Bike helmet size by head size
A soft tape, thirty seconds and one number: your head circumference.
Short answer: measure around your head, level, across your brow. In Giro's adult chart, 55–59 cm is a medium, 51–55 cm small and 59–63 cm large; Trek's Bontrager chart puts medium at 54–60 cm.[1 2 · Nov 11, 2026] Sizes overlap and differ by brand, so check the exact helmet's chart and try it on.
Helmet size by head circumference
| Head circumference | Giro bike helmet size[1 · Nov 11, 2026] | Trek / Bontrager size[2 · Nov 11, 2026] |
|---|---|---|
| 46 cm (18.1 in) | Youth XS | Toddler |
| 48 cm (18.9 in) | Youth XS | Toddler or Child |
| 50 cm (19.7 in) | Youth XS or Youth S | Toddler or Child or Youth/XS |
| 52 cm (20.5 in) | Youth S or S | Child or Youth/XS or Small or S/M |
| 54 cm (21.3 in) | Youth S or S | Youth/XS or Small or S/M or Medium |
| 56 cm (22.0 in) | M | Small or S/M or Medium or M/L |
| 58 cm (22.8 in) | M | S/M or Medium or M/L or Large |
| 60 cm (23.6 in) | L | Medium or M/L or Large or X-Large |
| 62 cm (24.4 in) | L | Large or X-Large |
| 64 cm (25.2 in) | XL | X-Large |
| 66 cm (26.0 in) | outside chart | X-Large |
Our reading of each published chart at every second centimeter; inches are converted and rounded. "Or" means the charts' size ranges overlap there: try both.
How to measure your head
- Tape level: wrap a soft tape around your head, level from front to back, across the brow and over the ears.[1 2 · Nov 11, 2026] No soft tape? Use a string, then lay it along a ruler.[2 · Nov 11, 2026]
- Fit with the dial loose: the helmet should go on with a little room when its fit system is loose, then sit low enough to protect your forehead without blocking your view.[1 · Nov 11, 2026]
- Snug, not tight once you tighten the rear fit system.[2 · Nov 11, 2026]
The full charts
| Giro size[1 · Nov 11, 2026] | Head circumference |
|---|---|
| Youth XS | 45–51 cm (17.7–20.1 in) |
| Youth S | 49–55 cm (19.3–21.7 in) |
| S | 51–55 cm (20.1–21.7 in) |
| M | 55–59 cm (21.7–23.2 in) |
| L | 59–63 cm (23.2–24.8 in) |
| XL | 63–65 cm (24.8–25.6 in) |
| Trek / Bontrager size[2 · Nov 11, 2026] | Head circumference |
|---|---|
| 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) |
Giro youth sizes and Trek's toddler, child and youth sizes are included above. For bikes, see the kids bike size chart and bike size by height.
Sources: 1. Giro, Size chart: bike helmet size guide (checked Nov 11, 2026). 2. Trek, How to fit your bike helmet (Bontrager size chart) (checked Nov 11, 2026). Trademarks belong to their owners; charts are restated in our own tables, not reproduced.