Watts, Amps and Volts Calculator
Fill in two of watts, amps and volts, and leave the third blank.
This result is an estimate only. Double-check important figures before relying on them.
How it is calculated
- Single phase: W = V × A × pf
- Three phase: W = √3 × V × A × pf
- The calculator rearranges the formula for the value you leave blank
For the full set of resistance and power formulas, use the Ohm’s law calculator. For kVA and kW, see the power factor calculator.
Worked example
A load drawing 10 A on 240 V single phase with a power factor of 1:
- Power = 2,400 W, which is 2.4 kW
Why your number may differ
Real loads change with temperature and voltage. Always check the nameplate rating, and have wiring work done by a competent person.
Frequently asked questions
What is the formula for watts?
For a single-phase supply, watts = volts × amps × power factor. For three phase it is √3 × volts × amps × power factor.
How do I convert watts to amps?
Divide the watts by the volts, and by the power factor. A 2,400 W heater on 240 V draws 10 A.
What power factor should I use?
Use 1 for heaters and kettles. Motors, compressors and some lighting have a lower power factor, often 0.8 to 0.9.