All our kits are based on head circumference. There are a couple of ways to measure this if you don't already know the size of your head. Either you can use a cloth tape measure or a piece of string.

Measure the circumference just above your eyebrows touching the top of your ears as you go around and try to keep the tape measure / string level as you go around the head. 

Read off the measurement or measure the string and bingo!

Check this measurement against the table below and now you know!