Measure your head circumference with a soft measuring tape

The most reliable way to find your helmet size is to measure the circumference of your head with a soft measuring tape — the kind used for sewing, not a metal ruler. Wrap the tape around your head about half an inch above your eyebrows, across the fullest part of your head, and around the back of your skull. Keep the tape snug but not tight enough to leave an indent. Write down the measurement in both inches and centimeters.

Most helmet manufacturers size their products in small, medium, large, and extra-large, and each size corresponds to a range of head circumferences. A small helmet typically fits heads measuring 51 to 55 centimeters (20 to 21.5 inches), a medium fits 55 to 59 centimeters (21.5 to 23 inches), a large fits 59 to 63 centimeters (23 to 24.75 inches), and an extra-large fits 63 centimeters and above (24.75 inches and up). These ranges vary slightly between brands, so always check the specific manufacturer's sizing chart before you buy.

Key Takeaways

  • Measure your head circumference with a soft tape measure placed half an inch above your eyebrows and around the fullest part of your skull.
  • Helmet sizes correspond to head circumferences in centimeters, and each brand publishes its own sizing chart that you should consult before purchasing.
  • If your measurement falls between two sizes, choose the larger size and use the helmet's internal padding adjustments to achieve a snug fit.
  • A properly fitting helmet should sit level on your head, not rock forward or backward, and you should be able to fit one finger between the helmet and your forehead.

Check the manufacturer's sizing chart for your specific brand

Every helmet brand publishes a sizing chart, usually found on the product page or in the instruction manual that comes inside the box. Some manufacturers use centimeters, others use inches, and a few provide both. Look for the chart that matches the specific helmet model you are considering, because sizing can differ between a road helmet and a mountain bike helmet from the same brand.

If your head measurement falls between two sizes — for example, you measure 55 centimeters and the small goes up to 55 and the medium starts at 55 — choose the larger size. You can always add padding inside a larger helmet, but you cannot make a smaller helmet bigger. Most helmets come with removable foam pads that let you fine-tune the fit.

Try the helmet on and check for proper fit

Once you have the helmet on your head, it should sit level, covering your forehead without tilting backward or forward. The front edge should sit about one to two finger-widths above your eyebrows. Fasten the chin strap and tighten it so that you can fit exactly one finger between the strap and your chin — not so loose that the helmet moves when you shake your head, and not so tight that it cuts off circulation.

Rock the helmet gently side to side and front to back. It should move slightly with your head but not slide around independently. If the helmet shifts more than half an inch in any direction, it is too large. If you cannot get it on your head without forcing it, it is too small. The interior should feel snug but not painful, and no part of the helmet should press on your temples or the back of your head in a way that causes discomfort.

Understand how padding affects fit

Most modern helmets come with removable foam padding that sits between your head and the helmet shell. This padding serves two purposes: it improves comfort and it fine-tunes the fit. If you buy a helmet that is slightly larger than your measurement but within the size range, you can add extra padding to take up the space and create a snug fit.

Padding thickness varies, but most helmets come with pads ranging from 10 to 20 millimeters thick. If you find that a medium helmet is slightly loose, you can often purchase additional adhesive-backed foam pads from the manufacturer or from third-party retailers and stick them inside the helmet. This approach is cheaper and faster than returning the helmet and ordering a different size.

Know when to remeasure and replace padding

Your head size does not change much in adulthood, but the padding inside your helmet compresses over time and loses its cushioning. If you have worn the same helmet for several years, the padding may have flattened enough that the fit has become loose. You can replace the padding without buying a new helmet — most manufacturers sell replacement pad kits that match their helmet models.

If you have gained or lost a significant amount of weight, or if you have grown your hair much longer or shorter, remeasure your head. Hair thickness can affect how a helmet sits, and in rare cases a substantial change in head size may mean moving to a different helmet size. A quick measurement takes less than a minute and ensures you are still wearing the right size.

Account for hair thickness and winter gear

If you have thick, long hair or you plan to wear a winter hat or balaclava under your helmet, account for that extra bulk when you measure. Some riders measure their head with their hair in its normal style, then add a quarter-inch to the measurement to account for the helmet sitting on top of the hair rather than directly on the scalp.

If you ride in winter and wear a thin knit cap under your helmet, measure with that cap on your head. The same applies if you wear a helmet liner or a do-rag. The helmet should fit snugly over whatever you plan to wear underneath it, so your base measurement should reflect your actual riding setup.

Frequently Asked Questions

What if I measure between two helmet sizes?

Choose the larger size and use the helmet's removable padding to tighten the fit. You can always add padding to a larger helmet, but you cannot stretch a smaller one. Most helmets come with extra pads or you can buy replacement kits from the manufacturer.

Can I use a regular measuring tape instead of a soft one?

A soft measuring tape gives you a more accurate reading because it conforms to the shape of your head. A metal ruler or hard tape will not bend around your skull and will give you a false measurement. If you do not have a soft tape, you can use a piece of string, mark where it meets, and then measure the string against a ruler.

How often should I remeasure my head?

Head size is stable in adults, so you typically only need to measure once. Remeasure if you have made a major change to your hair length or thickness, or if your current helmet feels noticeably loose or tight after years of use. Padding compression is more common than head size change.

Does helmet size affect safety?

Yes. A helmet that is too large will shift during a crash and may not protect the area that needs it. A helmet that is too small will be uncomfortable and you may not wear it. A properly fitted helmet stays in place and provides the protection it was designed for.

What if the helmet feels tight at the temples?

Some helmets have thicker padding at the sides than others. If a helmet pinches your temples, try the next size up, or look for a different brand or model that has a wider internal shape. Helmet geometry varies, and what fits one person perfectly may not work for another even at the same size.