How To Measure For A Bike Helmet?

If you want to get the maximum impact protection from your helmet, you need to make sure that the size of your bike helmet is exactly right, be it either a road or a mountain bike helmet. A bigger helmet in size will instantly come off and hence not protect you from head injuries in case of a crash. To get the right size bike helmet, you need to have the right measurements for your it.

How To Size A Bike Helmet Properly?

Find out the circumference of your head before purchasing a bike helmet. For this, take a soft tape measure and wrap it around the fullest part of the head. It should be almost an inch or so above the eyebrows. Alternatively, use a lace/thread instead of the measuring tape and then measure the lace.

Sizing Parameters & Bike Helmet Size Guide

When it comes to either road or mountain bike helmets, the correct size does matter. It could be the difference between life and death. The protective part consists of the outer shell and the inner liner. A loose helmet can rock on your head exposing the forehead and the back of your head in case of a crash or collision. Furthermore, it could block your view and lead to an accident. Not to mention, a loose helmet can come off instantly during a crash exposing the head to some serious injuries.

On the contrary, if you get a helmet that is on the tighter side, you will notice that after a couple of rides the helmet would be the only thing you would be thinking about while cycling. For this reason, you need the perfect bike helmet fit. And for your proper fit, you need to find the circumference of your head, correctly.

How To Measure For A Bike Helmet?

Measure Your Head

It is a good idea to ask a friend to measure the circumference of your head. It will be more accurate to get some help as you may find it a little hard to check for the widest part of the head to measure. The tape needs to be flexible so you or your friend can easily move it around the head.

  • Ask your friend to wrap the tape just above your ears around the fullest part of your head. You can do that yourself too by looking into the mirror. This would normally be around half an inch (or one inch) above your brows. Do not squeeze the tape too tightly around the head.
  • It is also a good idea to measure it twice. Measuring your head for the second time will eradicate any chances of errors. Once you or your friend has taken the measurements, note them down.
  • If you have measured twice, both the measurements should match. If they do, you are good to proceed. If they do not, figure out why and repeat the measurement.

Sizing Parameters For Your Bike Helmet

Once you have the correct head measurement with you, find a bike helmet that matches your specs. Here are some general parameters for bike helmets size:

  • If the circumference of your head is below 51 cm (20 inches), your helmet size is “Extra Small”.
  • If the circumference of your head is between 51 (20 inches) and 55 cm (21.75 inches), your helmet size is “Small”.
  • If the circumference of your head lies somewhere between 55 cm (21.75 inches) and 59 cm (23.25 inches), your helmet size is “Medium”.
  • If the head measurement is between 59 cm (23.25 inches) and 63 cm (24.75 inches), your helmet size is “Large”.
  • If the circumference is above 63 cm (24.75 inches), your helmet size will be “Extra Large”.

Some bike helmets have a very flexible fitting mechanism and can be adjusted to different sizes. These bike helmets come in “One Size” option. Here are their sizing specifications:

  • For men: “One size” covers a head circumference between 54 cm (21.25 inches) and 61 cm (24 inches).
  • For women: “One size” covers the head circumference between 50 cm (19.75 inches) and 57 cm (22.5 inches)
  • For children: “One size” ranges between the head circumference of 46 cm (18 inches) and 57 cm (22.5 inches).

Adult Helmet

Your proper helmet size may depend upon your head shape. There are three main types of head shapes: oval, round, and egg-shaped.

The oval-shaped heads are the most common ones. They are also referred to as long-faced heads. The round-shaped head is wider in the middle part of the head as compared to the chin or forehead. The egg-shaped heads are wide at the forehead side (top part) and narrow towards the chin.

Here are general adult helmet guidelines:

Helmet SizeMeasurement (in cm)Measurement (in inches)
XS53 to 5420 ⅞ to 21 ¼
S55 to 5621 ⅝ to 22
M57 to 5822 ½ to 22 ⅞
L59 to 6023 ¼ to 23 ⅝
XL61 to 6224 to 24 ⅜
2XL63 to 6424 ¾ to 25 ¼
3XL65 to 6625 ⅝ to 26

Youth Helmet

It is very natural for youths to grow out of their helmet sizes. Make sure that the bike helmets they are wearing are not too tight and buy a new one when you feel they have grown out of it. Also, do not buy them larger sized helmets just because “one day they will grow into it”. This is a dangerous practice that may save some money but can be very dangerous in case of an accident.

Here are the youth helmet sizes:

Helmet SizeMeasurement (in cm)Measurement (in inches)
Youth S49 to 5020 ⅞ to 21 ¼
Youth M51 to 5221 ⅝ to 22
Youth L53 to 5422 ½ to 22 ⅞
Youth S/M49 to 5023 ¼ to 23 ⅝
Youth L/ XL51 to 5224 to 24 ⅜

Child Helmet Sizing

It is important to develop the habit of wearing helmets in your children. However, if your child is not comfortable in his/ her helmet, chances are they will try to avoid it as much as they can. For this reason, make sure you measure their head properly (from the steps mentioned above).

Here are some general measurements for boys and girls according to their ages. Check the following chart to see what kids helmets work best for your children.

AgeMeasurements for BoysMeasurements for Girls
12 months46 cm / 18.1 inches45 cm / 17.7 inches
18 months47.5 cm / 18.7 inches46.5 cm / 18.3 inches
2 years47.5 cm / 18.7 inches47 cm / 18.5 inches
2.5 – 3 years49 cm / 19.3 inches48 cm / 18.9 inches
3.5 years50 cm / 19.7 inches49 cm / 19.3 inches
4 years50.5 cm / 19.9 inches49.5 cm / 19.5 inches
4 – 5 years51 cm / 20.1 inches50 cm / 19.7 inches
6 years52 cm / 20.5 inches51 cm / 20.1 inches
7 years53 cm / 20.9 inches52 cm / 20.5 inches
8 years54 cm / 21.3 inches53 cm / 20.9 inches

While these charts help immensely with bike helmet sizes when you order the helmet online or want to surprise-gift the helmet to a loved one, different brands may have different fitting standards. Remember not to be a slave to the numbers and measurements. Although these measurements provide a good insight into helmet fitting, yet the best way to know if the size is perfect is when it perfectly sits on your child’s head without your child complaining about it being too tight or too wobbly.


Your head size and shape will determine which bike helmet size will work best for your head before you buy a new helmet. Your ideal bike helmet needs to fit comfortably. You also need to wear it properly as many people do not take the buckles, removable pads, and chin strap as seriously as they should. There is no point in buying a perfectly measured helmet if you are not going to buckle up properly. Once you have measured your head and purchased the helmet, make sure your helmet sits well on your head and all the straps are properly adjusted. And then, you are ready to go!

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