How do I measure my head for a bike helmet?

Wrap a flexible cloth tape measure around the widest part of your head - roughly 1 cm (about a finger width) above your eyebrows and around the back. Keep the tape level. Repeat twice and use the larger number; hair adds variability.

What if my head measurement falls between two helmet sizes?

Always size up. Helmets can be dialled down with the retention system, but they cannot be dialled up. A helmet that is slightly loose on the dial is correctable; one that is too small will cause pressure points and never fit properly.

Do bike helmet sizes match between brands?

Roughly, but not exactly. Giro, Bell, Specialized, POC, Lazer, and Kask all use slightly different head-shape moulds (round, oval, and intermediate). Use our calculator to pick your starting size, then always try the specific model on before purchasing if possible.

How often should I replace my bike helmet?

Every 3-5 years even without a crash - UV, sweat, and temperature cycles break down the EPS foam and straps. After any crash that involved head impact, replace it immediately, even if the shell looks undamaged; the foam compresses only once.

Are youth and adult helmets interchangeable?

No. Youth helmets are certified to different impact standards (CPSC and ASTM F1447 for youth, plus sometimes EN 1078 for Europe) and have smaller retention ranges. A youth head that measures within an adult XS range should still use a youth-certified helmet until the child grows into the adult range.

How should a properly fitting helmet feel?

It should sit level - front edge about 1 cm above the eyebrows - with no front-to-back rocking. The side straps should form a Y meeting just below and slightly forward of your ear. Chin strap snug enough to fit only 1-2 fingers between strap and chin.

Can I adjust a helmet that feels slightly too tight?

You can only adjust *looseness* down using the dial retention system - not tightness up. If the helmet creates pressure points even at maximum loose, it is the wrong size. Switch to the next size up or try a different brand whose head shape better matches yours.

Bike Helmet Size Calculator - Adult, Youth & Toddler

Bike helmets are sized by head circumference in centimetres. Every major brand - Giro, Bell, Specialized, POC, Lazer, Kask — publishes a cm-to-letter chart (XS through XXL). The ranges overlap slightly between brands because head shape (round vs. oval) varies as much as size.

Our calculator uses the widely-adopted industry ranges: Adult S at 55 cm, M at 56–58 cm, L at 59–60 cm. Youth helmets run smaller (S starts at ~49 cm), and toddler helmets start at 44 cm and are built to child-specific CPSC and ASTM standards.

Fit goes deeper than the number. Use our general bike size calculator to match your bike too, and check the kids bike sizing guide when outfitting a younger rider.

Find your bike helmet size from head circumference - adult, youth, and toddler ranges included.

Size	Head circumference (cm)	Head circumference (in)	Typical riders
XS	51–54 cm	20.0–21.2 in	Small adults, teens
S	55 cm	21.6 in	Many adult women
M	56–58 cm	22.0–22.8 in	Most common adult size
L	59–60 cm	23.2–23.6 in	Many adult men
XL	61–62 cm	24.0–24.4 in	Larger adult heads
XXL	63+ cm	24.8+ in	Extended fit

Pro tip: Helmet sizes vary between brands. Use this result as a starting point, then try on the specific model and confirm it sits level, ~1 cm above the eyebrows, with the chin strap snug under the ear.

Fit goes deeper than the number. Use our general bike size calculator to match your bike too, and check the kids bike sizing guide when outfitting a younger rider.

Find your bike helmet size from head circumference - adult, youth, and toddler ranges included.

Bike Helmet Size Calculator

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Helmet size chart (cm & inches)

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How helmet sizing works

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Recommended helmets

Bell Nomad 2 MIPS

Giro Register MIPS

Joystar DRBIKE Kids Multi-Sport Helmet (Ages 3-8)

Kask Protone Icon

Mongoose All Terrain Kids Bike Helmet (Ages 5-8)

POC Omne Air SPIN

Know the rules where you ride

Frequently Asked Questions

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Helmets buying guide

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Ready to calculate