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Ohm's law links voltage, current and resistance, and the power formula adds watts. Fill in two boxes, leave the other two empty, and the missing values are calculated.

Enter your values

Volts (V). Leave empty if you want it calculated.
Where do I find this?

Voltage is the electrical 'pressure': a car battery is about 12 V, a wall socket 230 V (Europe) or 120 V (North America).

Measure it with a multimeter set to V, or read it from the label.

Amps (A). Leave empty if you want it calculated.
Where do I find this?

Current is the flow of electricity. It is on the device's label, or measured with a clamp meter or a multimeter in series.

Ohms (Ω). Leave empty if you want it calculated.
Where do I find this?

Resistance is printed or colour-coded on a resistor, or measured with a multimeter set to Ω with the circuit switched off.

Watts (W). Leave empty if you want it calculated.
Where do I find this?

The device's label shows its power in W or kW (1 kW = 1000 W).

Result

Enter your values to see the result.

Voltage
– V
Current
– A
Resistance
– Ω
Power
– W

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How it is calculated

  • Voltage: V = I × R
  • Power: P = V × I
  • Combined: P = I² × R and P = V² ÷ R
  • Current: I = V ÷ R and I = P ÷ V; Resistance: R = V ÷ I

Example

12 V across a load that draws 2 A: the resistance is 12 ÷ 2 = 6 Ω and the power is 12 × 2 = 24 W.

What this estimate leaves out

Ohm's law holds for simple resistive loads on DC. For AC circuits with motors or electronics, use the power factor (see the Watts to Amps Calculator).

Written and reviewed by QLines, Cologne · Last reviewed

FAQ

Frequently Asked Questions

What is Ohm's law?

Ohm's law says voltage equals current times resistance: V = I × R. With the power formula P = V × I, any two of the four values give you the other two.

How do I calculate watts from volts and amps?

Multiply them: watts = volts × amps. 12 V at 2 A is 24 W.

How do I calculate resistance?

Divide the voltage by the current: ohms = volts ÷ amps. 12 V and 2 A give 6 Ω. If you know power and current instead, resistance = power ÷ current².

Can I enter more than two values?

No: two values are enough, and a third could contradict them. Clear the boxes you want calculated.

More free calculators

Watts to Amps Calculator

Convert watts to amps or amps to watts for DC, single-phase and three-phase circuits.

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