Measure your head circumference at the widest point—just above your eyebrows and around the back—using a soft tape measure. Most adult helmets come in three size brackets: Small (51–55cm), Medium (55–59cm), and Large (59–63cm), with some brands offering XS or XL. One-size-fits-all models typically cover 54–61cm.
Getting the right size matters more than you might think. A helmet that's too large can shift during a crash, reducing its protection, while one that's too small creates pressure points that won't improve over time—helmets don't break in like shoes. Don't assume your hat size translates to helmet sizing either, since they use different measurements.
If you fall between sizes, size up and use the included thicker pads to fine-tune the fit. And if you regularly ride with a cap or headband, bring it along when trying on helmets to make sure everything feels comfortable together.