Before purchasing a bike helmet, you need to know your head circumference. Use a flexible tape measure to wrap around your head, about one inch above your eyebrows, ensuring the tape is level all the way around. Most helmet sizes are based on head circumference, so this measurement will help you select the right size. Choosing the Right Helmet Size Helmets come in different sizes,... https://www.uniquebikezone.com/how-bike-helmet-should-fit-2/#more-2462