As a general rule of thumb:
S1 English 16.5" / Western 15"
S2 English 17.5" / Western 16"
S3 English 18" / Western 17"
S4 English 19" / Western 18"
I also like to incorporate jean size as it gives us a good idea of the actual "butt mass" sitting in the seat.
S1 Up to jean size 4 and 6
S2 Jean size 8 to 10/12
S3 Jean size 12 and over
S4 Jean size over 16 (special order)
When we go with the Super Soft Seat upgrade, the padding will take up about 1/2" of the seat so it will snug the seat size a bit.
For example, the S2 Super Soft Seat becomes approximately a 17" English (15.5" Western).
There are more ways to zero in on the perfect seat size. If you are still unsure, contact me so we can discuss. [email protected]