A cross section of your horse's heart girth/barrel shows this part of the body is oval shaped. Generally the widest point is midway between the top of the withers and the chest floor (see diagram).
Your western cinch buckles should be 2-3 inches above the point of the elbow, yet below the widest point of the heart girth. This measurement gives you the length of cinch needed.
*To simplify measurements for everyone, the Length Measurement should be taken from the outside edge to outside edge of the buckles. ~Leslie
Now observe your horse's heart girth and barrel conformation from the side. Look at the chest floor just behind the elbow.
#1- Is he streamlined and tucked up like a greyhound as in the photo on the left, with his rib cage getting smaller towards the flank? This horse shape may benefit from the use of a breast collar.
#2- Does he have a level floor of heart girth as in the center photo? This body type is easiest to fit, girth width consideration are based on the saddle riggin.
Or #3- does he have a grove where the cinch would lay as in the photo on the right? with a well sprung rib cage that is wider than the heart girth area? This horse needs extra attention that the cinch doesn't slide up towards the elbows and cause extra pressure on the scapula.
If your cinch slants forward, as in this photo, then it is pulling your saddle forward while riding, putting pressure on the scapula (shoulder blade) and exerting excess pressure to the bottom of the heart girth, and causing excess friction of the hair and skin.
In this photo we have a 3/4 rigged saddle with a store-bought roper style (supposedly) 100% mohair 36" long cinch that is 3.5" wide at the buckle. It has leather reinforced center Dee rings. When we removed this cinch and measured the length it had stretched to 39 inches! This tells us it is a lower percentage of Mohair.
The vertical tape measure is being used as our plumb line because it was quite windy and we couldn't keep a string from blowing! The wider horizontal tape shows that the plumb line is 1-1/8" off center.
When using this cinch, it is tightened several times during a ride and the saddle has a tendency to roll. This horse has been quite crabby (cinchy) due to girth pain while being saddled even though he routinely receives bodywork and physical therapy.
So 1-1/8" + 1-1/8" = 2-1/4" or 2.25"
2.25" + 3.5" cinch width @ buckle = NEEDS 5.75" wide cinch.
As you can see in this picture, we have changed cinches to a 23 strand hand tied 99% Mohair cinch that is 34" long. The plumb line now hangs vertical and dissects the center of the riggin Dee and the Cinch Buckle. The center Dee rings are tied in (no leather or nylon webbing material). This horse never flicked an ear while cinching up, standing pleasantly while being saddled. After initial cinching, but not overly tightened, the saddle never rolled, even while riding up and down steep hills and ponying colts. He moved out much more freely. ~Leslie