Bonnets look best when they fit close to your child's head. For example size 6-12 months would be a bit loose on a 6 month old baby but will fit nicely on a 12 month old baby.
Please use these measurements to determine size.
0-3 months - head circumference 14-15.5" 3-6 months - head circumference 15.5-17" 6-12 months - head circumference 17-18.5" 12-18 months - head circumference 18.5-19.5" 18-24 months - head circumference 19.5-20" 2-4 years - head circumference 20-21.5"