Mobile & Electronics

USB Power Calculator

What a USB connection comes to in watts, and why that figure is a ceiling rather than a promise.

4 inputs Free, no sign-up

Your figures

Basic USB is 5 V. Higher voltages only happen when both ends negotiate them.

From the port or charger rating. Often quoted in milliamps.

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

USB power is volts times amps, the same as anything else. A port supplying 5 V at 2.4 A is delivering 12 W.

What makes USB confusing is not the arithmetic but the chain. The charger, the cable and the device each have a limit, and what you get is the lowest of the three. A calculation on two of those numbers tells you what that pair comes to, not what your setup will actually deliver.

How to use this calculator

  1. Enter the voltage. Basic USB is 5 V; a higher figure means a fast-charge mode has been negotiated.
  2. Enter the current, in amps or milliamps — chargers usually quote milliamps, so 2400 mA rather than 2.4 A.
  3. Read the wattage, and treat it as the ceiling for that pair of figures.

The formula

Power = Voltage x Current       P = V x I

For reference, the fixed voltages defined by the USB specifications are 5 V, 9 V, 15 V, 20 V, 28 V, 36 V and 48 V, with higher ones only available on cables and devices built for them. Which of those a particular link uses is negotiated between the two ends; it is not something a calculator can determine from a wattage.

SymbolMeaningUnit
V Link voltage V
I Link current A
P Power W

Worked example

A USB-A port rated 5 V, 2.4 A:

P = 5 x 2.4 = 12 W

A link that has negotiated 9 V at 2 A:

P = 9 x 2 = 18 W

The second delivers half as much again while carrying less current, which is exactly why fast charging raises the voltage rather than the current.

Notes and limits

  • This is what those two figures come to. It is not proof that a particular port, cable or device can deliver it safely.
  • The cable is usually the limit. A cable without the right marking or the right internal wiring will carry less than the charger could supply, and there is nothing on the outside to tell you.
  • Raising the voltage is how USB carries more power without more current, because current is what heats a cable.
  • A device draws what it asks for. Plugging a phone into a high-power port does not push more into it.
  • For USB-C specifically, the USB-C power calculator covers the negotiated-profile case.

What this calculation assumes

  • The figures are taken as entered; no USB specification or profile is inferred from them.
  • The result is a ceiling for that voltage and current pair, not what a given setup delivers.
  • The cable and both devices each impose their own limit, and the lowest of them wins.

Frequently asked questions

How many watts is a USB port?

It depends on the voltage and current it supplies. A basic 5 V 500 mA USB 2.0 port is 2.5 W; a 5 V 2.4 A charging port is 12 W; a negotiated 20 V 5 A link is 100 W. There is no single answer.

Why does higher wattage need higher voltage?

Because current is what heats a cable. Raising the voltage lets the same cable carry more power without carrying more current, which is why fast-charging standards negotiate 9 V, 15 V or 20 V rather than simply pushing more amps at 5 V.

Does this tell me if my cable can handle it?

No, and nothing calculated from voltage and current could. A cable's capability depends on its conductors and, for higher power, on an identifier chip inside it. Use the figure as the ceiling for the charger, then check the cable separately.

Is 2.4 A the same as 2400 mA?

Yes. Milliamps are thousandths of an amp, so 2400 mA is 2.4 A. Chargers usually print milliamps, which is why the unit selector is there.

Related calculators