Mobile & Electronics

Voltage, Current & Power Calculator

Enter any two of voltage, current and power and get the third, in whatever units you have them in.

7 inputs Free, no sign-up

Your figures

Leave blank to calculate it.

Leave blank to calculate it.

Leave blank to calculate it.

1 for DC and resistive loads. Lower for motors and switching supplies, from the rating plate.

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

Voltage, current and power are three views of the same thing, and any two of them give the third. Rather than three separate pages, this one takes whichever two you have.

Leave the unknown field empty. The result names which value was calculated, so an input and an answer are never confused with each other — which matters when you are checking a rating plate against a measurement.

How to use this calculator

  1. Fill in the two values you know, each with its own unit.
  2. Leave the third blank.
  3. Read the result. The calculated value leads, and the ones you supplied are shown underneath so you can check them.
  4. Filling in all three is allowed: nothing is recalculated and your figures are shown back, which is useful for sanity-checking a label.

The formula

P = V x I        power from voltage and current
I = P / V        current from power and voltage
V = P / I        voltage from power and current

Each is the same equation rearranged. On AC, a power factor below 1 means the real power in watts is lower than volts times amps; leave it at 1 for DC and for resistive loads.

Any figure supplied as zero counts as not supplied, because a zero voltage or current does not describe a working circuit and dividing by it has no answer.

SymbolMeaningUnit
V Voltage V
I Current A
P Power W
pf Power factor, AC circuits

Worked example

A laptop charger labelled 20 V and 65 W, and you want the current:

I = 65 / 20 = 3.25 A

A 12 V supply drawing 1.5 A, and you want the power:

P = 12 x 1.5 = 18 W

Notes and limits

  • Two values are the minimum. One value gives nothing, whichever one it is — and the page will say which figure it needs rather than producing a number.
  • Watch the units. A current in milliamps entered as amps is out by a thousand, which is why each field has its own unit selector.
  • Rating plates give maximums. A supply rated 20 V 3.25 A can deliver 65 W; the device connected to it draws what it needs.
  • For three-phase supplies there is an extra root-three factor that this does not apply.

What this calculation assumes

  • Any value left blank or entered as zero is treated as the one to calculate.
  • A power factor of 1 treats the circuit as DC or as a purely resistive AC load.
  • Single-phase only; three-phase needs an additional root-three factor.

Frequently asked questions

Which two values do I need?

Any two of the three. Voltage and current give power; power and voltage give current; power and current give voltage. One value on its own is never enough.

What if I enter all three?

Nothing is recalculated and your three figures are shown back as entered. That is deliberate - it lets you check a rating plate without the page quietly correcting one of the numbers.

Why does entering zero not work?

A zero voltage or current does not describe a working circuit, and the calculation would divide by it. A zero is treated as a value you have not supplied.

Does this work for AC?

Yes, with the power factor set. At a power factor of 1 the result is apparent power in VA and real power in W at the same time, which is correct for DC and resistive loads. Below 1, the watts figure is the real power.

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