Ohm's Law Calculator

Choose which two values you know. The calculator finds the third and the power dissipated.

Only the two values you pick are used; the third field is ignored.
Ignored when solving from current and resistance.
Ignored when solving from voltage and resistance.
Ignored when solving from voltage and current.

Voltage

12 V

Current

2 A

Resistance

6 Ω

Power

24 W

How it works

Ohm's law says that the current through a resistance is the voltage across it divided by that resistance: I = V ÷ R. Rearranged, it also gives V = I × R and R = V ÷ I, so knowing any two of the three fixes the third.

Pick which two values you know in the Solve From box. The calculator reads only those two fields and ignores the third completely, so you can leave it at whatever it was. It then finds the missing value and the power, P = V × I, which is the rate at which the circuit turns electrical energy into heat, light or motion.

Power is what matters for sizing: a resistor must be rated above the watts it dissipates, and an appliance's watts determine what it costs to run and how much current it pulls from a circuit.

Formula

V = I × R        (voltage, volts)
I = V ÷ R        (current, amps)
R = V ÷ I        (resistance, ohms)
P = V × I        (power, watts)

Example

A 12 V supply pushing 2 A through a load means the load's resistance is 12 ÷ 2 = 6 Ω, and the power is 12 × 2 = 24 W.

A 120 V space heater drawing 12.5 A has a resistance of 120 ÷ 12.5 = 9.6 Ω and dissipates 120 × 12.5 = 1,500 W.

Assumptions and limitations

  • Ohm's law as used here applies to DC circuits and to purely resistive AC loads such as heaters and incandescent lamps. Motors, transformers and electronics have impedance and a power factor below 1, so the watts shown will overstate their true power.
  • Resistance is treated as constant. Real conductors and filaments change resistance as they warm up.
  • Power is the electrical power delivered to the resistance, not the useful output of a device.
  • A calculation is not code compliance. Conductor sizing, overcurrent protection and any installation work are governed by the National Electrical Code (NEC) and local amendments; consult them and a licensed electrician before doing any wiring.

Frequently asked questions

Why does the third field do nothing?

Only two of the three values are independent; the third is determined by them. The calculator uses the two you select in Solve From and ignores the other field so a stale value there cannot contaminate the result.

Can I use this for a 120 V household appliance?

Yes for resistive appliances such as heaters, kettles and incandescent bulbs. For anything with a motor or electronics the nameplate watts or amps are a better guide than V ÷ R, because the load is not a simple resistance.

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