Rad River Co.
Size 1 (50 cm): Designed to fit kids aged 9 to 36 months and measures 46 to 50 cm. Velcro is flush at 48 cm.
Size 2 (54 cm): Designed to fit kids aged 3 to 5 years and measures 50 to 54 cm. Velcro is flush at 52 cm.
Size 3 (56 cm): Designed to fit kids aged 6 to 10 years and measures 52 to 56 cm. Velcro is flush at 54 cm.
If you’re unsure of sizing and don’t have a soft measuring tape, place a string/ribbon/shoelace around your or your child's head just above the ears and across the mid-forehead (where they would naturally wear a hat). You want the string to be held in place around the head firmly, but not super tight. Don't stress too much — remember, they’re adjustable! From there, use a ruler or measuring tape to measure the string.
Another option is to measure the inside of a hat that already fits you or your child.
Please note that measurements are in cm.