Having a properly fitted cinch for a horse is important. Girth or cinch size can vary with different types and shapes of saddles.
If you have an old cinch that fits both the horse and the saddle well, then you would get an accurate measurement by straightening out that cinch and measuring it from buckle to buckle. However this is not always possible.
If you don’t have an old cinch that fits, there is another method. First, measure your horse’s heart girth by taking one end of a measuring tape and placing it at your horse’s withers. Wrap the tape around your horse’s left side, down to a couple of inches behind the horse’s front legs, under the belly and around the right side until you reach the place where you started. The tape should be at a slight angle. Pull the tape snug, but not too tight. This gives you the measurement of the horse’s heart girth. Take the heart girth measurement (in inches) and divide it by 2. Then subtract 3 from your answer. If your figure isn’t a whole number, round it up. This should give you an estimate for your horse’s cinch size. Most cinches are sized in increments of 2 (34, 36, 38 inches etc). Note: dressage girths are shorter than this, and other English girths are usually longer, so this method will only work for a Western cinch.