Glad you asked...
How to measure a v belt? This is done by holding the belt tight and measuring the belt from curve to curve and then doubling the number.
Then again, this answer might help you...
The easiest way to measure a V belt or any other type of pulley belt is to first mark a starting spot on the belt. Place your mark lined up with a mark on a board or wall, then roll the belt until you get back to the beginning. Measure the distance, then subtract 2 inches.

