Head sizes:
Infant (3-12 months) - 17-18”
Toddler (1-3 years) - 19”
Child (3-10 years) - 20”
Teen/Adult small - 21 “
Adult large - 23”

For fitted hat:
Infant - 20 grams
Toddler - 25 grams
Child - 35 grams
Teen/Adult small - 40 grams
Adult large - 60 grams

If you choose to make this into a slouchy hat you will need to adjust your yarn requirements. For the slouchy adult large hat I used 80 grams.