I may have totally missed it in their description but it's a flex fit design so there's some give if your head size is within the range. I wish these companies would use one standard measurement but they don't. If it helps, I actually googled the size chart difference and my head of 7 1/4 sits in the smack middle of the S/M fit of this cap. Otherwise, I like Hurley's minimalistic and subtle cap design so this one's definitely one I'll be wearing for a long time.