That's a really tough call since everyone has difference shaped heads. When I measured my head with a piece of paracord,I was in between a 22" (med) and 23" (lrg). I went with the large in case and it did fit but strong winds would definitely knock it off. I plan to wear it to the beach and hikes and that can get windy, so I exchanged it for a medium, perfect fit for my head.

Did you play football? I was a green helmet if that helps, haha.