Measure your chest around the fullest part of your chest keeping the tape horizontal and your waist around the slimmest part of your waist.

Size Chest (in) Waist (in)
XXS 31 - 34 24 - 26
XS 33 - 36 26 - 28
S 35 - 38 28 - 30
M 37 - 40 30 - 32
L 39 - 42 32 - 34
XL 41 - 44 34 - 36
2X 43 - 46 36 - 38
3X 45 - 49 38 - 41
4X 48 - 52 41 - 44