Our Snapback hats are true-to-size based on the child's age. What this means is the average sized kid--about 50% on the growth charts--is used as the barometer for head circumference.
If you have a child whose age fits evenly between the ranges then sizing will follow as: Infant (4months-12months), Toddler (1 years-4 years), Youth (older than 4 years). If you know your child's head is larger than average, it might be best to size up rather than to size down.
The snap back helps make the hats fit tighter or looser but adjusting the fit in the back.