Ohm’s law and power calculator
Fill in any two of the four values (voltage, current, resistance, power). The other two are calculated. Leave the remaining boxes empty.
How the numbers are worked out
Ohm’s law states V = I × R. Electrical power in a resistive element is P = V × I. Combining the two gives P = I² × R and P = V² ÷ R. Every pair of inputs below maps to those relations: V and I give R = V/I and P = V·I; V and R give I = V/R and P = V²/R; V and P give I = P/V and R = V²/P; I and R give V = I·R and P = I²·R; I and P give V = P/I and R = P/I²; R and P give V = √(P·R) and I = √(P/R). The relations hold for resistive circuits and for DC or for RMS values of AC in a purely resistive load; reactive loads (coils and capacitors) need impedance and a power factor, which this tool does not model. The tool works in SI base units internally and converts the prefixes you choose.