Bike helmet size chart and calculator
Enter your head circumference to see the helmet size in the REI, Giro, Trek, Nutcase and Thousand charts. The tables below show every range.
Shop this size
Enter your head circumference above to get shopping links for your helmet size.
Affiliate links. We may earn a commission at no cost to you. See the disclosure.How to measure your head
- Use a flexible tape measure, or a string and a ruler.
- Wrap it around the largest part of your head, about 1 inch (2.5 cm) above your eyebrows, level and snug but not tight.
- Measure twice and use the larger number (Bell).
Bike helmet size chart by head circumference
Sizes overlap, and brands differ. Inch values are converted from the centimetres each source gives.
REI general guide
| Size | Head circumference | In inches (converted) |
|---|---|---|
| Extra small | under 51 cm | under 20.1 in |
| Small | 51–55 cm | 20.1–21.7 in |
| Medium | 55–59 cm | 21.7–23.2 in |
| Large | 59–63 cm | 23.2–24.8 in |
| Extra large | over 63 cm | over 24.8 in |
Giro (adult, youth and universal)
Giro's page gives no separate toddler table; its smallest listing is Youth XS at 45 to 51 cm.
| Size | Head circumference | In inches (converted) |
|---|---|---|
| Adult S | 51–55 cm | 20.1–21.7 in |
| Adult M | 55–59 cm | 21.7–23.2 in |
| Adult L | 59–63 cm | 23.2–24.8 in |
| Adult XL | 63–65 cm | 24.8–25.6 in |
| Universal Adult | 54–61 cm | 21.3–24.0 in |
| Universal Women's | 50–57 cm | 19.7–22.4 in |
| Universal X-Large | 58–65 cm | 22.8–25.6 in |
| Youth XS | 45–51 cm | 17.7–20.1 in |
| Youth S | 49–55 cm | 19.3–21.7 in |
| Universal Child | 47–54 cm | 18.5–21.3 in |
| Universal Youth | 50–57 cm | 19.7–22.4 in |
Trek
| Size | Head circumference | In inches (converted) |
|---|---|---|
| Toddler | 46–50 cm | 18.1–19.7 in |
| Child | 48–52 cm | 18.9–20.5 in |
| Youth / XS | 50–55 cm | 19.7–21.7 in |
| Small | 51–57 cm | 20.1–22.4 in |
| S/M | 51–58 cm | 20.1–22.8 in |
| Medium | 54–60 cm | 21.3–23.6 in |
| M/L | 55–61 cm | 21.7–24.0 in |
| Large | 58–63 cm | 22.8–24.8 in |
| X-Large | 60–66 cm | 23.6–26.0 in |
Nutcase and Thousand
Public Bikes lists these two brands. Its page gives only centimetres.
| Size | Head circumference | In inches (converted) |
|---|---|---|
| Nutcase Small | 52–56 cm | 20.5–22.0 in |
| Nutcase Medium | 56–60 cm | 22.0–23.6 in |
| Nutcase Large | 60–64 cm | 23.6–25.2 in |
| Thousand Jr. (XS) | 49–53 cm | 19.3–20.9 in |
| Thousand Small | 54–57 cm | 21.3–22.4 in |
| Thousand Medium | 57–59 cm | 22.4–23.2 in |
| Thousand Large | 59–62 cm | 23.2–24.4 in |
How a helmet should fit
- Level and low. The front edge sits one inch or less above your eyebrows, not tilted back (REI, Trek).
- Snug, not tight. Push the helmet side to side and front to back; movement of an inch or more means adjust it (REI).
- Straps. They form a V under each ear (REI, Trek). Trek says only two fingers should fit between the chin strap and your chin.
- Mouth test. Buckle the strap and open your mouth wide. The helmet should press down on top of your head (REI).
Questions
How do I measure my head for a bike helmet?
Wrap a flexible tape measure around the largest part of your head, about one inch (2.5 cm) above your eyebrows, level from front to back and snug but not tight (REI, Bell). Measure twice and use the larger reading (Bell). With no tape, wrap a string around your head and measure it against a ruler (REI).
What if I am between two helmet sizes?
The sources disagree. REI says to choose the smaller size, while Public Bikes says to choose the larger one for Nutcase and Thousand helmets. Try both on if you can: the helmet should sit level and snug without moving more than about an inch when you push it.
What size bike helmet is 56 cm?
56 cm is a Medium in the REI and Giro charts (55 to 59 cm). It also fits Trek's Small, S/M, Medium and M/L ranges and the top of a Nutcase Small or the bottom of a Nutcase Medium, so check the maker's own range.
Are bike helmet sizes the same for every brand?
No. REI notes that values vary slightly among brands, and the charts above show it: a 53 cm head is a Small in the REI chart (51 to 55 cm) but sits at the top of Thousand's Jr. (XS) range (49 to 53 cm). Always check the head range printed for the specific helmet.