Computers & Laptops

PSU Headroom Calculator

How much of your power supply is spare, in watts and as a percentage.

2 inputs Free, no sign-up

Your figures

The wattage printed on the label on the side of the unit.

Everything the supply has to carry, added up. The PC power calculator works it out from a component list.

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

Headroom is what is left over. It is the number that decides whether a faster graphics card fits the supply you already own, and whether the machine has any room to grow.

Enter the rating and the load, and this gives the spare capacity in watts and as a share of the supply.

How to use this calculator

  1. Enter the supply's rating from its label.
  2. Enter what the system currently draws.

To size a new supply rather than check an old one, use the PSU wattage calculator.

The formula

Capacity left    = Rated capacity - System power
Headroom percent = Capacity left / Rated capacity x 100
SymbolMeaningUnit
Rated The supply's label figure W
System power What the machine draws W

Worked example

A 650 W supply carrying 470 W:

Capacity left = 650 - 470            = 180 W
Headroom      = 180 / 650 x 100      = 27.7%

Notes and limits

  • Headroom is not wasted capacity. It is what absorbs a component upgrade, a hot day and the short spikes a graphics card produces under load.
  • A negative figure means the load already exceeds the rating, which is a sizing problem rather than a headroom one.
  • Supplies lose a little capacity as they age, particularly the cheaper ones, so a unit several years old has less real headroom than its label suggests.
  • Connectors constrain an upgrade as much as watts do. Check what the new part needs before relying on the spare capacity.

What this calculation assumes

  • Both figures are ones you entered.
  • The rating is treated as continuous capacity at the label's stated conditions.

Frequently asked questions

How much headroom should a power supply have?

Enough for the load, a hot day and anything you plan to add. Beyond that it is a judgement - there is no figure that is correct for every machine, which is why this page reports what you have rather than grading it.

Does spare capacity waste electricity?

Barely. A supply draws what the components ask for, not what it is rated at. The only effect is efficiency: units are slightly less efficient at very low load, so a 1,000 W supply running a 100 W machine wastes a little more than a smaller one would.

Can I fit a bigger graphics card with this headroom?

If the card draws less than the capacity left, the wattage allows it. Also check that the supply has the connectors the card needs, and leave something in hand for transient spikes.

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