How to use the Ohm's law calculator
Pick the two values you know, type them in, and the other two appear instantly. The calculator also shows which formulas it used, so you can follow the working. It assumes a DC circuit or a purely resistive AC load such as a heater or incandescent lamp.
The formulas
V = I × R I = V ÷ R R = V ÷ I
P = V × I P = I² × R P = V² ÷ R
| You know | Find | Formula |
|---|---|---|
| V and I | R, P | R = V ÷ I, P = V × I |
| V and R | I, P | I = V ÷ R, P = V² ÷ R |
| V and P | I, R | I = P ÷ V, R = V² ÷ P |
| I and R | V, P | V = I × R, P = I² × R |
| I and P | V, R | V = P ÷ I, R = P ÷ I² |
| R and P | V, I | V = √(P × R), I = √(P ÷ R) |
Worked example: 120 V and 10 A
- Resistance: R = 120 ÷ 10 = 12 Ω.
- Power: P = 120 × 10 = 1,200 W.
- Check with the other forms: I² × R = 100 × 12 = 1,200 W, and V² ÷ R = 14,400 ÷ 12 = 1,200 W.
Everyday uses
- Will it trip the breaker? Add up the watts on a circuit and divide by the voltage. A 15 A, 120 V circuit carries 1,800 W, and continuous loads should stay at or below 80% of that (1,440 W).
- Heater or element resistance. A 240 V, 4,500 W water heater element has R = 240² ÷ 4,500 = 12.8 Ω. Measure it with a meter: an open reading means a failed element.
- LED resistors. A 5 V supply driving a 2 V LED at 20 mA needs R = (5 − 2) ÷ 0.02 = 150 Ω.
- Running cost. Once you know the watts, the electricity cost calculator turns them into dollars.
AC circuits and power factor
On AC, motors, compressors, transformers and power supplies store and release energy each cycle, so current and voltage don't peak together. Volts × amps then gives apparent power in volt-amperes (VA), and the real power in watts is VA × power factor. A fridge drawing 2 A at 120 V is 240 VA; with a power factor of 0.7 it uses 168 W. Heaters, kettles and incandescent lamps have a power factor of about 1, so the simple formulas hold exactly for them.
Units and prefixes
- 1 kV = 1,000 V; 1 mA = 0.001 A; 1 kΩ = 1,000 Ω; 1 kW = 1,000 W.
- Enter values in base units (volts, amps, ohms, watts). For 250 mA, enter 0.25 A.
Questions people ask
What is Ohm's law?
Voltage equals current times resistance: V = I × R. Combined with the power law P = V × I, it lets you find any two of voltage, current, resistance and power from the other two.
How do I calculate amps from watts and volts?
Divide watts by volts: I = P ÷ V. A 1,500 W heater on 120 V draws 12.5 A.
How do I find resistance from volts and amps?
Divide volts by amps: R = V ÷ I. A device drawing 2 A from 12 V has an effective resistance of 6 Ω.
Does Ohm's law work for AC?
Exactly for resistive loads like heaters and incandescent lamps. For motors, transformers and electronics, AC current also depends on reactance and power factor, so P = V × I gives apparent power (VA), not true watts.
What is the power formula with resistance?
P = I² × R or P = V² ÷ R. Doubling the current through a resistor quadruples the heat it makes, which is why undersized wires get hot.
Sources and assumptions
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.