Ohm's Law & Power
The V·I·R·P wheel — enter any two of voltage, current, resistance and power and get the other two — plus DC, single-phase and three-phase AC power (kW, kVA, kVAr) and the current a known load draws. Live as you type, fully offline.
Open Ohm's Law & Power →What is the Ohm's law & power calculator?
A free electrical calculator that runs entirely in your browser. It solves the classic V·I·R·P wheel: type any two of voltage, current, resistance and power and it works out the other two, showing the exact formula it used. A second tab turns volts, amps and power factor into real power in kW, apparent power in kVA and reactive power in kVAr for DC, single-phase and three-phase supplies, together with the phase angle. A third tab answers the question electricians ask most often on site — how much current will this load draw? — from the load in kW, the supply voltage and the power factor. Three-phase voltages are treated as line-to-line throughout. Everything updates as you type, your last entries are saved on your own device, and nothing is ever uploaded.
How to use Ohm's Law & Power
- Pick a tab — Ohm's law for the V·I·R·P wheel, AC power for kW/kVA/kVAr, or Amps from power to size a supply.
- Enter any two values — On the Ohm's law tab, fill in any two of the four boxes. The two you used are highlighted and the other two are calculated instantly.
- Choose the supply type — On the AC tabs, select DC, single-phase or three-phase. Power factor is hidden for DC because it does not apply, and three-phase voltage is line-to-line.
- Read the results — Headline figures appear beneath the inputs, with the formula used shown on the Ohm's law tab. Use Clear to reset the wheel and start again.
Frequently asked questions
Which two values should I enter?
Any two will do. If you type into a third box the calculator uses the two you edited most recently, so you can keep changing your mind without clearing anything.
What is the difference between kW, kVA and kVAr?
Real power in kW is the part that does useful work. Apparent power in kVA is what the supply and cabling must actually carry. Reactive power in kVAr is the part that flows back and forth without doing work. Power factor is the ratio of kW to kVA.
Should I enter line-to-line or line-to-neutral voltage?
For three-phase, enter the line-to-line voltage — 400 V, 415 V or 480 V, for example. The calculator applies the √3 factor for you.
Why is power factor ignored on DC?
Power factor describes the phase difference between voltage and current in an alternating supply. DC has no phase angle, so power is simply volts times amps and the field is hidden.
Tips
- The Ohm's law tab is unit-agnostic in scale — enter volts, amps, ohms and watts and read the same units back.
- If you know a motor's rating in kW, use the Amps from power tab with its nameplate power factor to size the cable and protective device.
- Reactive power is what a power-factor correction capacitor cancels out, so the kVAr figure tells you roughly how much correction a load would need.