There’s more to sizing snowboard boots than just a number. While you’ll typically wear the same snowboard boot size as your standard street shoe size, the boot won’t fit your foot unless it’s also the right shape. Because some boots have wider or narrow heels or toe boxes, you may need to size up or down slightly from your street shoe size, or shop through several boot brands, until you find the one that fits just right.
Start with your normal shoe size. As with all footwear, boot shapes will vary between manufacturers, so you may need to try on several pairs of boots no matter what--but starting with your shoe size will at least get you in the ballpark.
Try on the boots. Make sure to wear the same socks you intend to use for snowboarding--padding at the heels and shins is a bonus. Try the boots on in the afternoon or evening when your feet are at their largest, typically swollen after the day’s activities.
Tie the boot laces snugly or, if they have a dial-in lacing system or quick laces, snug them as tight as possible. Pay close attention to whether the boots rub or pinch anywhere; if that’s the case, you should size up or try a different boot.
Stand up and rock forward on your toes. You might want to hold on to something or someone sturdy while you do this since snowboard boots typically flex little, if at all, in the ankles. If your heels come up before the boot heel comes up, you need to tighten the boot laces or try on a slightly smaller boot.
Walk around for a couple of minutes to see if the boot laces remain as snug as they were when you first tied them. If they loosen significantly, you should try a different boot. That sudden looseness on the slope could be a real hazard.
Try on the boots in your snowboard bindings just before purchase or just after if you couldn't do it before to make sure the two are compatible.