Check the correspondence with your usual pant size. If you are between two sizes, choose the larger size to be more comfortable.

BIOpads US/CA UK/AU EU Waist (in) Hips (in) Height (in) Waist (cm) Hips (cm) Height (cm)
9-10 yr 28.7–30.7 4'5" 73-78 138
11-12 yr 31.1–33.1 5' 79-84 152
13-14 yr 33.5–35.8 5'3" 85-92 160
XS 2 6 34 23–25 35–36 60-64 90-94
S 4 8 36 25–26 37–38 64-68 95-98
M 6 10 38 26–28 39–40 68-72 99-103
L 8 12 40 28–30 41–42 72-76 104-108
XL 10 14 42 30–31 43–45 76-80 109-113
2X 12 16 44 31–33 46–47 80-84 114-120
3X 14 18 46 33–35 48–49 84-88 121-127
4X 16 20 48 35–36 50–52 88-92 128-133
5X 18 22 50 36–38 53–54 92-96 134-139
6X 20 24 52 38–40 55–57 96-100 140-145

Body measurements, not garment measurements. If in doubt between two, choose the larger size for comfort.