Free Physics tool · Step by step

Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to get the other two using V = IR and P = VI, with mA, kΩ and other units. Free physics tool for GCSE, AP Physics and electronics.

Fill in any two values — the other two are calculated.

Voltage (V)

12 V

given

Current (I)

3 A

calculated

Resistance (R)

4 Ω

given

Power (P)

36 W

calculated

  1. 1

    Ohm’s law and power

    V=IR,P=VI=I2R=\ racV2RV = IR,\quad P = VI = I^2R = \ rac{V^2}{R}
  2. 2

    Current

    I=VR=124=3 AI = \frac{V}{R} = \frac{12}{4} = 3\ \mathrm{A}
  3. 3

    Power

    P=V2R=1224=36 WP = \frac{V^2}{R} = \frac{12^2}{4} = 36\ \mathrm{W}

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FAQ

What is Ohm's law?

Voltage equals current times resistance: V = IR. Power is P = VI, which can also be written P = I²R or P = V²/R.

How many values do I need?

Any two of V, I, R and P — the other two follow from the formulas.

Does it work for every component?

It assumes a resistor that obeys Ohm’s law (constant resistance). Lamps and diodes change resistance with current.

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