Volts, Amps & Watts Calculator
Enter any two values – voltage, current, power or resistance – and get the rest instantly.
Fill in any two fields. The last two you typed are used; the others are calculated.
Results
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Formula cheat sheet
Watts to amps at common voltages
Resistive loads (power factor 1). Useful for checking a circuit breaker or extension lead rating.
| Power | 12 V DC | 120 V | 230 V | 400 V 3‑phase |
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How volts, amps, watts and ohms relate
Ohm's law says voltage equals current times resistance (V = I × R), and electrical power is voltage times current (P = V × I). Combining the two gives every other formula, so knowing any two of the four values lets you calculate the other two. Example: a 2,000 W kettle on 230 V draws 2000 ÷ 230 ≈ 8.7 A.
AC circuits and power factor
With alternating current, motors, transformers and many power supplies cause the current to shift out of step with the voltage. The power factor (cos φ, between 0 and 1) is the share of the apparent power (volt-amperes, VA) that does real work (watts): P = V × I × PF. Heaters and incandescent lamps have a power factor of about 1; motors are often 0.8–0.9. The rest is reactive power, measured in VAR.
Three-phase power
For a balanced three-phase load, enter the line-to-line voltage (for example 400 V in Europe or 208/480 V in North America): P = √3 × V × I × PF, where I is the current in each line.
Results are theoretical. Always follow local electrical codes and leave a safety margin when sizing cables, fuses and breakers.