I'm fairly sure that I took my measurements correctly, but the Medium seemed too large (I have a 7-1/8 hat that fits me well, and several hats in Medium that fit me well by different manufacturers). I know the style is supposed to be baggy, but I thought I had about a centimeter or so of give, and the brim was noticeably wide my face. I might have asked to exchange it for a small, but I feared the small might be too small, and the brown tweed wasn't quite what I wanted when I saw it real life. The picture is pretty accurate, but the weave has specks of turquoise in it, which I just didn't think looked right.
HOWEVER: If it fits you and you like the look of the weave, it's a VERY nicely made hat. I order from Historical Emporium because I like styles that society has unwisely decided to abandon, not because I like to wear costumes, and Historical Emporium sells real clothes in these styles. This is one of them.