PSU wattage calculator
Select your parts. We add the CPU's maximum turbo power, the GPU's board power and about 100 W for the rest of the system, then apply 30% headroom and round up to a common PSU size.
Why 30% headroom
Modern graphics cards draw brief spikes well above their rated board power — a 300 W card can pull 450 W for a few milliseconds. Power supplies are also most efficient and quietest around 50–70% load. Sizing to about 1.3× the steady-state estimate covers both without paying for wattage you will never use.
What the estimate leaves out
Heavy overclocking, many hard drives, RGB controllers and pump-driven water cooling each add tens of watts. If your build has several of these, step up one tier.