Based upon other buyer feedback about the hats running small, we bought a bigger size. Our daughter is 9 months old with a head ~50% percentile and this fits (bought a 2T) a bit big but surprisingly not ridiculously big. It keeps her warm and she looks adorable but we do not fasten the chin strap and always have her in view when she wears it because it does fall over her eyes which makes her mad. Probably could have gone a size down for a perfect fit right now, but we wanted to make sure she fits this through the winter. She has actually had it for a month and in such a short time fit has improved so we have not regrets with our size choice.