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
YXXS 19 - 22 18 - 20.5 3/4
YXS 22 - 24 20.5 - 22 5/6
YS 24 - 26.5 22 - 23.5 7
YM 26.5 - 28.5 23.5 - 25 8/10
YL 28.5 - 30 25 - 26 12
YXL 30 - 32 26 - 28 14
Y2X 32 - 35 28 - 31 16