Ohm's Law Calculator

V = I × R, I = V ÷ R, R = V ÷ I and P = V × I. Substituting the first into the last gives the two other power forms, P = I² × R and P = V² ÷ R — six useful rearrangements in total, and knowing any two of the four quantities lets you reach the other two through them.

For purely resistive AC loads — heaters, incandescent lamps — yes, using RMS voltage and current. For motors, transformers and other reactive loads the relationship involves impedance and power factor rather than resistance alone; the Watts to Amps Converter on this site handles the power-factor version of that calculation.

Because two values are what the maths needs and all it can honestly use. One value leaves the other three undetermined, and three or four values would usually contradict each other slightly through rounding — with exactly two, the solution is unique and exact.

Volts for voltage, amps for current, ohms for resistance and watts for power — the four SI units this calculator uses. Milliamps, kilohms and the like must be converted first (20 mA is 0.02 A); the Unit Converter handles those prefixes.

It can show the power it is dissipating: solve for P from your voltage and resistance (for example 12 V across 4 Ω dissipates 36 W) and compare it with the resistor's rated wattage. A standard small resistor is rated at 0.25 W — dissipating far more than the rating is why a part runs hot, discolours or fails.