Saddles are measured in many ways. The “side" of a saddle refers to the seats size. When someone says a saddle is 18 inches it refers to how big the saddle is from pommel to cantle (front to back). Do not confuse saddle sides with tree size, gullet size, or any of the other measurements horse saddles are sold in. Saddle size refers to hot it fits the rider one. For example, a size 17 saddle will almost always fit a rider who measures for a size 17 seat, but only some size 17 saddles will fit that rider's horse. When considering a saddle to ride in, tree size is just as or more important than the size of the seats. If the tree does not fit a horse, the saddle size is irrelevant.
To determine the size saddle you need you may ask “what size saddle do I need? The best way to determine in your seat sides is to sit in several different saddles. A saddle that fits your seat size should feel comfortable, you should feel that you are sitting in the center of the saddle, in the deepest part. Your pelvis should not feel tilted forward or backward. You should be able to fit three or four fingers width in front of you and behind you in English horse saddles. This is how you determine the size of saddle you need.