Equestrians employ the deep seat, long stirrups and straight-cut flaps of a dressage saddle to maintain close contact with the horse's back and sides, enabling them to control the forward impulsion and lateral body movements necessary to the sport. The longer saddle billets -- the straps that buckle and hold the girth -- necessitate that riders use a shorter girth than they do with jumping or close contact saddles. By measuring correctly for girth size, dressage enthusiasts eliminate spending money on tack that won't work for their saddle or their horse.
Halter your horse and tie the lead rope to a secure spot in its stall or place your horse in crossties.
Set your dressage saddle on your mount's back minus the saddle pad. Ensure the saddle is in the correct spot by fitting the deepest part of the seat into the deepest part of your horse's back.
Position your measuring tape on the third hole from the bottom of the left billet and secure it in place with a piece of masking tape.
Pull the measuring tape under your horse's ribs up to the third hole from the bottom of the right billet, fitting the tape snugly as you would a girth. This measurement gives you the size of girth you need to purchase.
If you're using a fleece girth or a fleece girth cover, measure as you would for a flat, leather girth. The fleece compresses against the horse's ribs so that the girth can't cause rub sores, but doesn't extend the length. Dr. Jessica Jahiel, American Riding Instructor certified dressage trainer and author of "Riding for the Rest of Us: A Practical Guide for Adult Riders," recommends subtracting 50 cm (20 inches) from the size of your regular girth to get the size of your dressage girth. She notes that riders should check the condition of their billets to ensure they haven't stretched, which would cause the measurement to be incorrect.
Measure your girth so that when you buckle it, you can fit two fingers between the girth and your horse's ribs. A too-long girth can cause your saddle to slip when you mount and be unsafe for riding. A too-short girth binds your mount's ribs and lungs and can cause sores where it rubs the skin.