The proper way to measure the seat size of a Sports Saddle is to measure between the cantle and the pommel at the nearest points and add 6 inches. Don't include the trim rolls in your measurement.

So, if you measure 9½” between the two nearest points, not including the pommel and cantle trim rolls, add 6 inches, resulting in a 15 ½” seat. Easy once you know the formulae. Take a close look at the white line between the cantle and pommel in the photo below.