These sizes come from each brand’s official size guide, checked in October 2026. Giro publishes one chart for all its adult bike helmets; Bell, MET and Lazer list the chart on each model, so we took their best-selling road helmets and checked that other models in the same line use the same ranges. The brands set their ranges in centimeters; the inches here are converted to the nearest quarter inch. If your helmet is a different model, check its own chart before you buy.
Head circumference by size and brand
| Size | Giro | Bell (XR Spherical, 4Forty, 5Fifty) | MET (Trenta, Rivale, Vinci) | Lazer (Z1, Vento, Strada) |
|---|---|---|---|---|
| S | 20-21.75 in (51-55 cm) | 20.5-22 in (52-56 cm) | 20.5-22 in (52-56 cm) | 20.5-22 in (52-56 cm) |
| M | 21.75-23.25 in (55-59 cm) | 21.75-23.25 in (55-59 cm) | 22-22.75 in (56-58 cm) | 21.75-23.25 in (55-59 cm) |
| L | 23.25-24.75 in (59-63 cm) | 22.75-24.5 in (58-62 cm) | 22.75-24 in (58-61 cm) | 22.75-24 in (58-61 cm) |
| XL | 24.75-25.5 in (63-65 cm) | — | — | 24-25.25 in (61-64 cm) |
Dashes are sizes that model doesn’t come in. Giro also sells one-size helmets: 21.25-24 in (54-61 cm) for the adult version and 19.75-22.5 in (50-57 cm) for the women’s. Bell also has a generic adult chart with other ranges (S 52-56, M 56-58, L 58-60, XL 60-62 cm, or about 20.5-22, 22-22.75, 22.75-23.5 and 23.5-24.5 in), and some of its full-face helmets, like the Full-10, run narrower; the XR Spherical, 4Forty and 5Fifty use the ranges in the chart. At Lazer, the XL only exists on some models, like the Z1 and the Strada.
Which size fits your head
| Head circumference | Giro | Bell XR Spherical | MET | Lazer |
|---|---|---|---|---|
| 20.5 in (52.1 cm) | S | S | S | S |
| 20.75 in (52.7 cm) | S | S | S | S |
| 21 in (53.3 cm) | S | S | S | S |
| 21.25 in (54.0 cm) | S | S | S | S |
| 21.5 in (54.6 cm) | S | S | S | S |
| 21.75 in (55.2 cm) | M | S or M | S | S or M |
| 22 in (55.9 cm) | M | S or M | S | S or M |
| 22.25 in (56.5 cm) | M | M | M | M |
| 22.5 in (57.2 cm) | M | M | M | M |
| 22.75 in (57.8 cm) | M | M | M | M |
| 23 in (58.4 cm) | M | M or L | L | M or L |
| 23.25 in (59.1 cm) | L | L | L | L |
| 23.5 in (59.7 cm) | L | L | L | L |
| 23.75 in (60.3 cm) | L | L | L | L |
| 24 in (61.0 cm) | L | L | L | L or XL |
| 24.25 in (61.6 cm) | L | L | — | XL |
| 24.5 in (62.2 cm) | L | — | — | XL |
| 24.75 in (62.9 cm) | L | — | — | XL |
| 25 in (63.5 cm) | XL | — | — | XL |
“S or M” means your measurement falls where two sizes overlap and either one can work. If you’re right on the line between two sizes, as at 21.75 in (55 cm) on a Giro, try both.
How to measure your head
- Use a soft measuring tape (a sewing tape). If you don’t have one, use a piece of string and then a ruler.
- Wrap it around your head just above your eyebrows, about 1 inch (2.5 cm) above according to Bell, and around the widest part of the back of your head. The tape should sit level front to back, as Giro specifies, snug but not tight.
- Measure twice and go with the larger number.
- Try the helmet on with the rear fit system open. Giro says it should go on with a little room before you tighten the dial.
MET points out that size isn’t just circumference: head shape (rounder or more oval) decides whether a helmet in your size fits well or pinches at the temples. If you’re between two sizes, try both.
Whether you’re required to wear one depends on where you ride: helmet laws vary by state and city, and many only cover kids. Check the bike laws section for the rules that apply to you. For the rest of your fit, the bike size calculator gives your frame size from height and inseam, and the guide to finding your bike size explains how to measure yourself. The names of the helmet and every other part in six languages are in the components glossary.
Frequently asked questions
What size bike helmet do I need for a 22.5-inch head?
A medium in all four brands in the chart: Giro (21.75-23.25 in, 55-59 cm), Bell XR Spherical (21.75-23.25 in), MET (22-22.75 in, 56-58 cm) and Lazer (21.75-23.25 in).
What if I'm between two helmet sizes?
Try on both if you can, because head shape matters too, as MET points out. If you can't try them, Bell suggests measuring twice and going with the larger measurement.
Do you measure your head the same way for every brand?
Yes: a tape around your head, level and just above your eyebrows (Bell says about 1 inch, or 2.5 cm). What changes between brands is the range for each size.