Easiest way to determine belt size is look at the back of the belt.Most manufacturers
put their name and belt size on the back of the belt. If it is to obscure to read then take the belt off and take it to a auto parts store and they can measure it and give you a new duplicate back. Try to buy the heavy duty version if you want them to last more than 10,000 miles, they cost a little more but you only have to change them once and not multiple times for the same miles. I just got some from Fleetpride to
replace the NAPA belts on my 8.3 Cummins they offered both the belt numbers in
a regular and a HD version for the same belt number.