Measure your chest around the largest part of your chest and your waist around the slimmest part of your waist.

Size Chest (in) Waist (in) Youth Size
XS 22-24 20.5-22 5/6
S 24-26.5 22-23.5 7
M 26.5-28.5 23.5-25 8/10
L 28.5-30 25-26 12
XL 30-32 26-28 14
2X 32-35 28-31 16