I think I wear 7.5", and I always order S/M. I hate it when the hat is too large and the band pushes the temples of my sunglasses down on my ears, or the brim hits them. The circumference of the hat has never been an issue. They are stretchy and/or adjustable. It's the size of the dome, and I don't like them roomy. E.g. The S/M First Spear Forager hat I just got is borderline too big in the dome.