I ordered this hat in size medium for my three year old girl with head circumference of 20.5". It fits snug. I should of went with size large. It was my mistake. I tried to measure her head when sleeping and I measured it wrong (19"). Just for curiosity I stuck the hat on my eight year old son with head circumference of 20.25" and it fit much better (he would need size large too). The hat is very lite, swim like material and my daughter loves it.