Nothing kills a new PC build faster than a power supply that cannot feed it. The machine boots, the game loads, the graphics card spins up — and then the whole system blinks off under load. Nine times out of ten, the PSU was sized by guesswork.

The PC Power Supply Calculator above replaces guessing with arithmetic. Pick your processor and graphics card, count the smaller parts, and it totals the peak draw, adds a 30 percent cushion, and recommends the right standard PSU size.

Under-sizing is the famous failure, but over-sizing wastes money too. The goal is the middle: enough headroom for peaks and upgrades, without paying for watts you will never use. A few minutes with the calculator before ordering parts is the cheapest insurance a build can have.

What Does the PC Power Supply Calculator Do?

You select your CPU and GPU from dropdowns that carry each part's peak draw. Then you enter counts for RAM sticks, NVMe drives, hard drives, and case fans, and optionally tick the overclocking box.

The calculator adds the CPU and GPU draw, applies a 25 percent uplift if overclocked, adds the small parts at fixed per-unit values, multiplies by 1.3, and rounds up to the nearest standard size: 450, 550, 650, 750, 850, 1000, 1200, or 1600 watts.

How to Use the PC Power Supply Calculator

Choose your processor and graphics card from the dropdowns. Each entry shows its peak draw — an RTX 4090 contributes 450 W, while integrated graphics contributes nothing.

Tick the overclocking box only if your CPU or GPU is actually overclocked. Enter the counts for RAM sticks, drives, and fans. Press Calculate for the recommendation, or Reset to start a different build.

What Counts Toward the Load

The CPU and GPU make up 80 to 90 percent of a gaming PC's peak draw. Everything else is small parts, billed at fixed values: 5 W per RAM stick, 8 W per NVMe or SSD, 9 W per hard drive, 3 W per fan.

Motherboard, USB devices, and lighting draw single-digit watts each and are absorbed by the 30 percent headroom. Only something exotic, like a multi-pump water loop, deserves a manual addition to the total.

CPU Draw: Rated Power vs Real Peaks

The wattage shown per CPU is its peak under full load, not the lower figure on the box. Modern chips briefly exceed their nominal ratings, and the PSU must survive those bursts — sizing to the box number is a classic way to end up 100 W short.

CPU draw here assumes all cores loaded. Gaming rarely does that, but rendering and compiling do, and a PSU sized only for gaming peaks can shut down mid-render.

GPU Draw: The Biggest Slice

The graphics card dominates most builds. A flagship card can draw more than the rest of the system combined, which is why GPU choice moves the final recommendation more than any other decision.

The values are board power figures, and brief transient spikes can exceed even those — one more reason the 30 percent cushion exists. Integrated graphics shows 0 W because its draw is already inside the CPU's number.

The Small Parts That Add Up

Two RAM sticks, one NVMe drive, and three fans total 27 W — trivial next to a 200 W GPU. But four hard drives, six fans, and four sticks reach 80 W, which can tip the recommendation from 650 W to 750 W.

When in doubt, round counts up. An extra fan you might add later costs 3 W in the estimate and nothing until installed — cheap insurance against redoing the sum.

Why the 30 Percent Headroom Rule Exists

The formula is:

recommended PSU = peak load x 1.3, rounded up to the next standard size

Three things justify it: components age and deliver slightly less clean power over time, GPU transient spikes exceed rated draw for milliseconds, and supplies run most efficiently — and quietly — near half load.

How to Interpret Your Result Correctly

The headline number is a shopping instruction: buy this size, in 80 PLUS Bronze or better. Your actual draw is the "estimated peak load" line, and everyday use sits well below even that.

Around 25 to 35 percent headroom is the sweet spot. Much above 50 percent means rounding pushed you up a size — acceptable, since smaller standard sizes may not exist.

The Overclocking Checkbox

Tick it only for real overclocks. Factory "OC edition" cards are already covered by their board power figure, and undervolting needs no box at all — the calculator's answer is simply conservative for undervolted builds, which is what you want.

Twenty-five percent is deliberately generous: a mild overclock adds about 10 percent, an aggressive one up to 30. The calculator takes the cautious end since it cannot know your settings.

When the Recommendation Surprises You

Office builds surprise downward: a 113 W peak still recommends 450 W, simply the smallest standard size. The headroom line will show around 75 percent, which is fine.

High-end builds surprise upward. A 942 W peak needs 1225 W with the cushion, and the next standard size above that is 1600 W. The gap looks wasteful, but those are the sizes shops actually sell.

Worked Example: A Mid-Range Gaming Build

First: an Intel Core i5-14600K (181 W) with a GeForce RTX 4060 (115 W), no overclock, plus 2 RAM sticks, 1 NVMe drive, and 3 fans.

Then: core draw is 296 W, small parts add 27 W, so peak load is 323 W. Times 1.3 is 420 W, rounding up to 450 W.

Finally: the calculator recommends a 450 W PSU with about 28 percent headroom. A quality 550 W unit would be a sensible roomier buy.

Worked Example: A Flagship Build, Overclocked

First: an Intel Core i9-14900K (253 W) with an RTX 4090 (450 W), overclocked, plus 4 RAM sticks, 2 NVMe drives, 1 hard drive, and 6 fans.

Then: core draw of 703 W rises 25 percent to 879 W; small parts add 63 W. Peak load is about 942 W. Times 1.3 is 1225 W, rounding up to 1600 W.

Finally: a 1600 W PSU. The jump looks large, but 1200 W would leave almost no cushion — and flagship GPUs have the nastiest transient spikes.

