Sizing Reference:
(0-3 Months) Circumference: Up to 15"
(3-6 Months) Circumference: Up to 16"
(6-12 Months) Circumference: 16 - 18"
(2T-4T) Circumference: 19" (Approximate) 
(SM Adult) Circumference: 20"
(MED Adult) Circumference: 21"
(LG Adult) Circumference: 22"
(XL Adult) Circumference: 23"