Bonnets are designed to fit snug around the face, cover the ears, and show a little tuft of hair at the hairline. They run true to age/size. Sizing does not have to be exact due to the natural stretch of the cotton yarn. Below you will find a helpful size chart based on measurement. If your child is on the border of an age/size-- order down if you prefer a snug fit for one season; order up if you don't mind a roomier fit and the possibility of wearing it through multiple seasons.
0-3 months: about 13.5" - 15.5"
3-6 months: about 15" - 17"
6-12 months: about 17" - 18.5"
12-24 months: about 18" - 19.5"
2-4t: about 19.5" - 21.5"