Ohm's Law Calculator
Enter any two values — voltage, current, resistance, or power — and get the other two instantly, with the formula used.
DC or purely resistive AC (RMS values). For motors and electronics, see the AC note below.
All four values
—
formula will appear here
For planning and study. Always verify circuits with a meter and follow local electrical codes for anything mains-powered.
The Ohm's law wheel, in plain terms
Two formulas connect the four electrical quantities: V = I × R (Ohm's law) and P = V × I (power). Rearranged, they give you all twelve combinations — this calculator picks the right pair automatically:
- From V and I: R = V ÷ I, P = V × I
- From V and R: I = V ÷ R, P = V² ÷ R
- From I and R: V = I × R, P = I² × R
- From P and V: I = P ÷ V, R = V² ÷ P
- From P and I: V = P ÷ I, R = P ÷ I²
- From P and R: V = √(P × R), I = √(P ÷ R)
AC note: these hold exactly for DC and for resistive AC loads (using RMS values). For inductive or capacitive loads — motors, LED drivers, anything electronic — resistance becomes impedance and real power is V × I × power factor. For those cases, use our Watts to Amps calculator, which includes power factor.
Ohm's law FAQ
What is Ohm's law?
Current through a conductor is proportional to the voltage across it: V = I × R. Combined with the power formula P = V × I, any two of the four values give you the other two.
How do I calculate watts from volts and amps?
Watts = volts × amps. A 12 V circuit drawing 5 A delivers 60 W. Exact for DC and resistive AC; multiply by power factor for reactive AC loads.
How do I find resistance from volts and amps?
R = V ÷ I. If 12 V pushes 2 A through a component, resistance is 6 Ω.
Does Ohm's law work for AC circuits?
For purely resistive loads, yes — use RMS values. For motors and electronics, resistance becomes impedance and real power depends on power factor, so treat the simple form as an approximation.