Many riders find that the best way to determine their seat size is to start with their current seat size. From there, you can determine if you want more or less room.
While a variety of fits are acceptable, for most disciplines we recommend at least 2" or space between the rider's thigh and the closest point on the swell.
It’s easy to measure a western saddle’s seat size.
Use a retractable tape measure. Start the tape measure behind the swell and stretch it across the seat to the front of the cantle. That measurement is the seat size.
When you’re trying to decide if a certain seat size will fit you, you should consider the cantle height and angle before making up your mind. These tips will help you pick the perfect seat size.
If you can sit in the saddle to see how it feels, that’s always best. Often times most riders (especially trail riders), feel better in a saddle that gives a little extra room rather than feeling too snug. On the other hand, speed events riders typically like their saddle seats to fit snug.
There is no one right or wrong way, as long as you can sit comfortably in the saddle with your leg at least 2 fingers from the closest point of the swell.
Here is a quick tip from Julie Goodnight