I measured my head circumference - and the figure was between the size M and L. I then decided to use L and that was the right decision. It fits perfectly.
My wife I got the size M. also perfect.
Although in the description, "teenager helmet", but adults can wear it the same way if they want to look sporty.
I would buy the helmet again.
Enclosed were still 2 sets of spare batteries, because the back of the "jog dial" a light-emitting diode is that you can turn on in the dark (just fixed on the wheel button (light on / flashing / out). Then you will be better seen.
-Buy, Attract, satisfied grin, keep, go :-)
Have fun with it.