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PSU Calculator
Estimates the power supply wattage a PC build needs from its CPU, graphics card, drives and fans, with headroom for transient spikes.
Calculators
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Free, unlimited
Enter your figures and press Calculate.
In short
PSU Calculator is a free online tool on inputtesthub.com that estimates the power supply wattage a PC build needs from its CPU, graphics card, drives and fans, with headroom for transient spikes. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.
How to use the PSU Calculator
- Enter your figures in the input fields.
- The result updates as you type, together with the full working.
- Check the breakdown table to see exactly how the number was reached.
About the PSU Calculator
The PSU Calculator estimates the power supply wattage a PC build needs from its CPU, graphics card, drives and fans, with headroom for transient spikes. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.
What people use it for
- Sizing a PSU for a new build.
- Checking an existing supply before a GPU upgrade.
- Understanding why a "sufficient" PSU trips under load.
Details
| Headroom applied | 30% |
|---|---|
| Best efficiency | 40–60% load |
| RTX 4090 transients | Can exceed 500 W briefly |
| Motherboard and misc | ≈ 40 W |
The maths behind it
This tool uses Continuous draw = GPU + CPU + 40 W board + 3 W per RAM stick + 8 W per drive + 3 W per fan. Recommended supply adds 30% headroom for transient spikes. Every intermediate value is shown on screen, so the result can be checked by hand.
Modern GPUs draw brief spikes far above their rated power, and a supply sized to the average will trip its protection on those transients even though the average wattage looks fine. Running at 40–60% load is also where most supplies are quietest and most efficient.
Frequently asked questions
What wattage PSU do I need for an RTX 4070?
A mid-range card around 200 W with a mainstream CPU puts a system near 400 W continuous, so a 650 W supply gives comfortable headroom and sits in its efficient range. Enter your own parts above.
Why does my PC restart under load when the PSU wattage looks fine?
Almost always transient spikes. A GPU can pull double its rated power for a few milliseconds, and a supply with no headroom sees that as an overload and shuts down to protect itself.
Are there any usage caps on the PSU Calculator?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the utility.
Is my data uploaded anywhere?
No. The PSU Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
Can I check the maths myself?
It uses Continuous draw = GPU + CPU + 40 W board + 3 W per RAM stick + 8 W per drive + 3 W per fan. Recommended supply adds 30% headroom for transient spikes. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What problem does the PSU Calculator solve?
In practice it gets used for three things: sizing a PSU for a new build. Checking an existing supply before a GPU upgrade. Understanding why a "sufficient" PSU trips under load. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Is there a catch?
Modern GPUs draw brief spikes far above their rated power, and a supply sized to the average will trip its protection on those transients even though the average wattage looks fine. Running at 40–60% load is also where most supplies are quietest and most efficient.
What figures does the PSU Calculator work from?
Headroom applied is 30%. Best efficiency is 40–60% load. RTX 4090 transients is Can exceed 500 W briefly. These are printed on the page itself, so you do not have to take them on trust.
How precise is the result?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.
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