How to Measure Crash Helmet Size? | Get the Right Fit First Time

Getting the correct crash helmet size starts with measuring your head circumference with a soft tape, then matching that number to the brand’s sizing chart — a process that takes less than two minutes but is the single most important step for safety.

Sliding into a helmet that’s too loose or squeezing into one that’s too tight wastes money and compromises protection. The measure-first method is straightforward, and it works for motorcycle helmets, bicycle helmets, and any head protection you need to fit properly. Before shopping, you need one number: your head circumference in centimeters.

How to Measure Your Head for a Helmet

The only tool you need is a flexible measuring tape — the soft cloth or plastic kind sewers use. If you don’t have one, a length of string and a ruler will do the same job. Place the tape just above your eyebrows, run it level around your head across your temples and above your ears, and wrap it around the thickest part at the back of your skull. Keep the tape snug — not tight enough to dent skin, but firm enough that it doesn’t sag. Record the measurement in centimeters, since most helmet size charts use that scale.

Measure at least twice. If the numbers differ by even a few millimeters, use the larger measurement. If you plan to wear a balaclava, head sock, or have thick hair under the helmet, take one more measurement with that layer on and use that larger figure.

Match Your Measurement to a Size Chart

Helmet sizing is not universal: a medium from one brand can feel tighter or looser than a medium from another. Your circumference number is the anchor. NHTSA provides a general bicycle helmet size range as a starting point:

These are general ranges. Always check the specific brand’s chart before ordering, because variations exist between manufacturers and between helmet types (motorcycle vs. bicycle, full-face vs. open-face).

Common Measuring Mistakes to Avoid

The most frequent error is placing the tape too low — down at eyebrow level rather than just above them. This gives an artificially small measurement and a too-tight helmet. Pulling the tape overly tight compresses soft tissue and produces a false reading. Relying on a single measurement instead of the largest of two or three tries is also common.

Perhaps the most expensive mistake: skipping the measurement entirely and buying based on a size label alone. Always try the physical helmet in your size before purchasing. A properly sized helmet sits low on the forehead without blocking upward vision, feels snug all the way around, and does not shift when you shake your head. If you’re ready to see top-rated options, our
roundup of the best crash helmets
compares fit, safety ratings, and features across major brands.

Fit Check: How the Helmet Should Feel

Snug but not painful. The helmet should contact your head evenly — no pressure points on the temples or the crown. If it leaves red marks after a minute of wear without being fastened, it may be too small. NHTSA’s fitting guidance recommends measuring first, then trying several helmets in that size until you find the one that feels right.