Waist Circumference:
Measure around your waist where your pant loop holes are. Usually this is right by your hip bones.

The below measurements are from the belt prong point to the 1st Hole and then the measurement from the belt prong point to the 2nd Hole.
This will provide you with the size range between the 1st and 2nd Holes for each size belt.

  S M L XL
Waist Circumference 28" - 32 7/8" 32 1/8" - 37 1/8" 36" - 40 7/8" 40 1/8" - 45 1/8"