PSU Wattage Calculator
Find the recommended power supply wattage.
Default 30%
Results update as you type.
How it works
Choosing the right power supply for a PC means adding up component power draw and adding headroom, and this calculator does the sizing. You enter the total wattage of your components and the desired headroom percentage, and it returns the recommended PSU wattage. The total component wattage is the sum of the power drawn by your CPU, GPU, motherboard, RAM, storage drives, fans, and peripherals under load — the graphics card and processor dominate, so those matter most. Adding headroom, commonly around 30 percent, is important for several reasons: components can briefly spike above their rated draw, especially GPUs; power supplies run most efficiently at roughly 50 to 80 percent of their rated capacity rather than maxed out; and extra margin leaves room for future upgrades. So a system drawing 400 watts is well served by a roughly 520-watt supply. Undersizing a PSU can cause instability, shutdowns, or an inability to handle peak loads, while a modest oversize is safe and efficient — though a hugely oversized unit wastes money and runs less efficiently at very low loads. Also consider the 80 PLUS efficiency rating and quality of the unit, not just wattage. This calculator gives a solid starting recommendation. Enter your component wattage and headroom to find the recommended PSU size.
Recommended = total component wattage × (1 + headroom %)
The recommended power supply wattage is the sum of your components' power draw multiplied by one plus the headroom percentage. Headroom of around 30 percent leaves margin for power spikes, future upgrades, and keeps the PSU running efficiently below its maximum rating.
Source:PC power sizing
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter total component wattage.
- 2
Adjust the options
Adjust headroom if they apply — sensible defaults are already set.
- 3
Read your result
Recommended PSU updates instantly as you type — no button to press.
Frequently asked questions
How do I calculate the PSU wattage I need?
Add up the power draw of all your components under load, then multiply by about 1.3 to add roughly 30 percent headroom. A 400-watt system is well matched to a 520-watt supply. The headroom covers power spikes, keeps the PSU in its efficient range, and leaves room for upgrades, so do not size it exactly to the component total.
Why add headroom to the PSU size?
Headroom matters because components, especially GPUs, can spike briefly above their rated draw, and power supplies run most efficiently around 50 to 80 percent of capacity rather than at their maximum. Extra margin prevents instability under peak loads and leaves room for future upgrades. Around 30 percent headroom is a common, sensible choice for most builds.
What draws the most power in a PC?
The graphics card (GPU) and processor (CPU) are by far the biggest power consumers, often together accounting for most of a system's draw under load. High-end GPUs alone can pull 300 watts or more. Motherboard, RAM, drives, and fans use comparatively little. So sizing a PSU centers mostly on your CPU and GPU choices.
What happens if my power supply is too small?
An undersized PSU can cause system instability, random shutdowns or restarts under load, failure to boot, or an inability to handle peak power demands, potentially stressing the unit. In the worst cases it can shorten the PSU's life or damage components. Sizing with adequate headroom avoids these problems and keeps the system running reliably.
Does PSU efficiency rating matter?
Yes. The 80 PLUS rating (Bronze, Gold, Platinum, etc.) indicates how efficiently the PSU converts wall power to DC, with higher ratings wasting less energy as heat. A more efficient, quality unit runs cooler and quieter and can lower electricity use. Wattage sizing and efficiency both matter, so choose a reputable PSU with an appropriate rating.