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Fan Measurements

Q. Which part of the fan do I measure? A. Fan size is determined by measuring the outer edge of the frame, straight across.

Fan Measurement Don'ts:

  • don't measure diagonally
  • don't measure from the frame corners. The corners are rounded, so the measurement will come up short
  • don't measure from the holes This is the most common mistake made. Mounting hole placement is standardized by frame size, so that measurement should be unnecessary anyway. A perfect excuse to fool around with Photoshop, we've created an overly dramatic graphic showing the right direction measure from.

Fan Measurement Guide

Q. Why shouldn't I use the measurement between the mounting hole, I think it's more accurate and I wanna be sure everything will line up?
A. You can use those measurements if you like, but there's one thing to keep in mind; the majority of fan manufacturers use model numbers which incorporate the frame diameter. For example, fan model: F6010B12HS isn't just a random number; it's also a physical description of the fan

F 60 10 B 12 HS
12vdc High Speed

Expect some busy signals if you call distributors and ask them to measure the distance between the mounting holes. But, don't despair, some manufacturers include schematics of each model on their sites that show the mounting hole pitch (Delta is one example).

Q. Should I measure fans in inches or millimeters?
A. Best practice is to stick with metric, but standard measurements are still common, particularly with North American distributors and resellers. The best reason to avoid inches, when possible, is the wide variations you'll see in rounded-off measurements and the confusion it causes (real-world example: "I need to replace the 2.5" fan in my computer...")

25mm 0.98"
40mm 1.57"
50mm 1.97"
60mm 2.36"
70mm 2.75"
80mm 3.15"
90mm / 92mm 3.54" / 3.62"
120mm 4.72"

Here's a conversion chart for some popular fan diameters.

Q. I'm trying to replace a 90mm fan but I can only find 92mm models; what should I do?
A. 90mm and 92mm fans are treated as the same size and used interchangeably. It's not uncommon for frames to be slightly larger than their specs indicate. 50mm fans that actually measure 52mm are common. Differences of 2mm in the frame length rarely matter.

However, differences in frame width/thickness are something to pay attention to, especially if the fan will be attached with a snap in style mount. For example, Cooler Master produced a bunch of Slot 1 models with 50mm x 11mm fans that snap-on. It's difficult to visually tell 50x10mm and 50x11mm fans apart, but a 50mm x 10mm fan will rattle if you try to use it a replacement; very annoying.