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I recently picked up an old system with fairly noisy fans. I'd like to replace them, but the fans themselves don't have any mention of their size. Now, I know that there's standard sizes like 40, 60, 80, 92, 120 (and non standard ones, like sticking a box fan on the side of your system).

My question is, how do I measure the size? Is it by the size of the blades, or the square casing they're in?

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It's the length or width (doesn't matter because it's a square) of the frame in millimeters, not the blades.

image of 92mm fan from other, deleted answer.

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what is the size in your example? – Louis Rhys Sep 12 '11 at 6:24
92mm, i edited that in, as it was in a deleted answer, and while this was the most correct answer, the diagram made it clearer. – Journeyman Geek Sep 12 '11 at 7:23
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You measure a computer fan along one edge, from the points where the screw holes are.

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My question is, how do I measure the size? Is it by the size of the blades, or the square casing they're in?

What difference does it make? The frame is at most a few mm larger than the blades. For example, if the blades have a diameter of 76mm, then the fan is obviously an 80mm fan. Because for all intents and purposes, (ie, regular, consumer use) fans are standard sizes, so you can just round to the nearest size (there are no standard sizes that close together).

One time, I saw what I thought was a 40mm fan on a video-card which turned out to be in actuality 36mm:

Regardless, the fan itself had no frame, so with one, it would still have been a 40mm fan anyway.

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on a case fan, it makes a difference in terms of the holes on the fan fitting the holes on the case -which need to be a precise fit – Journeyman Geek Oct 17 '11 at 0:29
Are you saying that a fan with 78mm blades will have different screw-holes than a fan with an 80mm frame? My point is that most general-use fans are standard sizes, and because the difference between each size is more than the difference between the blades and the frame, you can round to the nearest size to figure out the “official” size of the fan using either the blades or the frame. – Synetech Oct 17 '11 at 4:31
well, there are iirc, 80 and 82 mm fans - which are pretty close in size. If bringing an example fan was not an option, and i had measured the blades of a 92mm fan, it would measure as an 92 mm fan, and you'd get the wrong size. In this case, there IS a convention which is going by case size. – Journeyman Geek Oct 17 '11 at 6:16
But are the 82mm or 92mm fans in common use in consumer systems? How often do you come across them? For most people, rounding is sufficient. If you work in a field with proprietary sizes a commonality, then you’ll need your tape measure to measure each and every aspect since if one part is non-standard, then other parts (like the screw hole distances, spacing between blades and frame, etc) are probably non-standard as well. – Synetech Oct 18 '11 at 17:49
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