SIZE SMALL - UNDERARM TO UNDERARM IS ABOUT 13 INCHES OR ABOUT 26 INCHES ALL AROUND SIZE MEDIUM - UNDERARM TO UNDERARM IS ABOUT 14 INCHES ACROSS OR ABOUT 28 INCHES ALL AROUND SIZE LARGE - UNDERARM TO UNDERARM IS ABOUT 15 INCHES ACROSS OR ABOUT 30 INCHES ALL AROUND SIZE X-LARGE BOUT 16 INCHES OR ABOUT 32 INCHES ALL AROUND 2XL UNDER ARM TO UNDER ARM IS 17 INCHES- 3XL UNDERARM TO UNDERARM IS A 18 INCHES