How do you determine bike helmet size?

Answer 1

Bicycle helmets come in a variety of sizes, which is important because proper fit is essential to their effective protection. Aside from toddler and child sizes, adult bike helmets can generally be bought in any size from extra small through extra large. By measuring one's head size and referencing a manufacturer's chart, one can determine the best fit.

Answer 2

You can do a crude guesstimate by measuring the circumference of your head with a tape measure about where the helmet would go.

It's not really reliable though. Some heads are rounder other more oval.

You're really better off trying before buying.