

Actually I always calculated belt length by measuring the distance between the center of flywheel to the the center of the other drive pulley and measuring the diameter of both. After getting the length, I multiplied it with 0.95 to accomodate for some tension from stretching. The calculation always came right to select the belts.
Measure the diameter of each pulley/flywheel and divide by 2 to get the radius of the pulley
Measure the distance from the center of the pulley/flywheeel to the center of the other pulley
Total length of the belt = [(2 X distance from the center of the pulley/flywheel to the center of the other pulley + 3.14 X radius of the flywheel/pulley + 3.14 X radius of the other pulley)] X 0.95
Basically we are adding the 1/2 of the circumference (2Ï€r) of two pulleys, and adding twice the distance between the center of two pulleys, then reducing the total length by 5% to get the snug fit.
If the diameter of the flywheel is a lot bigger than the other drive pulley, then another calculation will come onto play. The belt messurement becomes a little smaller in that case. I will soon come with another calculation to accomodate that.