Ohm's Law Calculator: Volts, Amps, Ohms and Watts
Pick the two values you know, enter them, and get the other two straight away with the formula used.
- Nothing you type is stored
- Free
- No signup
Current (I)
2 A
- Voltage (V)
- 12 V
- Current (I)
- 2 A
- Resistance (R)
- 6 Ω
- Power (P)
- 24 W
Show working
- I = V ÷ R = 12 ÷ 6 = 2 A
- V = 12 V, I = 2 A, R = 6 Ω, P = 24 W
How to use
- Choose which two values you know.
- Enter them in volts, amps, ohms or watts.
- Read all four values and the working.
Good to know
Ohm's law: V = I × R. Power: P = V × I = I² × R = V² ÷ R. With any two known, the other two follow; for example I = V ÷ R and P = V² ÷ R.
Results use engineering prefixes: 4.7 kΩ = 4,700 Ω, 20 mA = 0.02 A. Enter plain numbers in V, A, Ω and W (for 20 mA type 0.02).
For AC mains these formulas hold for resistive loads like heaters and bulbs. Motors, fans and electronics also have power factor, so real power is V × I × PF; use the Electrical power calculator for those.
Questions
How much current does a 100 W bulb draw at 230 V?
I = P ÷ V = 100 ÷ 230 = 0.43 A, and its hot resistance is V² ÷ P = 529 Ω.
What is the formula for resistance?
R = V ÷ I, or R = V² ÷ P, or R = P ÷ I².
How do I enter milliamps?
Convert to amps: 20 mA = 0.02 A.
Does it work for AC?
For resistive loads yes. For motors and electronics include power factor; see the Electrical power calculator.
Why does a bulb measure lower resistance cold?
Filament resistance rises with temperature; Ohm’s law applies at the operating temperature.