Voltage equals current times resistance — V = I × R. Power is a separate relationship, P = V × I, and combining the two gives P = I²R and P = V²/R. Between them, any two of the four quantities determine the other two, which is what this page does.

How do I convert amps to watts?

You cannot, without the voltage. Watts are volts times amps, so 2 amps is 24 watts at 12 volts and 240 watts at 120. Any table claiming to convert amps to watts has silently assumed a voltage, and it is usually 120 V.

Why does a resistor need a power rating?

Because the energy it removes from the circuit leaves as heat. A 6 ohm resistor carrying 2 amps dissipates 24 watts — enough to destroy a quarter-watt part instantly. Checking the power line above before choosing a component is the difference between a working circuit and a burnt one.

Does Ohm’s law apply to everything?

To ohmic materials, which is most wire and most resistors at ordinary temperatures. Diodes, transistors, lamps and anything whose resistance changes with temperature or voltage are not ohmic, and the law describes them only at a single operating point. A filament bulb’s cold resistance is a fraction of its hot one.

Ohm's Law Calculator

With voltage 12 V, current 2 A, ohm's law comes to 24.000 W — power. It is reached in 6 steps, the last of which is 12 * 2, and each one is printed on the page with its numbers filled in. The formula is the one published by NIST Special Publication 811, not an approximation fitted to it.

Voltage, current, resistance and power from any two of them, with the whole set printed rather than the one you asked for.

Formula and sources checked 27 August 2026 · How we check

What you know Voltage and current, Voltage 12 V, Current 2 A

24.000 W

Power for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

What you knowVoltageVCurrentA

Power

24.000 W

Voltage: 12 V
Current: 2 A
Resistance: (12 / 2)6 Ω
Power: 12 * 224 W
Current in milliamps: 2 * 10002,000 mA
Kilowatt-hours if left on all day: 24 * 24 / 10000.576 kWh

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Worked example

12 volts across a load drawing 2 amps is 24 watts, and the load is 6 ohms. Left running for a day that is 0.576 kilowatt-hours — the figure an electricity bill is actually made of.

How to work it out yourself

1.Pick the pair you actually measured. A multimeter gives volts and amps directly; resistance is usually calculated rather than measured in a live circuit.
2.Read all four values, not just the one you were after. Most electrical mistakes come from knowing the current and never checking the power the component has to dissipate.
3.Watch the units: milliamps and kilohms are the everyday units of electronics, and mixing them with volts gives answers a thousand times out.

The formula

Voltage
Current
Resistance(12 / 2)
Power12 * 2
Current in milliamps2 * 1000
Kilowatt-hours if left on all day24 * 24 / 1000

Source: NIST Special Publication 811 — Guide for the Use of the International System of Units

Questions people actually ask

What is Ohm’s law?: Voltage equals current times resistance — V = I × R. Power is a separate relationship, P = V × I, and combining the two gives P = I²R and P = V²/R. Between them, any two of the four quantities determine the other two, which is what this page does.
How do I convert amps to watts?: You cannot, without the voltage. Watts are volts times amps, so 2 amps is 24 watts at 12 volts and 240 watts at 120. Any table claiming to convert amps to watts has silently assumed a voltage, and it is usually 120 V.
Why does a resistor need a power rating?: Because the energy it removes from the circuit leaves as heat. A 6 ohm resistor carrying 2 amps dissipates 24 watts — enough to destroy a quarter-watt part instantly. Checking the power line above before choosing a component is the difference between a working circuit and a burnt one.
Does Ohm’s law apply to everything?: To ohmic materials, which is most wire and most resistors at ordinary temperatures. Diodes, transistors, lamps and anything whose resistance changes with temperature or voltage are not ohmic, and the law describes them only at a single operating point. A filament bulb’s cold resistance is a fraction of its hot one.

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