How to Measure Bike Helmet Size? | One Tape Measure Fix

Measure your head circumference with a flexible tape about one inch above your eyebrows to find your bike helmet size.

Getting the right bike helmet size starts with one number: your head circumference. Wrap a flexible tape measure around the largest part of your head — horizontally across your brow, over your ears, and around the occipital bump at the back of your skull. That measurement, matched against a brand’s size chart, tells you which helmet to grab. Here’s the full method, the common mistakes to avoid, and why the same nominal size can fit differently across brands.

The Right Way To Measure Your Head

Use a cloth or flexible tape measure — the kind a tailor uses. Position it about one inch above your eyebrows, running level across your forehead and just over your ears. Pull it snug but not tight, and read the number where the tape overlaps. That’s your head circumference in centimeters or inches.

No tape measure handy? Grab a piece of string, wrap it the same way, mark the overlap, then lay it flat against a ruler. The measurement is just as accurate.

Match Your Numbers To A Size Chart

Every helmet brand publishes its own size chart, and those charts are not interchangeable. Giro’s sizing, for example, runs S at 51–55 cm, M at 55–59 cm, L at 59–63 cm, and XL at 63–65 cm. REI’s general bike helmet guide splits it differently: XS below 20 inches (51 cm), S at 20–21.75 inches (51–55 cm), M at 21.75–23.25 inches (55–59 cm), L at 23.25–24.75 inches (59–63 cm), and XL above 24.75 inches (63 cm).

Youth sizes run smaller — Giro’s XS youth starts around 45–51 cm — but children’s sizing often includes age ranges as a rough guide only. Measure the child’s head the same way; age is a starting point, not a fit.

These are typical ranges, not universal law. Always check the specific brand’s chart before you buy, because sizing genuinely varies by manufacturer — a medium in one brand may feel like a large in another.

How A Helmet Should Fit On Your Head

Once you’ve got a helmet in your measured size, put it on level — sitting horizontally across your brow, not tilted back or forward. The front edge should rest about two finger-widths above your eyebrows. Adjust the sizing pads or the fit ring at the back until the helmet feels snug all the way around. It should grip your head firmly enough that it doesn’t move when you shake your head, but it must never feel painful or constricting.

The National Highway Traffic Safety Administration’s fitting guide stresses that a helmet sits level and snug, covering the forehead without rocking. If it leaves red marks after a few minutes, it’s too tight.

Try on several helmets in your measured size before settling. Different brands shape their shells for different head profiles — some use Western fits, others Asian fits — so the same circumference can feel completely different. If one model pinches at the temples, another in the same size may fit perfectly. If you’re shopping for a lid that keeps the sun off your face, our roundup of the best bike helmets with sun visors covers tested options in the right sizes.

Common Sizing Mistakes To Skip

Most helmet-fit problems trace back to a few predictable errors. Measuring too high on the head is the most common — the tape needs to run across the brow and over the ears, not up near the crown, because that’s where the helmet actually sits. Another frequent miss: trusting a size label without checking that brand’s own chart. A “medium” from one manufacturer is not the same as a “medium” from another. And choosing a helmet that’s loose because it felt comfortable in the store sets you up for a helmet that shifts in a crash. A snug fit is the safe fit.

Helmet sizing is brand-specific, and fit charts apply to bike helmets only — they don’t transfer to skate, ski, or climbing helmets, which use different shapes and standards. When in doubt, a properly fitted helmet is worth more than any feature list.