Bike helmet sizes explained: how to measure your bike helmet size for a perfect fit

How to get the correct-sized helmet for your head

Making sure your bike helmet fits you correctly is vital in ensuring maximum protection.


A loosely-fitting helmet may not remain in place during an accident, increasing your risk of a dangerous head injury. On the other hand, an excessively tight-fitting helmet, or one that is too small, will be both uncomfortable and possibly unsafe.

In this guide, we will take you through how to correctly size a road bike helmet or mountain bike helmet.

How are helmets sized?

Helmet sizing sticker
All helmets will have their size marked with a sticker or decal.
Felix Smith / Immediate Media

Size notation on helmets varies between manufacturers.

While more affordable helmets sometimes offer a one-size-fits-all design, many bike helmets come in a range of sizes, which in turn helps provide a secure and comfortable fit.

Some opt for a range of head diameters (for example, size 52-56cm), while others go for variations on a small/medium/large scale.

For those manufacturers that denote their sizing in words rather than a numerical value, they are always linked to a sizing chart, from which you can find the diameter range of each size.

MET, for example, offers three sizes for its Trenta MIPS road helmet: small, medium and large, corresponding to head diameters of 52-56cm, 56-58cm and 58-61cm respectively.

All sizing charts are brand-specific and many produce size guides for individual models of helmet, so do check before you buy a new helmet.

How to size a bike helmet - bike helmet sizing guide
Full-face helmets typically use removable pads to adjust size.
Will Jones / Immediate Media

The majority of bicycle helmets, with the exception of some full-face downhill models (which come with replaceable internal foam pads of different thicknesses), come equipped with some form of adjustable cradle to accommodate a variety of head sizes within a defined size range.

Finally, not all heads are the same shape, and, in a similar vein to cycling shoes, helmets can also vary in shape from one brand or model to the next. Once again, it’s best to try before you buy to find the perfect fit.

How to measure your head for a helmet

How to size a bike helmet - bike helmet sizing guide
A fabric tape is the best option for measuring your head.
Will Jones / Immediate Media

If you’re buying a new helmet and want to find the correct size, it’s advisable to measure you head and check it against the brand’s size chart.

Helmets are designed to fit just above your eyebrows and ears, and as such this is the best place to measure your head to guarantee proper fit.

The best thing to use is a flexible fabric tape measure (dig around in a sewing kit that doubtless lurks in a biscuit tin to find one).

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Sizing differs between brands and individual helmet models, so it’s best to try on a helmet before you buy.
Will Jones / Immediate Media

If you don’t have one of these, then a viable alternative is to use some string and to measure it afterwards.

The fit of a helmet should be snug, but not constricting, so take this into account when measuring.

Avoid using a metal tape measure from the toolbox – they aren’t designed to measure curves and won’t lie flat around your head, meaning you may get things wrong.

How to adjust the fit of a helmet

How to size a bike helmet - bike helmet sizing guide
With the exception of full-face helmets, which typically use swappable foam inserts, all helmets will have some form of adjustment dial.
Will Jones / Immediate Media

If you’ve correctly measured your head, there should be minimal adjustment required to fit it correctly.

Simply pop it on your head so it sits above your eyebrows and ears, and adjust the cradle, usually with a dial at the rear. The adjuster on the rear of the helmet will either tighten the cradle or, on some helmets, tighten a band that runs around the full circumference of the helmet.

There will also often be the option to adjust the rear of the cradle up or down to fit comfortably. The chin strap should be snug, but not constricting.

If the helmet features replaceable padding to adjust the fit, you may have to try several iterations to get it right.

The fit should be snug and the helmet shouldn’t be able to rotate in any direction. As mentioned, it should sit just above your eyebrows and shouldn’t be tilted back or forward on your head.

If you want to learn more, check out our comprehensive guide on how to (and how not to) wear a helmet correctly.

Does MIPS affect helmet sizing?

How to size a bike helmet - bike helmet sizing guide
MIPS liners used to affect helmet sizing, but this is rarely the case these days.
Will Jones / Immediate Media

Recent advancements in tech have resulted in helmets where the outer shell has some freedom to rotate independently of the cradle, which is claimed to improve the overall safety of a helmet.

The most common form this takes is a MIPS liner (the yellow part in the photo above).

When MIPS was first introduced, the technology was retrofitted into helmets that were already available on the market. This impacted the size of the helmet because MIPS was adding a 0.5 to 0.8mm layer, which reduced available headspace.

This is no longer the case because MIPS now works with brands from the development stage of producing a helmet, so the technology is integrated into the overall design.

MIPS will introduce a small amount of movement to your helmet (around 10 to 15mm of overall movement), which can be slightly disconcerting when initially adjusting a helmet.

However, as long as this is small, and confined to the outer shell and not the cradle, you’ve nothing to worry about.

Can I have a ponytail or wear a hat with a helmet?

How to size a bike helmet - bike helmet sizing guide ponytail
Some helmets are compatible with a ponytail.
Will Jones / Immediate Media

Some things can prevent you from getting a proper fit with a bike helmet, a ponytail being the most common.

While some helmets are designed to accommodate this hairstyle with a large exit port at the rear, some aren’t. As such, you may have to style your hair to accommodate the helmet, or choose a helmet designed to suit.

Thin hats and cycling caps can be worn under helmets if needed, as long as they don’t alter the fit.


It’s best to avoid wearing a beanie or hood under your helmet for safety reasons. The thickness will also affect the fit of the helmet.