Measure the narrowest part of your waist and the widest part of your hips and buttocks.

Brand Size Waist (inch) Hip (inch)
S 22.8 - 28.3 33.4 - 35.4
M 24.3 - 29.8 35.4 - 38.5
L 25.9 - 31.4 37.3 - 40.5
XL 27.5 - 33.4 39.3 - 42.4