Voltage, current, resistance, power - fill in any two of the four fields below and leave the other two blank. The full Ohm's Law + power wheel figures out the right formula automatically.
Enter V=120 and P=100, leave current and resistance blank: R = V² ÷ P = 120² ÷ 100 = 144Ω, and I = P ÷ V = 100 ÷ 120 = 0.833A. Try it above.
| Find | From V & I | From V & R | From I & R |
|---|---|---|---|
| V | - | - | V = I×R |
| I | - | I = V÷R | - |
| R | R = V÷I | - | - |
| P | P = V×I | P = V²÷R | P = I²×R |
Ohm's Law itself is just V = I×R. Combined with the power equation P = V×I, algebra gives twelve total formulas covering every pair of known values - which is why this tool only needs two numbers, not three, to solve everything. One real limitation worth knowing: this assumes a simple resistive circuit. It doesn't directly apply to components like diodes and LEDs, whose resistance isn't constant - which is exactly why LED circuits need an external resistor to limit current, since the LED can't limit itself the way Ohm's Law assumes.