A bike helmet that fits you properly will feel better on every ride and, most importantly, can help make sure it can do its job in case it needs to. This step-by-step guide explains how to select the right size bike helmet and adjust it correctly for the best fit.


How to find the right bike helmet size at home

Measure the circumference of your head using a cloth tape measure or a piece of string and a ruler. Be sure to measure horizontally across your brow and over your ears. Use the bike helmet size chart below to find the proper helmet size based on your measurement.

Headform shape Helmet size Fit range (cm) 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
STANDARD Toddler 46-50 Toddler
Child 48-52 Child
Youth/XS 50-55 Youth/XS
Small 51-57 Small
S/M 51-58 S/M
Medium 54-60 Medium
M/L 55-61 M/L
Large 58-63 Large
X-Large 60-66 X-Large
ASIA S/M 51-58 S/M
M/L 55-61 M/L

Wear the helmet in the correct position, with your forehead protected

How to check bike helmet sizing

Some Bontrager helmet models are available in different sizes to better fit different sized heads. To check for the right size, put it on your head and adjust the rear fit system. A good fit is snug, but not tight.

How to make helmet adjustments

In order to provide protection, a helmet must be worn correctly and properly maintained. Read the instructions below and follow them carefully.

Step 1

Place the helmet on your head. It should be level so that it sits just above your eyebrows and covers your forehead. Then unlock the strap dividers.

Step 2

On the left side, hold the buckle under your chin and slide the strap divider just beneath your earlobe. The straps should sit flat against your head. Lock the strap divider— – it should sit just below your ear lobes.

Step 3

From the right side of the helmet, pull any left-side slack across the back of the helmet. Then repeat Step 2 on the right side.

Step 4

Make sure that the chin-strap is correctly threaded through the right buckle and fasten the buckle. To keep the chin-strap from loosening or flapping, it should be tucked through the keeper.

Step 5

Make sure that the buckle is correctly fastened. Listen for a click as the buckle engages. Then attempt to pull the buckle apart with the chin-straps. It should not come apart.

Step 6

Adjust the chin-strap so that it is positioned snugly against the bottom of your jaw as shown. The chin-strap should be snug enough to only squeeze two fingers between your chin and the strap.

How to test the adjustments

A helmet should fit and be worn correctly. To check for correct fit, place the helmet on your head and make any adjustments indicated. Securely fasten the retention system. Grasp the helmet and try to move it to the front and rear.

A helmet that fits correctly will be comfortable and will not move forward to obscure your vision or backward to expose your forehead. Consult your local Trek retailer if you cannot correctly adjust your helmet.

How to secure excess chinstrap

Depending on the size of your head, you may have excess chin-strap after adjustments. You can fold it up and tuck it into the strap keeper, or you can cut it. If you choose to cut the chinstrap, leave at least 25 mm past the buckle.

After cutting, melt the end of the chinstrap to prevent fraying. Be careful with high heat near the helmet. The strap is flammable. Avoid contact with the melted nylon until it cools because it can burn.

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