To measure an English saddle's seat, measure from the nail on the pommel to the center of the cantle. To measure a western saddle, measure from the center of the cantle to the center of the pommel below the horn.
Never measure English saddles from the center of the pommel because that results in a too-small seat size. Be sure to measure from the actual nail head and not one of the D-rings or other attachments that the saddle might have, which are generally placed more forward and can give an inaccurate reading. Cutback saddles, which are used for saddleseat riding, are generally measured the same way as other English styles.
When you measure a western saddle, measure from the leading edge of the cantle to the point just above the swell of the gullet, which is usually marked by a hole in the pommel. Do not measure from the base of the horn, which often is set slightly forward, or from the outer edge of the cantle, which can add more than half an inch to the measurement.
Sometimes, the same seat size can feel different, depending on the manufacturer, style of saddle, size of the tree or the way it fits your horse. Always at least sit in a saddle before buying it, even if it is the same size as saddles you have used in the past. Most saddle shops allow customers to take new saddles home on trial, which can help ensure you get the right size.