Bike Helmet Size Chart: Find Size by Head Circumference
Measure around the head, enter it in cm or inches, and see the youth and adult sizes Giro and Bell list for it.
Bike helmet size by head circumference finder
A starting point; check fit and your league rules.
Prefer no scripts? The chart tables below hold every number the finder uses.
Bike helmet size chart by head circumference
Helmet sizes are by head measurement, not by age. Neither maker ties a size to an age, so use the tape, not the birthday. Bell lists inches; the finder converts to cm.
Giro
| Size | Range |
|---|---|
| Youth XS | 45 to 51 cm |
| Youth S | 49 to 55 cm |
| Universal Child | 47 to 54 cm |
| Universal Youth | 50 to 57 cm |
| Adult S | 51 to 55 cm |
| Adult M | 55 to 59 cm |
| Adult L | 59 to 63 cm |
| Adult XL | 63 to 65 cm |
| Universal Adult | 54 to 61 cm |
Source: Giro: Size charts (bike helmets). Checked .
Bell
| Size | Range |
|---|---|
| Universal Toddler | 17 3/4 to 20 1/2 in |
| Universal Child | 18 7/8 to 21 5/8 in |
| Universal Youth | 20 1/8 to 22 7/8 in |
| Youth XS | 19 1/4 to 20 7/8 in |
| Youth S | 20 1/8 to 21 5/8 in |
| Adult XS | 20 7/8 to 21 1/4 in |
| Adult S | 21 1/4 to 22 in |
| Adult M | 22 to 22 7/8 in |
| Adult L | 22 7/8 to 23 5/8 in |
| Adult XL | 23 5/8 to 24 3/8 in |
Source: Bell Helmets: Size charts (general youth and adult bike). Checked .
A helmet should fit now
Buy the size that fits today, not one to grow into. Giro says the helmet should fit with a little room when the fit system is loose, and sit low enough to protect the forehead without blocking vision. Replace a bike helmet after a crash, and follow the maker's instructions.
How to use it, in 3 steps
Measure the head
Bell: use a soft tape or string, wrap it about 1 inch (2.5 cm) above the eyebrows, level and snug but not tight, and measure twice using the larger reading.
Match the number to a size
Enter it in the finder. Giro and Bell ranges overlap, so more than one size can match. Try the sizes on.
Check the fit and the label
Giro says to try the helmet on before riding. Look for the CPSC bicycle helmet statement inside the helmet, then adjust the fit system.
Gear that helps Sponsored
bike helmet sizes
Links go to Amazon search results for each type of gear and are affiliate links: we may earn from qualifying purchases. Details.
-
Search on Amazon
Kids bike helmet
A search for adjustable kids' bike helmets that list CPSC certification.
Search on Amazon -
Search on Amazon
Toddler bike helmet
A search for toddler-size bike helmets.
Search on Amazon -
Search on Amazon
Soft measuring tape
A search for soft tape measures, used to measure around the head.
Search on Amazon -
Search on Amazon
Youth bike helmet
A search for youth bike helmets in the 50 to 57 cm range.
Search on Amazon
Frequently asked questions
How do I measure my child's head for a bike helmet?
Bell says to use a soft tape (or string and a ruler), wrap it about 1 inch (2.5 cm) above the eyebrows, level and snug but not tight, and measure twice, using the larger reading. Giro's instruction is to keep the tape level from front to back.
What size bike helmet for a 5-year-old?
Neither Giro nor Bell lists a size by age, so the tool does not either. Measure the head and match it to the chart. As a guide, Giro's Universal Child is 47 to 54 cm and Bell's is 18 7/8 to 21 5/8 inches.
What head size is small, medium and large?
Giro's adult sizes: S 51 to 55 cm, M 55 to 59 cm, L 59 to 63 cm, XL 63 to 65 cm. Giro's youth sizes: XS 45 to 51 cm and S 49 to 55 cm. Bell lists its own ranges in inches.
What if the head measures between two sizes?
Giro and Bell's ranges overlap, so a measurement can match two sizes. Bell says to use the larger of two readings. Try both sizes on, and check the fit with the straps adjusted.
What does the CPSC label mean?
CPSC's business guidance says bicycle helmet requirements are published in 16 CFR Part 1203, and that bicycle helmets failing them violate the Consumer Product Safety Act. Look for the CPSC statement printed in the helmet and check the maker's product page.
Should my child grow into the helmet?
No. Buy the size that fits now. Giro says the helmet should fit with a little room when the fit system is loose; adjust the fit system for a snug, level fit. Buy a bigger size when the head has grown.
When should a bike helmet be replaced?
Replace a bike helmet after a crash, and follow the maker's instructions for age and wear. This tool does not give medical or injury advice.
Do Giro and Bell sizes match?
Not exactly. Giro's youth S is 49 to 55 cm. Bell's youth S is 20 1/8 to 21 5/8 inches, or about 51 to 55 cm. Check the chart on the model's own page.
More size tools
- Baseball bat size chartBat length and drop by height, weight and league stamp, plus a tee ball section.Open →
- Baseball glove size chartGlove size by age and position, from two makers' charts.Open →
- Softball bat size chartFastpitch bat length and drop by height, weight and age.Open →
- Football size by agePee wee, junior, youth and official footballs by age, from Wilson and Nike.Open →
- Hockey stick size chartStick length and flex by height, with how to check the length.Open →
Link to this chart
Share it or save your answer: the link below includes the numbers you entered.
Sources and last checked
Last checked: .
- Giro: Size charts (bike helmets): Bike helmet sizes by head circumference; how to measure. Checked November 3, 2026.
- Bell Helmets: Size charts (general youth and adult bike): Bike helmet sizes by head circumference; how to measure. Checked November 3, 2026.
- CPSC: Bicycle helmets business guidance: The federal bicycle helmet requirements (16 CFR Part 1203). Checked November 3, 2026.
These are makers' and leagues' published guidance, not measurements of your child. Where makers differ, the tool shows the span between them. Rules and charts change, so check the source pages and your league.
FindSportsUSA is not a coach, league or manufacturer. This tool is a starting point, not a measurement. Some links on this page are Amazon affiliate links; see our affiliate disclosure.