How to Measure Snap Sizes?

[topshelfstuff]

I have been searching for an explanation of how to determine the size of a snap without any luck on snap websites and Youtube. For instance when a snap is listed as 20mm is that the cap or the stud? Oddly, this detail is left out of descriptions of snap sizing even in tutorials I've found online.

The reason I ask is because I am trying to reverse engineer a broken snap on a discontinued leather item in which the snap stud broke off the snap post. So I am trying to replace the snap post but I think I need to determine the size of the snap in order to determine which snap post I need to get to reattach the snap stud. 

Thanks for the help!

[fredk]

Where I buy snaps, aka poppers, from the millimeter measure given is the diameter of the nice head