My son will be 11.5 month kid loves this hat. I have to turn the brim up slightly but after I do that he will wear this hat for hours without pulling it off. It is easy to stuff in a pocket or a purse.  We went by the measurements on their website.

Age, Weight, Head Measurement, Hat Size

0-6 mos 10-15 lbs 14"-17" Small

6-12 mos 15-20 lbs 17"-18.5" Medium

12-18 mos 20-25 lbs 18".5-20" Large

18-24 mos 25-30 lbs  18".5-20" Large

2 years 30-35 lbs 20"-22" X-Large

3 years-up 35 lbs and up 21"-22.5" XX-Large