Ohm's law calculator
Any two of voltage, current, resistance and power determine the other two. Say which pair you know and the rest follow — the calculation behind choosing a resistor, a fuse or a cable size.
- Current (A)
- 0.5000
- Resistance (Ω)
- 24.000
- Power (W)
- 6.000
- Current in mA
- 500.0
- Energy if run all day (kWh)
- 0.144
Working on mains electricity is dangerous and, in most countries, regulated. These figures are for design and study, not a substitute for a qualified electrician.
The formula
Ohm's law: V = I · R. Power: P = V · I, which rearranges to P = I²R and P = V²/R. Every combination here is one of those four identities solved for the missing pair.
Worked example
12 V across a 24 Ω resistor draws 0.5 A and dissipates 6 W. Running continuously, that is 0.144 kWh a day.
How to use it
- 1Pick the pair you actually measured, not the pair you can guess — a multimeter gives voltage and current most easily.
- 2Check the power figure against the rating of your components. A quarter-watt resistor dissipating 6 W will burn.
- 3For alternating current, these formulas hold for resistive loads. Reactive loads need impedance and a power factor.
Questions people ask
Which resistor wattage should I choose?
At least twice the calculated power, and more in an enclosed space. A 6 W dissipation wants a 10 W or larger part, with airflow.
Does this work for AC mains?
For a purely resistive load such as a heater or an incandescent lamp, yes, using RMS values. Motors, transformers and electronics draw reactive current, where apparent power and power factor enter the picture.
Last reviewed: 2026-10-20