Electric Power Calculator: P = VI

Watts, volts and amps: find any one from the other two.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • P = V\,I
  • V = \frac{P}{I}
  • I = \frac{P}{V}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the power: power = voltage x current
  2. Put the values in (every value in its SI unit): power = 230 x 10
  3. The answer: power: 2300 W
  4. The same answer in other units: 2.3 kW; 3.08435 horsepower

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

How do I convert amps to watts?

Multiply by the voltage. 10 A at 230 V is 2300 W.

How do I convert watts to amps?

Divide by the voltage. A 2300 W heater on 230 V draws 10 A.

Does this work for three-phase?

No. Three-phase power also has a factor of the square root of 3 and the power factor.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any school physics or electrical text (electrical power).

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