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Ohm’s law calculator

Volts, amps, ohms and watts from any two.

Try the circuit in the labBuild it on the simulator's bench and measure it.

How it is worked out

V = I × R: the voltage across a resistance is the current through it times the resistance. Rearranged, I = V ÷ R and R = V ÷ I. Power is P = V × I, which is also I² × R and V² ÷ R.

Example: 12 V across 4.7 kΩ drives 12 ÷ 4700 = 2.55 mA, and the resistor turns 12 × 0.00255 = 31 mW into heat, so a ¼ W resistor is plenty.

Inputs accept prefixes: 4.7k, 2.2M, 500m (milli), 10u (micro).

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