Start by measuring your head circumference with a soft tape measure. Wrap it around the widest part of your head, just above your eyebrows and ears—this gives you the centimeter measurement that matches helmet sizing. Most adult helmets fall into these ranges: Small (52-55cm), Medium (55-58cm), Large (58-61cm), and XL (61-64cm).
A properly fitted helmet should feel snug all around without any pressure points or hot spots. When you shake your head, the helmet shouldn't shift or slide around. If it does, go down a size. Also, always try your helmet on with your goggles to check for a 'gaper gap'—that uncomfortable space between your goggles and helmet that lets cold air straight in.
Don't assume your hat size translates directly to helmet size, and avoid buying loose thinking you'll wear a beanie underneath. A helmet that's too loose won't protect you properly in a crash.