Worked Example: A Basic Office PC

First: an Intel Core i3-14100 (89 W) with integrated graphics, plus 2 RAM sticks, 1 NVMe drive, and 2 fans.

Then: core draw is 89 W, small parts add 24 W, so peak load is 113 W. Times 1.3 is 147 W, but 450 W is the smallest standard size.

Finally: a 450 W PSU with about 75 percent headroom. Typical office work draws 40 to 60 W — oversized by necessity, since smaller sizes barely exist.

Worked Example: A Creator Workstation

First: an AMD Ryzen 9 7950X (200 W) with an RTX 4070 Ti Super (285 W), plus 4 RAM sticks, 2 NVMe drives, and 5 fans.

Then: core draw is 485 W, small parts add 51 W, so peak load is 536 W. Times 1.3 is 697 W, rounding up to 750 W.

Finally: a 750 W PSU with about 29 percent headroom — a textbook fit for a machine that renders with CPU and GPU loaded together.

The Efficiency Sweet Spot Explained

Power supplies convert wall power to the DC voltages components use, and the conversion is never perfect — the rest becomes heat. Efficiency peaks around 50 percent load, which is why the calculator's result screen mentions the 40-to-60 percent band.

At very low loads, efficiency sags: a 1600 W unit feeding a 100 W office PC wastes a larger share as heat than a 450 W unit doing the same job. At very high loads, heat and fan noise climb steeply. The 30 percent headroom lands most builds in the efficient middle without overshooting it.

Common PSU Sizing Mistakes

The classic mistake is sizing to average draw instead of peak. Averages are for electricity bills; peaks — plus spikes — are for power supplies.

The second is trusting a cheap no-name unit's headline wattage. Without an 80 PLUS rating it may not deliver cleanly on the 12 V rail the CPU and GPU use. The calculator's number assumes an honest, rated unit.

The third is forgetting the future. If a GPU upgrade is likely within the unit's five-to-seven-year life, size for the card you will own then, not the one you own today. Replacing a PSU mid-life costs more than buying one size up now.

Where PSU Calculations Are Useful

New builds are the obvious case: the calculator turns a parts list into a PSU size before anything is ordered, avoiding the return-and-replace cycle of an undersized unit. Upgrades are the second — a hungrier GPU in an existing machine is the most common cause of sudden shutdowns.

Troubleshooting is the third. Random reboots under load, where the calculator says the unit is undersized, point strongly at the PSU — a cheaper fix to test than a motherboard swap.

Frequently Asked Questions

1. How much power does a typical gaming PC need?

Most mid-range gaming PCs peak between 300 and 450 W and pair well with a 550 to 650 W PSU. High-end builds with flagship GPUs peak near 700 to 950 W and need 1000 W or more, which is why the calculator's headroom math matters most at the top end.

2. Is a bigger PSU always better?

No. Beyond the recommended size you gain nothing but cost. The 30 percent headroom the calculator builds in is the sweet spot.

3. What does 80 PLUS Bronze mean?

It certifies at least 82 to 85 percent efficiency at typical loads, meaning less wall power wasted as heat. Higher tiers — Gold, Platinum — waste less still. The calculator recommends Bronze as the minimum worth buying.

4. Should I include monitor power?

No. Monitors plug into the wall separately. Only components inside the case count toward the PSU size.

5. Does overclocking really add 25 percent?

It can. The calculator applies 25 percent to the CPU-plus-GPU subtotal as a cautious allowance for raised voltages and clocks.

6. Why did a 113 W build recommend 450 W?

Because 450 W is the smallest standard size the calculator rounds to. The headroom line will show around 75 percent.

7. Can I reuse my old PSU?

Often yes, if it is a quality unit with enough wattage and the right connectors — modern GPUs may need newer 12VHPWR cables that old units lack. Re-run the calculator with the new parts first, and mind its age, since capacitors degrade over five to seven years.

8. What are transient power spikes?

Millisecond bursts where a GPU exceeds its rated board power. Invisible to monitoring software, they can trip an undersized PSU's protection. The headroom absorbs them.

9. How accurate are the CPU and GPU wattages?

They are peak figures — maximum sustained draw, not averages. Real gaming loads run 10 to 20 percent lower, which is intentional: the PSU must survive the peaks, and the 30 percent headroom covers the transient spikes on top of them.

10. Should RGB lighting be counted?

It is covered by the headroom. A lit strip setup draws single-digit watts. Only multi-pump custom water loops deserve a manual addition.

11. What if I plan to add a second GPU?

Add its board power to your mental total first, or run the calculator and step up one standard size. Multi-GPU setups are rare today but double the dominant term.

12. Is it safe to run a PSU at 90 percent load?

Briefly, on a quality unit — but not as a steady state. Heat, noise, and wear climb steeply near maximum load, which is why the calculator targets 70 percent or less.

13. Do I need a new PSU when upgrading the CPU?

Re-run the numbers. CPU upgrades typically move the total 30 to 80 W — sometimes enough to cross into the next standard size, often not. The calculator settles the question in seconds, before the new chip is even ordered.

14. Why round up to standard sizes?

Because PSUs are sold in standard sizes. Recommending 697 W would be precise but useless; 750 W is what you can actually buy, and rounding up keeps the safety margin intact instead of trimming it.

15. Does the calculator account for PSU aging?

Indirectly, through the 30 percent headroom. Capacitors lose capacity over years of heat, and the cushion covers that slow decline along with spikes, upgrades, and the extra draw of dust-clogged fans working harder.