Ohm's law calculator: any two, get all four
Volts, amps, ohms and watts from any two you know, with the formulas highlighted so you can check the work or teach an apprentice.
For DC and resistive AC loads (heaters, incandescent lamps, elements). Motors, compressors and electronics have a power factor, so watts are less than volts × amps there.
V = I × RV = P ÷ IV = √(P × R)I = V ÷ RI = P ÷ VI = √(P ÷ R)R = V ÷ IR = V² ÷ PR = P ÷ I²P = V × IP = V² ÷ RP = I² × ROhm's law says voltage equals current times resistance (V = I × R), and power equals voltage times current (P = V × I). Any two values give the other two: a 1,500 W heater on 120 V draws 1,500 ÷ 120 = 12.5 A and has a resistance of 120 ÷ 12.5 = 9.6 Ω. With motors, power factor makes watts lower than volts × amps.
How to use the Ohm's law calculator
Enter two values
Any two of voltage, current, resistance and power.
Read the other two
They calculate as you type; the two you didn't enter are highlighted.
Check the formula
The formula wheel highlights the three formulas used for that pair.
What the Ohm's law calculator does
Any two inputs
All six pairs: V and I, V and R, V and P, I and R, I and P, R and P.
Formula wheel
The twelve Ohm's law and power formulas, with the ones in use highlighted.
Zero checks
Warns when a zero would make the result undefined.
Power factor note
Reminds you that motors and electronics don't follow P = V × I exactly.
- Ohm's law
- V = I × R
- Power
- P = V × I = I² × R = V² ÷ R
- Units
- Volts (V), amps (A), ohms (Ω), watts (W)
- Cost and privacy
- Free, no sign-up; runs in your browser
Ohm's Law Calculator FAQ
What is Ohm's law?
The relationship between voltage, current and resistance in a circuit: voltage equals current times resistance. Combined with the power formula, P = V × I, it lets you find any two of volts, amps, ohms and watts from the other two.
How do I find amps from watts and volts?
Divide watts by volts. A 4,500 W water heater element on 240 V draws 4,500 ÷ 240 = 18.75 A.
How do I find resistance from volts and watts?
Square the voltage and divide by the watts. That 4,500 W element at 240 V is 240² ÷ 4,500 = 12.8 Ω.
Does Ohm's law work for AC circuits?
Yes for resistive loads such as heaters and elements. Motors, compressors, transformers and electronics have inductance or capacitance, so you need impedance and power factor, and watts are less than volts × amps.
Why does a cold element read less resistance than calculated?
Most heating elements' resistance rises as they heat up, so a meter reading at room temperature is usually a little below the value worked out from the rated watts and volts.
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