US to cm Helmet Size Conversion
| US Size | Head (cm) |
|---|---|
| S | 54-55 |
| M | 56-57 |
| L | 58-59 |
| XL | 60-61 |
Sizing conventions vary by brand and by cut — this table shows the standard published conversion. Always check the garment's own size chart when one is available. See how these tables are compiled.
A US 'M' helmet size corresponds to a 56-57cm head circumference on the standard chart, the reverse lookup from centimetres to the US letter label — useful when a US retailer's size guide only shows the letter, not the underlying measurement.
Because different helmet brands cut the same letter size around slightly different internal shell shapes (oval vs. round head shapes especially), the centimetre figure from your own tape measurement is the more dependable reference than the US letter across brands.
Frequently asked questions
Does a US 'M' helmet always fit a 56-57cm head?
As a starting point, yes, but different brands shape their shells for slightly different head profiles (rounder vs. more oval), so the same centimetre measurement can still fit two brands' 'M' helmets differently.