How the size is worked out
The load is the sum of the three figures you enter. Headroom is added as a percentage, because a supply runs most efficiently and quietly below its maximum and because graphics cards spike above their rated draw.
The total is then rounded up to the next 50 watts, which is how supplies are sold.
Worked example
A processor at 125 W, a graphics card at 250 W and 75 W of everything else gives a load of 450 W. With 30 per cent headroom that is 585 W, which rounds up to a 600 W supply. At 120 volts the load draws 3.75 amps.
These figures are illustrative. Use the peak package power from your own specifications.
Why the figures are entered, not looked up
Component power draw varies with the exact model, the BIOS, the cooling and the workload. A database would be wrong for many readers and right for a few.
Reviews that measure board power are the best source. A manufacturer's figure is usually a board power limit rather than a typical draw.
Limits of this calculator
It sums what you enter and does not measure anything. It does not model transient spikes, capacitor ageing, cable ratings or connector counts.
It uses 120 V mains, which is the US convention. The current figure is a guide, not a circuit design.
For the other country's mains voltage, use the UK power supply calculator.
Inputs for the US power supply calculator
Processor power draw
Peak package power from the specification, not the idle figure.
Graphics card power draw
Board power from the specification, or the figure a review measured.
Everything else
Motherboard, drives, fans and USB devices together. 75 W is a fair allowance.
Headroom to add
Extra capacity above the load. 30 per cent leaves room for peaks and ageing.
CALCULATOR QUESTIONS
Power supply calculator US questions
Is a bigger supply always better?
No. Efficiency is best at part load, so a grossly oversized supply wastes money without benefit.
Why add headroom?
Graphics cards spike above their rated draw, and a supply near its limit runs hotter and louder.
Does the efficiency rating matter?
It affects heat and running cost, not the size you need. This page sizes the supply, not the bill.
Can I use a supply rated just above the load?
It will run, but with little margin for spikes. The suggested size is deliberately conservative.