I use for electrical power?

In SI units: power in W, current in A, resistance in Ω. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange P = I² R?

Rearranging gives these forms: to find power: P = I² R. to find current: I = √(P / R). to find resistance: R = P / I².

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More electricity equations: Charge, current and time, Ohm’s law, Electrical power, Energy transferred, Resistors in series, Resistors in parallel, Resistivity, EMF and internal resistance. See the full list on the physics equation calculator page.