PC power supply calculator: size a PSU part by part

This PC power supply calculator adds up what is in the case, divides the total by the share of the rating you want to use, and rounds up to a standard power supply size. Beneath the size you get the estimated load, the minimum rating, the 12 V current and how hard the chosen size would work. It suits a revived Core 2 desktop and a current build alike, with one caveat for modern graphics cards.

How to use the PC power supply calculator

• CPU power (W): on older chips, the thermal design power (TDP) from the maker's spec page. Many current chips draw well above TDP while boosting, so for those the field hint asks for the higher sustained limit (Intel's Maximum Turbo Power, AMD's PPT). Sizing a CPU cooler by TDP explains both.
• Graphics card board power (W): the card maker's board power figure, or 0 for onboard graphics. If you cannot find one, the plugs give a ceiling: the slot supplies up to 75 W, a 6-pin plug 75 W and an 8-pin plug 150 W.
• Counts: memory modules, SATA SSDs, 3.5 inch hard drives, case fans, optical drives and USB devices powered by the PC.
• Share of the rating: 50, 60 or 70 percent of the power supply's rating at full system load.

Every field needs zero or a positive number.

The allowances

The calculator turns each count into watts with a fixed allowance:

These are planning allowances, not measurements. They are round figures for a typical part under load, and real parts land above and below them. The page does not let you edit them, so when a label tells you a part draws more, add the difference to the CPU line. That slightly overstates the 12 V figure, which errs on the safe side. Two common cases: a CPU cooler fan has no field of its own, so count it as a case fan, and a bus-powered USB 3 drive may take up to 4.5 W (5 V at 0.9 A), so count it as two USB devices.

Why aim for 50, 60 or 70 percent

The minimum rating is the load divided by the share you pick. A switching power supply is most efficient in the middle of its range: the 80 PLUS program tests at 20, 50 and 100 percent load (Titanium adds 10 percent), and every level's requirement peaks at 50 percent. Planning for 50 to 70 percent at full system load keeps the unit near that peak, lets its fan run slower, leaves room for brief load spikes and absorbs the slow loss of capacity as its capacitors age.

Sixty percent is the balanced default. Pick 50 for a quiet machine or one you expect to upgrade. Pick 70 for a fixed build on a unit you trust.

Standard sizes

The calculator rounds up to the first size at or above the minimum from this list: 300, 350, 400, 450, 500, 550, 650, 750, 850, 1000 and 1200 W. Units in other sizes, such as 600 or 700 W, follow the same rule. The list stops at 1200 W. If the minimum rating comes out above that, the page still shows 1200 W but adds a red warning that the build is outside what the estimator covers.

The 12 V estimate

Since the Pentium 4, the CPU has been fed from 12 V through the board's voltage regulator, and graphics cards take nearly all their power from 12 V as well. The calculator adds the CPU, the graphics card, the fans and half of each hard drive allowance (the motor side), then divides by 12. Compare the result with the combined 12 V rating on the power supply's label, not with the big number on the box. Older designs put much of their rating on the 3.3 V and 5 V rails, so a healthy total can still hide too few 12 V amps. The board regulator also loses some power as heat, so leave margin.

Worked example: the default settings

The calculator opens with an 80 W CPU (the TDP of a Xeon E5450), a 75 W graphics card that runs on slot power alone, two memory modules, one SATA SSD, one hard drive, three case fans, no optical drive, two USB devices and the 60 percent target.

Board and chipset              25 W
CPU                            80 W
Graphics card                  75 W
Memory       2 x 3 W     =      6 W
SATA SSD     1 x 3 W     =      3 W
Hard drive   1 x 8 W     =      8 W
Case fans    3 x 2 W     =      6 W
Optical      0 x 15 W    =      0 W
USB          2 x 2.5 W   =      5 W
Estimated load                208 W

Minimum rating   208 / 0.60                = 346.7, shown as 347 W
Pick             first size at or above 347 = 350 W
Load at 350 W    208 / 350                 = 59%
12 V draw        (80 + 75 + 6 + 4) / 12    = 13.75, shown as 13.8 A

The page shows 350 W in large type, with 208 W, 347 W, 13.8 A and 59% beneath it and a line noting the 25 W board allowance. Change the share and the pick moves. At 50 percent the minimum is 416 W, the pick 450 W and the load 46%. At 70 percent the minimum is 297 W, the pick 300 W and the load 69%. The 12 V draw stays at 13.8 A, because the share changes the size, not the parts. Current units carry nearly their whole rating on 12 V, so that is easy for them; on an old unit, read the label.

Limits

• Transient spikes. Modern graphics cards can draw far more than their board power for fractions of a millisecond. Intel's ATX 3.0 design guide answered with power excursion requirements: units above 450 W must ride through twice their rating for 100 microseconds. The calculator works with sustained figures, so for a current high-end card, also check the card maker's recommended power supply size and take the larger answer.
• Old units and capacitor aging. A label describes the unit when it was new. Electrolytic capacitors dry out with heat and years, and units from the capacitor plague years around 1999 to 2007 carry extra risk.
• Quality matters more than the label. Two 500 W units can differ widely in regulation, ripple and protection circuits. The 80 PLUS badge measures efficiency and power factor, none of those; published reviews that load and measure a unit tell you more.
• Plugs. The calculator does not check connectors (24-pin main, 4-pin or 8-pin CPU, PCIe).

Choosing a power supply for an older PC covers rails, connectors, aging and how to test a unit.

Questions

Is the recommended size a minimum or a target?

A minimum. It is the smallest standard size that keeps the full-load estimate at or below your chosen share. A bigger unit is fine; the main cost is that it spends more of its life at low load, where efficiency is lower.

Why does a 208 W build get a 350 W supply?

Because 208 W is the estimate at full load, and the 60 percent setting asks for a unit that would carry it at 60 percent of its rating. A 300 W unit would run at 69 percent, which the 70 percent setting accepts.

Can I keep the power supply that came with an old PC?

The calculator tells you whether its rating is enough, not whether it is healthy. Check its 12 V amps against the estimate, then look for the warning signs: a burnt smell, a rattling fan, bulging capacitors or random reboots.

What do I enter for an overclocked CPU?

Its estimated heat at the new clock and voltage. The cooler TDP headroom check works that out from stock TDP, clock and voltage, and its estimate is a fair stand-in for the CPU line here.