More or less, but why do you need such a precise measurement?
If your rim is around 19", it's a 19 inch rim. There are 17, 18, 19, 21, 23 inch rims typically, and 23 is a rare bird used mostly on early Honda XL250's.
I've never encountered a 20 inch rim outside a BMX bike, or a 22. So odds are if your wheel is around that 19 inch mark, it's a 19 inch rim.
Front or rear will help narrow it down if you're not sure either and vintage. 18 inch fronts are not typical, although they do exist like on my wife's old TU250X.