How to choose the right bike helmet: Everything you need to know before you head out

A bike helmet fits correctly when it sits level on your head with the front edge about two fingers above your eyebrows, doesn’t move when you shake your head, and fastens with room for one to two fingers under the chin strap. You can choose the correct size by measuring your head circumference in centimetres around the widest part of your head. Besides the size, your head shape (round or oval) also determines which helmet will feel comfortable. In this step-by-step guide, we will tell you about your head size and shape, and help you choose and adjust your helmet.

What is a good fit and why does it matter?

A good fit means the helmet sits securely and stably on your head without pinching, and will not shift during a crash. This isn't just a matter of comfort; it's a matter of safety. A helmet that is too big or sits crookedly offers barely any protection because it cannot absorb the impact properly.

The fit is determined by three aspects: your head circumference, your head shape, and how you adjust the helmet. We will cover these one by one below. Additionally, many modern helmets feature an extra layer of protection against rotational forces, such as MIPS or Lazer’s KinetiCore system. At Mantel, you can filter for this directly on the left-hand side of our main bike helmets page under "Extra protection".

What size bike helmet do I need?

Your head circumference in centimetres determines your helmet size. Measure around the widest part of your head, about one centimetre above your eyebrows and ears. On the online helmets page, you can then filter directly by this measurement so you only see helmets that match your head circumference.

How to measure your head circumference in 4 steps:

1. Place a tape measure around the widest part of your head, roughly 1 cm above your eyebrows and above your ears. This is exactly where the helmet will rest.

2. Keep the tape measure level and comfortably against your head - not too tight.

3. Don't have a tape measure? Use a piece of string or a charging cable, mark where it overlaps, and measure it against a ruler.

4. Measure twice and note down the largest number in centimetres.

Most helmets are sold in size ranges. As a guideline, size S usually covers 51 to 54 cm, M covers 55 to 58 cm, and L covers 59 to 63 cm, though this varies by brand. If you fall exactly on the boundary between two sizes, choose the smaller one: you can usually open up the helmet a little more using the rear adjustment dial.

If you are doubting between two sizes or brands, feel free to drop by and try them on in person at one of the Mantel Superstores in Arnhem, Den Bosch, Rotterdam, or Utrecht.

Is my head round or oval, and why does it matter?

In addition to circumference, your head has a specific shape, which plays a major role in how comfortable a helmet feels. With a round head, the width and length are roughly equal; with an oval head, your head is noticeably longer from front to back than it is from side to side. Manufacturers design their helmets based on a specific shape, and a mismatch can be spotted by pressure points that won't go away, regardless of how you adjust it.

If you try on a helmet designed for an oval head on a round head, it will pinch at the sides while leaving gaps at the front and back. Vice versa, a round fit on an oval head will feel loose at the sides and press against your forehead and the back of your head. You will not recognise an incorrect shape by the size, but rather by these persistent pressure points, no matter how much you tweak the retention system.

How do you find out your own head shape? Have someone take a photo from above, or feel with your hands whether your head is clearly longer than it is wide. This also explains why a helmet from one brand might fit like a glove, while another in the same size never feels quite right. When in doubt, this is the strongest reason to try on different brands before making a decision.

How to wear your bike helmet correctly

A bike helmet fits properly when it sits level on your head, with the front edge about two fingers above your eyebrows, and cannot shift around. This ensures your forehead is protected while keeping your forward vision clear. This position is crucial for proper protection; a helmet that tilts too far back can shift more easily and only protects half of your head.

A quick check: Gently shake your head from side to side and up and down. The helmet should move with your head and not slide loosely across your forehead. If you can easily twist it with one hand, it is too loose, and the adjust system needs to be tightened. Make sure you actually wear your helmet on all rides, even the short ones - especially on those familiar trips where you think nothing will happen. A helmet left in the cupboard offers no protection at all!

How to adjust your bike helmet in 3 steps

You can adjust a bike helmet at three points: first, the retention system around the back of your head, then the side straps under your ears, and finally, the chin strap. Together, they ensure the helmet sits securely without pinching.

Step 1: The rear retention system. Put the helmet on level and turn the dial or adjust the strap at the back of your head until the helmet fits comfortably. Not so tight that it pinches or leaves a mark, but tight enough that the helmet stays in place when you lean forward and shake your head. This step makes the biggest difference in stability.

Step 2: The side straps. The two straps should meet in a Y-shape just below your earlobe. On most helmets, you can adjust this split point. Take your time with this, as misaligned straps will irritate you throughout your ride.

Step 3: The chin strap. Fasten the buckle and adjust it so that one to two fingers can fit between the strap and your chin. Too loose and the helmet can shift during a crash; too tight and it will pinch. Final check: Open your mouth wide as if yawning; you should feel the helmet pull down slightly on your head. If you feel that, the chin strap is correctly adjusted.

Frequently asked questions about bike helmet fit

How do I measure my head for a bike helmet?
Place a tape measure horizontally around the widest part of your head, roughly one centimetre above your eyebrows and ears. The number in centimetres is your head circumference and therefore your helmet size. Don’t have a tape measure? Use a piece of string or a charging cable and measure it against a ruler afterwards.

What bike helmet size do I need for my head circumference?
As a general guideline: size S for 51 to 55 cm, M for 55 to 59 cm, and L for 59 to 63 cm. This varies by brand, so always check the size chart of the specific model. If you are on the boundary, choose the smaller size or visit one of our Superstores to try it on.

How do I know if my bike helmet fits correctly?
The helmet fits well if it sits level, the front edge is two fingers above your eyebrows, and it doesn't move when you shake your head. There should be room for one to two fingers under the chin strap.

What is the difference between a round and oval head for a helmet?
With a round head, the width and length are roughly equal, whereas an oval head is longer from front to back. A helmet that doesn't match your head shape will cause pressure points that will not go away, even if the size is correct.

Do I need to replace my helmet after a crash?
Yes. The foam inside a helmet deforms or cracks to absorb the impact and can only do this job once. Even if you see no damage on the outside, the inner shell may be cracked. Always replace a helmet after a significant crash or fall.

Choosing the right helmet

With the correct size, a helmet that matches your head shape, and a proper adjustment, you will choose a helmet that is safe and so comfortable that putting it on becomes second nature.

Explore our full range in the bike helmets category, where you can filter by head circumference, extra protection (MIPS, KinetiCore), and buckle type. Looking for something specific? We have separate categories for road bike helmets, mountain bike helmets, e-bike helmets, speed pedelec helmets, and kids bike helmets. Would you prefer to try one on first? Pop into one of the Mantel Superstores, where our expert staff will be happy to be of assistance!