The only way to be sure of a saddle's fit is to actually sit in the saddle. However, measuring your leg can give you a quick idea of what size saddle you need so that you have a place to start on your saddle hunt. Proper saddle sizing not only keeps you comfortable as you ride, but it helps hold you in the saddle if the horse makes sudden, unexpected moves. When you've determined the correct size for either a Western or English saddle, it's easy to figure out what size you need in the other style.
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Sit on a flat chair so your thighs are parallel to the floor to measure for an English saddle. Have a friend measure the outside of your thigh from the front of your knee to the back of your hips. To measure for a Western saddle, measure the circumference of your thigh about 4 inches below your hips.
Record the numbers and match them to a saddle seat size, often provided by the manufacturer. For example, if your English saddle measurement is 19 inches, you probably need a 16 1/2-inch seat. If it's 22 inches, you probably need a 17 1/2-inch seat. For a Western saddle thigh measurement of 24 inches, you probably need a 16-inch saddle, but a measurement of 25 bumps up to a 17-inch saddle.
Sit on the saddle that matches the size determined by your measurements. The saddle should either be on a horse or a barrel-shaped dummy to allow you to sit as you would during normal riding. Check your seat spacing; there should be about one to two fingers of space between your back and the cantle, or raised rear portion of the seat, as well as between the front of your thighs and the front of the seat -- it's open on an English saddle, but a Western saddle has a pommel in the front that your thighs shouldn't be touching. Adjust the saddle size slightly as necessary for the proper fit.
Find your correct seat size in either Western or English saddles, then translate that number to the other style. In general, the Western saddle seat number will be 2 inches shorter than the English saddle one. For example, if you need a 16-inch Western saddle seat, you most likely need an 18-inch English version.