How do I measure case fan size?

[tbob]

I'm noob here,

Can someone give me a straightforward answer on how to measure a fan size? I've found the following answers in various web places, which are all different on my fan:

1) measure the distance between the mounting holes
2) measure the outside of the case
3) measure the blades from tip-to-tip.

Perhaps someone could take a fan that they know is an 80mm fan, for example, and measure to find out what exactly it is that's 80mm?

Thanks!

 

[MustISO]

It should be edge to edge.

 

[Zepper]

Generally the metric measure is across the flats, but that is just a convenient approximation as the mm are almost never truly at the 10s (i.e. the 120 is actually 119, etc.). The measure that needs to be accurate is the dimension between the mounting holes (on center) on the diameter. 80mm is 4" on centers, 120mm is 5-7/8" on centers, etc. The standards were pretty much set by the US companies which made the first muffin fans before metric took over the world...

Here's an example:
http://db.sanyodenki.co.jp/pro...ail.php?master_id=1652


.bh.

 

[Borealis7]

or...you can measure the diagonal and divide by the square root of 2