I want to replace an old Xpelair window fan (model FXC6) as it's too
powerful. See http://i1.ebayimg.com/01/i/06/9e/51/9d_1_b.JPG

I believe it's called a "6 inch fan". The blade diameter is 6inches
(150mm).

The diameter of the circular hole in the window is 184mm (seven and a
quarter inches).

Is this hole diameter standardised so that a replacement "6inch/150mm
fan" from BLOGGS & CO would be expected to fit a window hole of 184mm
diameter?