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

Brand Size Waist (in) Hip (in)
S 22.0-27.5 22.8-29.8
M 23.6-29.1 24.4-31.4
L 24.4-29.8 25.2-32.2
XL 25.9-31.4 26.7-33.8

Natural silk has little stretch. For the best fit, consider sizing up.