Watt calculator

What is a watt calculator?

A watt calculator works out electrical power from the quantities you already know about a circuit. Power tells you how fast electrical energy is being delivered or consumed, and it is measured in watts (W). One watt equals one joule of energy moved every second.

Most of the time you do not have all three of voltage, current and resistance. This tool lets you pick the pair you do have, then it returns the power for you. Behind the scenes it switches between three equivalent expressions so you never have to rearrange a formula by hand.

How does the calculator work?

Pick which two quantities you know from the dropdown:

1. Voltage and current — enter the voltage in volts and the current in amps.
2. Current and resistance — enter the current in amps and the resistance in ohms.
3. Voltage and resistance — enter the voltage in volts and the resistance in ohms.

The field that is not part of your chosen pair is greyed out, and the power result appears in watts as soon as both required inputs are filled in.

Formulas

The base relationship comes from Watt’s law, where power is voltage multiplied by current:

Here $P$ is power in watts, $V$ is voltage in volts, and $I$ is current in amps. Combining this with Ohm’s law, $V = I \cdot R$, gives two more useful forms. When you know current and resistance:

And when you know voltage and resistance:

where $R$ is resistance in ohms. All three return the same power for a given circuit; you simply use whichever matches the values on hand.

Worked examples

Voltage and current. A device draws 5 A from a 120 V supply:

Current and resistance. A 2 A current flows through a 3 Ω resistor:

Voltage and resistance. A 12 V source is applied across a 4 Ω resistor:

Practical notes

• These formulas describe direct current and purely resistive alternating-current loads. For reactive loads such as motors and transformers, real power also depends on the power factor.
• Resistance of zero is not allowed in the voltage-and-resistance form, because dividing by zero has no physical meaning.
• To size a fuse or supply, convert the resulting power back to current with amps = watts ÷ volts.