Simple! Check out our online sizing guide to help you find the perfect fit at www.ruroc.com/sizing. You need to measure around the widest part of your head (above your ears/ forehead) and don't forget to tick if you have any large features.. we won't judge 😉. Every RG1-DX comes with extra padding inserts so you can adjust the inside of your helmet to your head shape, you may not need it but it's there is you do! If you need any further help contact our customer service team firstname.lastname@example.org.