Both Western and English saddles come in different sizes to conform to the rider's size as well as the horse's back and girth. A saddle that is too big can slip on the horse and cause a rash or sores. A saddle that is too large for the rider can cause the rider to slide around in the saddle, making the rider more susceptible to injury. Saddles that are too small can also endanger the horse and the rider, as well as make for an uncomfortable experience.
Place a ruler or other straight object perpendicular to the seat against the base of the pommel of a Western saddle. The pommel is the handle at the front of the saddle. This gives you a point to start your measurement.
Stretch a tape measure from the straight object to the front of the cantle (the part of the saddle that slopes up in the back). This measurement is the seat size of the saddle. Most tack shops have these sizes printed on the saddle. Western saddles are often available in sizes ranging from 14 to 19 inches, though some tack shops can order or make speciality saddles in smaller or larger sizes.
Place the end of a tape measure against the front of the cantle on an English saddle and measure from this point to the centre of the nail head in the pommel, keeping the tape measure at an angle. This measurement gives you the seat size.
Saddles also have a tree size, which is the size of the bars inside the saddle. Tree size determines whether the saddle will fit your horse. Tree sizes are not standardised, and they are difficult to measure because they are inside the saddle. You will typically see tree sizes referred to as narrow, medium and wide. Sometimes tree sizes are referred to by the type of horse they are generally meant for, such as Arab, Quarter Horse and Haflinger. If you are not sure which tree size you need for your horse, ask a horse trainer or experienced tack salesperson to assist you. Many tack shops will let you take the saddle home for a few days to try it out before you make a full purchase.
Do not put the measuring tape against the pommel to measure a Western saddle. Most Western saddle pommels slope forward, which can result in an inaccurate measurement.