truthfully i would find a way to make it up to the nearest parts store, and just ask them to pull belts until u find one. measuring is not always very accurate and u will still spend a few trips to the store i bet. but if u wanna try measureing, get some string and route it the way u would ur belt, thenpull the tensioner all the way up, and tighten the string. measure and cut it to find the length of the belt u will need, then use it to have them find u a belt as close to that size as possible