The size chart below is a suggestion based on age, but it is always best to measure before buying. Remember, these are adjustable so they can range up to 1" outside of the measurements. If your child is on the upper end, use your discretion on how you would like the hat to fit. If you prefer a larger fit, you should size up.
Infant (9m-2y) 17"-19"
Toddler (2y-4y) 19"-21"
Child/Adult female (5+) 21"-23"
Adult (most males) 24"+
Keep in mind these are knitted hats. The measurements below are when laid flat, but these are made with the intention of being expandable. These will stretch several inches outside of the measurements.S/M (most children) 6.5"
M/L (most adults) 8.5"