How do you measure a bathroom extractor fan

My bathroom extractor fan has broken. The clips that hold it in the mount on the ceiling have snapped.
I’ll have to get a new one, but I am not sure how to measure it.

At first, I thought you measure the dimensions of the unit. For example:
(image 1)



However, the fans I have seen online are either 100mm or 150mm. They also only give 1 dimension, so I think this is wrong.

Do you instead measure the cylinder part on the back:
(image 2)


Or do you measure the side part of the cylinder on the back:
(image 3)


Or do you measure the circle part that is in the ceiling:
(image 4)


I think it is the cylinder part on the back (as shown in image 2) but I'd be grateful if someone could confirm.

I believe the 100/150mm refers to diameter of pipe to be using, so the one in your photo is 100mm.

The 100mm is the important dimension

That’s a 100mm fan or 4 inch in old money

Don’t be tempted to buy a cheap replacement .