A simple formula to remember is adding 1mm to rim internal width will add .5mm to the mounted tire width.

I just happen to have the same tire on two different rims of 21 and 25mm width (I borked my rear 21mm rim and put on the 25mm one as a substitute for now). I just measured my tires and the 21mm was 30.5mm wide and the 25mm was 32.5mm wide .. so it exactly matches this simple approximation .. not bad! The tires are labeled 700x30c so they must have been put on 20mm or some such rims for the 30c number.