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Ohm's Law Calculator

Enter any two of V, I, R and P and get the other two.

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Fill in any two of the four values (voltage, current, resistance or power) and the other two are calculated. You can type prefixes: 20m amps, 4k7 ohms, 1.5k watts.

The result shows which formula was used, so it's handy for homework and exam prep too.

How to use

  1. Type the two values you know, using the unit menus or prefixes (m, k, M).
  2. The other two fields fill in and are marked as calculated.
  3. Change any value and the calculation follows the last two you typed. Clear starts over.

Ohm's law formulas

Ohm's law: V = I × R. Power: P = V × I. Combining both: P = I² × R = V² ÷ R, I = V ÷ R = √(P ÷ R) and R = V ÷ I = V² ÷ P.

Worked examples

A 1 kΩ resistor on 12 V draws 12 mA and dissipates 0.144 W, fine for a 1/4 W resistor. A 2,000 W heater draws about 8.7 A on 230 V, or 16.7 A on 120 V. A 100 Ω resistor rated for 1 W can take up to 10 V.

Frequently asked questions

How do I calculate current from voltage and resistance?

Divide the voltage by the resistance: I = V ÷ R. 12 V across 1 kΩ gives 0.012 A, or 12 mA.

How do I convert watts to amps?

Divide the power by the voltage: I = P ÷ V. A 60 W load on 12 V draws 5 A. If you know the resistance instead, use P = V² ÷ R or P = I² × R.

Does Ohm's law work for LEDs?

Not for the LED itself, whose voltage stays almost fixed, but it does for the resistor in series with it. Use the LED resistor calculator.

Does Ohm's law work for AC?

For resistive loads like heaters and incandescent bulbs, yes, using RMS values. Motors and power supplies have a power factor, so the simple formula is only an estimate.

Related tools

LED Resistor CalculatorThe resistor value and wattage for one LED or a string in series. Voltage Divider CalculatorVout from two resistors, or the resistors for the Vout you need. Series and Parallel Resistor CalculatorTotal resistance of any list of resistors, and standard pairs for any value.