How to measure for a girth:

Stand your horse on a level surface. Pass the tape measure around his body where the girth will rest - between his elbows and withers. Make the two ends meet and pull it snugly. This is your horse's heart girth.

This simple formula will calculate your horse"s girth size. Take your measurement in inches, divide it by three, multiply by two and subtract 2. The resulting number will only be an estimate, but it is a good starting point for ordering the correct size.
Measurements include the buckles.

Deduct 24” off a GP girth to find out what size of dressage girth is needed