Mobile & Electronics

Watt to Volt Calculator

The voltage a given wattage runs at, when you know the current.

5 inputs Free, no sign-up

Your figures

From the rating plate or the label.

The current at that power. It cannot be zero.

Leave at 1 for DC and resistive loads.

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

Watts divided by amps gives volts. It is the least-used of the three rearrangements, and it comes up when a label gives a power and a current but not a voltage — or when you are checking whether a supply is the one a device expects.

Zero current has no answer here. A device drawing no current is not running at some voltage; it is not running, and the division has nothing to divide.

How to use this calculator

  1. Enter the power in watts or kilowatts.
  2. Enter the current, in amps or milliamps. It has to be above zero.
  3. Read the voltage. Compare it against what your device expects before connecting anything.

The formula

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

Zero current is refused rather than treated as a very small number, because the answer would grow without limit and the figure produced would be meaningless rather than large.

SymbolMeaningUnit
P Power W
I Current A
V Voltage V

Worked example

A 60 W supply delivering 5 A:

V = 60 / 5 = 12 V

A 65 W laptop charger delivering 3.25 A:

V = 65 / 3.25 = 20 V

Notes and limits

  • Voltage is the figure that has to match. A device expecting 12 V will not run properly on 20 V and may not survive it, whatever the wattages are.
  • A calculated voltage that is not a standard value — 5, 9, 12, 15, 19, 20 — usually means one of the input figures was a maximum rather than an operating value.
  • Polarity matters as much as voltage on barrel connectors, and no calculation shows it. Check the symbol on the label.
  • This is arithmetic, not a compatibility check. Matching a voltage does not make a supply suitable for a device.

What this calculation assumes

  • Current must be above zero; a zero is refused rather than divided by.
  • A power factor of 1 treats the circuit as DC or purely resistive.
  • Both figures are taken as operating values, not as maximum ratings.

Frequently asked questions

How do I calculate volts from watts?

Divide the watts by the amps. A 60 W supply delivering 5 A runs at 12 V. Watts alone are not enough - you need the current as well.

What if the current is zero?

There is no answer. Dividing by zero does not produce a very large voltage, it produces nothing meaningful, so the calculation stops and says so.

The voltage I calculated is not a standard value. Why?

Usually because one of your figures is a maximum rather than an operating value. Chargers are rated at their peak output, and a device rarely draws exactly that.

Can I use any supply with the right voltage?

Voltage matching is necessary but not sufficient. The connector, the polarity and the current capability all have to suit the device too, and this calculation says nothing about any of them.

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