Ohm's Law Calculator
Enter any two of voltage, current, resistance or power and get the other two instantly, with the classic Ohm's-law power wheel and the exact formula used.
V (V) · given
12
I (A) · given
0.5
R (Ω) · solved
24
P (W) · solved
6
One law, four quantities
Ohm's law is just V = I · R — voltage equals current times resistance. Add the power relation P = V · I and the four quantities — voltage (V), current (I), resistance (R) and power (P) — are fully linked: know any two and the other two are fixed.
That is what the “power wheel” captures: twelve rearrangements of these two equations, one for every pair of knowns. This tool picks the right one automatically.
Every formula in the wheel
| Find | From V, I | From V, R | From I, R |
|---|---|---|---|
| V | — | — | I · R |
| I | — | V / R | — |
| R | V / I | — | — |
| P | V · I | V² / R | I² · R |
Units & prefixes
The base SI units are volts (V), amperes (A), ohms (Ω) and watts (W). Real circuits span huge ranges, so prefixes matter: milliamps (mA = 0.001 A) for signal electronics, kilohms (kΩ = 1000 Ω) for most resistors, milliwatts to kilowatts for power. Keep every value in base units before applying the formulas, then scale the answer.
Where it breaks down
Ohm's law assumes a constant resistance. It holds well for metals and ordinary resistors but not for diodes, LEDs, transistors, lamp filaments (resistance rises with temperature) or anything “non-ohmic”. For those, the calculator still gives the operating-point relationship, but the device will not follow a single straight V–I line.