Mobile & Electronics

Amp to Watt Calculator

Watts from amps and volts, handling milliamps - the conversion for turning a charger label into a wattage.

5 inputs Free, no sign-up

Your figures

From the label. Watch whether it says A or mA.

The voltage that current flows at.

Leave at 1 for DC - which covers every USB charger and battery. Lower only for AC motors and similar.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A charger label gives volts and amps; the wattage is the two multiplied. A label reading 5 V 3 A is a 15 W charger, and one reading 9 V 2 A is 18 W — which is how a charger with a smaller current number can be the more powerful of the two.

This is also the conversion that catches people out on milliamps. A 500 mA supply is half an amp, so it is 2.5 W at 5 V, not 2,500 W.

How to use this calculator

  1. Enter the current, and check whether the label says amps or milliamps — they are a factor of a thousand apart.
  2. Enter the voltage that current flows at.
  3. For AC motors, set the power factor from the rating plate. For anything USB or battery, leave it at 1.

The formula

Power = Voltage x Current                    P = V x I
Power = Voltage x Current x Power factor     AC with a power factor

Multi-voltage chargers list several pairs — 5 V 3 A, 9 V 2.22 A, 12 V 1.67 A. Each pair works out to about the same wattage, because the charger's limit is its power rather than any one of the voltages.

SymbolMeaningUnit
I Current from the label A
V Voltage V
pf Power factor, AC only
P Power W

Worked example

A charger labelled 5 V, 3 A:

P = 5 x 3 = 15 W

A supply labelled 12 V, 500 mA:

500 mA = 0.5 A
P = 12 x 0.5 = 6 W

Notes and limits

  • Milliamps are the usual mistake. 500 mA is 0.5 A; entering it as 500 A gives an answer a thousand times too big, which is why the unit selector is there.
  • A charger's wattage is its maximum output, not what a device will draw. Plugging a phone into a 65 W charger does not push 65 W into the phone.
  • Where a charger lists several voltage and current pairs, work out each one: the highest is the charger's real capability, and the others are what it offers at lower voltages.
  • Wattage alone does not make two chargers interchangeable. The voltage has to match what the device expects, or the device has to be able to negotiate for it.

What this calculation assumes

  • Values are taken as entered, in the units selected beside each one.
  • A power factor of 1 treats the circuit as DC, which covers USB and batteries.
  • A label gives a maximum rating, not what a connected device draws.

Frequently asked questions

How many watts is 5 V 3 A?

15 watts. Volts times amps gives watts, so 5 x 3 = 15.

How do I convert milliamps to watts?

Divide the milliamps by 1,000 to get amps, then multiply by the voltage. 500 mA at 12 V is 0.5 x 12 = 6 W. Selecting mA above does that division for you.

My charger lists several ratings - which is its wattage?

Work out each voltage and current pair and take the highest. A charger listing 5 V 3 A, 9 V 2.22 A and 12 V 1.67 A is a 20 W charger that offers 15 W at 5 V.

Is a higher-wattage charger bad for my phone?

Not in itself - a device draws what it is designed to draw rather than what the charger could supply, provided the voltage is one the device expects or can negotiate. Matching wattage alone is not what makes a charger suitable.

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