People come into the shop every week asking what size saddle they need. My first question is "What kind of saddle do you want?"
English, Western, Australian saddles are all measured differently and the variations in the different disciplines within
those categories also determines the seat size that you need.
English saddles are measured from the saddle nail (a small round dot, often with a company logo) at the front of the saddle just below the pommel, to the middle of the cantle. Adult saddles are generally from 16" to 18.5", Polo and Lane fox cutback saddles will usually be 19" to 21". The type of English saddle will also determine the size you need. A close contact jumping saddle can usually be a full inch shorter than a good fitting all-purpose because of the depth of the seats.
You must sit in a saddle to determine if it's the right fit for you. The saddle needs to put you into the proper position for the type of riding you will be doing, with your seat bones in the saddle your legs in the proper position, and the flaps falling far enough below the tops of your boots so they don't catch under the edge of the flaps. Most people go by the general rule of laying your hand behind your tailbone and reaching the edge of the cantle. This usually is a good indication of proper seat length but should not be "carved in stone."
Western saddles are typically 14", 15" and 16", with some bigger and some smaller and some in half-inch seat sizes. Cutting saddles usually have long flat seats, team ropers want a fairly snug seat so they don't have too far to stand up to throw their loop. Barrel racers want a secure seat that places them near the horse's center of balance.
Western seat measurements are the most confusing and inaccurate of all the sizing. Traditionally, the western seat is measured level from the inside top of the cantle to the fort of the saddle below the horn. The problem with this measurement is it doesn't take into consideration the height of the cantle or the slop of the fork. You can have a 15" seat with a forward slope to the forks and a flat dish to the cantle and that will give you 11" of thigh room. Another saddle can be a 16" seat with a straight up fork and a 5" cantle that only gives you 9.5" of thigh room. The higher the cantle, the smaller your actual amount of room in the seat area will be unless you increase the seat size proportionately.
It is best to sit in a western saddle before buying, just as it is with any big ticket item. You don't want to be stuck with one that's too big or too small.
Remember, once you find a saddle that fits you and is really comfortable, you still need to fit it to Old Dobbin so that he is as happy as you are.