How to Choose the Right Size Bike Helmet | Match Your Head Measurement

One wrong measurement can leave you with a helmet that shifts during a ride or feels like a vice fifteen minutes in. The process takes about thirty seconds with a cloth tape measure. Here is exactly how to measure, what the numbers mean, and the one mistake that trips up most riders — plus a link to our tested top picks if you already know your size.

How To Measure Your Head For A Bike Helmet

Grab a flexible cloth tape measure or a piece of string and a ruler. Position the tape around the widest part of your head — roughly one inch above your eyebrows and just above your ears. Keep the tape level, snug against the skin but not tight enough to compress it. Read the measurement in centimeters (preferred by most helmet brands) or inches. If you ride with a thin balaclava, measure with that on; a helmet that fits the bare head can feel too snug when you add a layer.

Match Your Measurement To A Size Chart

Every helmet brand publishes its own size ranges, and those ranges differ. A “Medium” from one company may overlap with a “Small” from another. Visit the brand’s official size chart — REI’s general guide runs from XS (below 51 cm or 20 inches) up to XL (63 cm or 24.75 inches and above). Bell’s chart lists specific centimeter ranges per size, and HJC Helmets offers sizes from 3XS through 5XL. The number you wrote down must fall inside one of those published ranges. If it sits between two sizes, Bell advises choosing the larger size because a slightly roomy helmet can be tightened with the fit ring, while a too-small one cannot be stretched.

Head shape also matters. If one brand’s “Medium” feels tight at the temples but loose at the crown, try a different brand’s version of the same size — the measurement is the start, not the final answer.

How A Properly Fit Helmet Should Feel

Put on the helmet without fastening the chin strap. Nod your head and shake it gently side to side. The helmet should move with your head, not slide across your skin. It sits level on your forehead — not tilted back — and low enough to protect the brow bone without blocking upward vision. When you buckle the chin strap, tighten it until it feels snug when you open your mouth wide. Use the fit system (dial or pads) to eliminate any wobble. If you can fit more than two fingers between the strap and your chin, it is too loose.

Between Sizes? Try Before You Decide

If your measurement falls between two sizes on a chart, buy the larger size and use the fit system and supplied pads to dial in snugness. Try on the helmet for at least five minutes. A helmet that feels fine for thirty seconds can develop a pressure point after a few minutes. Swap pads or adjust the fit ring until it feels evenly snug with no hard spots. If you cannot get rid of the pressure point, try a different brand. If you are measuring above 63 cm, the selection narrows; we have a curated roundup of the best XL bike helmets tested for comfort that saves you trial-and-error.

Common Helmet Sizing Mistakes To Skip

Measuring too low on the forehead produces a circumference smaller than reality, leading to a helmet that sits too high. Letting the tape droop behind the ears skews the number the other way. Skipping the brand chart because you remember a previous size is another miss — helmet sizing is not universal. Never buy a kids’ or junior size based on age alone. A ten-year-old with a head circumference of 55 cm belongs in a small adult size. Check the chart every time.