How this calculator works
Ohm's law says the voltage across a resistance equals the current through it times the resistance. Power is voltage times current. Any two of the four quantities (volts, amps, ohms, watts) give the other two.
Worked example: 10 A at 120 V is a resistance of 12 Ω and 1,200 W. 12 V across 4 Ω is 3 A and 36 W. 2 A through 50 Ω is 100 V and 200 W. A 1,500 W heater on 120 V draws 12.5 A and has a resistance of 9.6 Ω. 5 A at 600 W is 120 V and 24 Ω, and 100 W across 144 Ω is 120 V and 0.833 A.
Ohm's law formulas
Each quantity in terms of the other pairs.
| To find | From V and I | From I and R | From V and R | From P and V, I or R |
|---|---|---|---|---|
| Volts (V) | — | I × R | — | P ÷ I or √(P × R) |
| Amps (I) | — | — | V ÷ R | P ÷ V or √(P ÷ R) |
| Ohms (R) | V ÷ I | — | — | V² ÷ P or P ÷ I² |
| Watts (P) | V × I | I² × R | V² ÷ R | — |
Tips before you buy
- Ohm's law holds for a resistor at a fixed temperature. A lamp filament, for example, has a much lower resistance cold than hot.
- Doubling the current through a fixed resistance makes four times the heat, since power goes with the square of the current.
- Power is what you pay for: kilowatt-hours are kilowatts times the hours the load runs.
- Check the units: milliamps (mA) are thousandths of an amp, and kilohms (kΩ) are thousands of ohms.
- For household AC, use the voltage rating of the circuit and, for motors, the power factor too.
Tools & materials for this project
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Frequently asked questions
What is Ohm's law?
Voltage equals current times resistance: V = I × R. With the power formula P = V × I it links volts, amps, ohms and watts.
How do you calculate amps from volts and ohms?
Divide the volts by the ohms. 12 V across 4 Ω is 3 A.
How do you calculate resistance from watts and volts?
R = V² ÷ P. A 1,500 W heater on 120 V is 120² ÷ 1,500 = 9.6 Ω.
How do you calculate watts from amps and ohms?
P = I² × R. 2 A through 50 Ω is 200 W.
Does Ohm's law work for AC?
Yes for resistive loads such as heaters. For motors and electronics the power factor matters too, because current and voltage are out of step.
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