Please measure above ears where a hat band would sit with a soft measuring tape or a string, then measure the string. Please take age into consideration and also know that if you want the snaps in the back to sit flush then order the smaller # in " for example if your Childs head measures 18.5" then order the INFANT. These hats adjust down to the smaller # of inches and will sit flush at the bigger # of inches. 
Suggested sizing: LOOK AT AGE MOSTLY.
Sun Hats:
Infant/Toddler: 2M-3YR 
16.5-18 inches 
Toddler/Child: 3YR-10YR 
18-22 inches
Trucker Hats:
If you know your child has a larger head than measure. After lots of reviews we have found our hats compared to other company’s like George Hats™ run one size bigger. If you are right on the cusp to switch then depending on the fit you want (perfect or tightened a little) size down or up. 
If your child is on the last inch of the measurements listed below, that is the size you will want.
NEWBORN (N): Most 3-9M 15”-17.5”
INFANT (XS): Most 6M-2YR 17.5-18.5”
TODDLER (S): Most 2YR-4YR 18.5-20”
CHILD (M): Most 5YR+ 21-23” (a lot of adult women) 
ADULT (L): 23-25”