Choosing a power supply is a balance: too small and the system crashes under load or fails to start, too large and money is wasted on capacity that never gets used. The right size depends on the components actually installed, because a high end graphics card can draw several times the power of the processor, and drives and fans add a smaller but real amount on top. This calculator adds up the estimated draw of your CPU, GPU, storage drives and fans, then recommends a power supply size with thirty percent headroom, rounded up to the nearest fifty watts. The headroom matters because power supplies run most efficiently and quietly below full load, and because component power spikes briefly above their rated draw.
Quick Answer: Enter CPU watts, GPU watts, drive count and fan count, then click Calculate PSU Size to get total draw and the recommended supply.
Free Online Tool
PC Power Supply Calculator
Component draw plus 30% headroom, rounded up to the nearest 50 W.
How to Use the PC Power Supply Calculator
- Find your CPU’s rated power draw from its specification, usually listed as TDP or processor base power.
- Find your graphics card’s rated board power from its specification page.
- Count your storage drives and cooling fans, since each adds a small fixed amount.
- Click Calculate PSU Size to see the estimated total draw and the recommended supply size.
- Choose a supply at or above the recommended size, from a reputable maker, with the connectors your parts need.
Key Features
- Component based total: CPU, GPU, drives at ten watts each and fans at three watts each.
- Recommended size with thirty percent headroom, rounded up to the nearest fifty watts.
- Clear split between estimated draw and recommended supply, so the safety margin is visible.
- Simple inputs that use published specification numbers rather than guesswork.
- Instant local calculation for build planning and upgrade checks.
Best Practices and Tips
- Use board power for the graphics card, not the minimum supply size a vendor suggests, since the calculator adds headroom itself.
- Add headroom for a future upgrade, such as a stronger graphics card, if you plan one within the life of the supply.
- Check connector requirements separately: wattage alone does not guarantee the right cables for your parts.
- Prefer a supply with an efficiency rating, since a more efficient unit wastes less power as heat at the same wattage.
Common Mistakes to Avoid
- Sizing from the CPU alone and forgetting the graphics card, which is usually the largest single draw in a gaming PC.
- Buying exactly the estimated draw with no headroom, which leaves no margin for power spikes or future parts.
- Trusting a generic wattage number for a whole system instead of adding up the actual components installed.
Frequently Asked Questions
Why thirty percent headroom?
Power supplies are most efficient and quietest when loaded well below their maximum, and components briefly draw more than their rated power during spikes. Thirty percent headroom covers those spikes, keeps the supply in its efficient range, and leaves room for a modest upgrade without replacing the supply.
What is TDP and is it the same as power draw?
TDP, or thermal design power, is a cooling specification rather than a precise electrical measurement, but for most consumer CPUs and GPUs it is the best published proxy for typical full load draw. High performance parts can exceed it briefly, which is one more reason the calculator adds headroom.
Does a bigger PSU waste electricity?
No, not meaningfully. A power supply only delivers what the system draws, so a larger unit idles at the same low draw as a smaller one. The cost of oversizing is the purchase price and possibly slightly lower efficiency at very light loads, not a large running cost.
Do drives and fans really matter?
Individually they are small, at about ten watts per drive and three watts per fan in this calculator’s estimates, but a system with many drives and fans can add fifty watts or more. Including them keeps the total honest rather than optimistic.