you can usually get a pretty accurate measurement with a piece of rope. find some close to the thickness of your belt, run it through the pully system and mark it off where it was sitting. then measure it. another thing is to do some math. say you had an 8 inch pully, and needed a 100 inch belt. your new pully is 4 inches, so you subtract 4 from your original belt, now you need a 96" belt. different numbers im sure, but same concept.