It isn't the "actual" width (it doesn't work that way) but it's stamped on the rim for tire fitment. For example a 21X2.500 is a 21" tire with an INTERIOR rim width of 2.5". If it's the tire width you're looking for and you have a metric size: (for example 120/70-17), the first number is the cross sectional width (in mm.) and the second is the aspect ratio (distance from bead to interior belts) being 70% of 120mm (or 84mm) for a 17" tire (and does not take into consideration thread thickness).
I was looking for INTERIOR rim width I did'nt know that rims were specified any other way. If you say its stamped on the rim I guess I need to take another my rims again.