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Ohm's law and power formulas
The formulas for volts, amps, ohms and watts, for single-phase and three-phase circuits, on one sheet.
For motors and other AC loads, multiply by the power factor. For sizing the wire for a load, use the wire size calculator.
Verify with your local code and AHJ. Reference only, no warranty.
Ohm's law
| Volts | V = I × R |
| Amps | I = V ÷ R |
| Ohms | R = V ÷ I |
Power, single phase
| Watts | P = V × I |
| Watts | P = I² × R |
| Watts | P = V² ÷ R |
| Amps | I = P ÷ V |
For motors and other AC loads, multiply by the power factor (PF).
Three phase
| Watts | P = √3 × V × I × PF |
| Amps | I = P ÷ (√3 × V × PF) |
| kVA | √3 × V × I ÷ 1000 |
| √3 | 1.732 |
Handy
| 1 hp | 746 W |
Open this sheet in the toolkit to search it, pin it and use it offline.
Questions
How do I convert watts to amps?
Single phase: I = P ÷ V. Three phase: I = P ÷ (√3 × V × PF).
What is Ohm's law?
V = I × R: voltage equals current times resistance.
How many watts in one horsepower?
One horsepower is 746 W.