How to use this chart
Measure your head circumference with a soft tape, then find that number in the table for the activity you need. Keep the tape level above the eyebrows and ears and around the widest part at the back. Repeat the measurement to check that the tape did not slip. The diagram identifies the measuring band; it is not a helmet-position test.
These are general US sizing references from REI’s cycling guide and Christy Sports’ ski and snowboard guide, not a particular helmet manufacturer’s chart or an official national sizing standard. Their S, M and L ranges agree, but individual models can differ. The inch column converts the centimeter endpoints using exactly 2.54 cm per inch; displayed values are rounded to two decimals.
Read the range, then check the model
A number inside a row identifies a reference size to examine, not a confirmed fit. Head shape, the helmet’s internal shape and its adjustment system are not measured here. Open the source and the specific model’s instructions before relying on a general chart.
The general M range ends at 59 cm and L begins at 59 cm. This shared endpoint is retained in both tables. Compare the actual model’s instructions and fit rather than assuming the larger or smaller letter must be correct. Use the unrounded centimeter measurement near a boundary; rounding should not move a measurement into another range.
Two examples
At 57 cm, both general tables show M, with a reference range of 55–59 cm. The equivalent circumference is about 22.44 inches. This is a starting point for comparing models, not confirmation that every medium cycling or snow helmet will fit the same head.
At 55.25 cm, both tables also show M. At exactly 59 cm, the measurement appears in both M and L. Cycling XS is below 51 cm and XL is above 63 cm; these are open reference categories, not a claim that a particular helmet adjusts to every smaller or larger head.
Supported equipment and limits
The cycling table includes XS through XL; the snow reference supplies S through L. No snow XS or XL limits are invented where that source gives none. These tables do not establish youth, motorcycle, football or race-specific sizing. A shared number does not make a cycling helmet suitable for skiing, or a snow helmet suitable for cycling. Check the intended activity and the actual model.
Use this guide to prepare for a fitting or understand a manufacturer’s size label. It does not select protective equipment for a particular sport, check a safety standard, assess impact damage or certify retention. A matching circumference is only one input to the manufacturer’s complete fitting process.
Should I choose the larger size at a boundary?
There is no universal answer here. The source ranges can overlap. Check the exact model’s fit and adjustment instructions and compare the adjacent sizes.
Can I use a hat size instead of measuring?
Measure directly. Hat labels and helmet letters are not a universal conversion. This page compares actual head circumference, and its United States label identifies the guidance’s country context rather than a mandatory sizing rule.
What if my tape only shows inches?
Use the inch column as a convenient comparison. For a boundary decision, multiply the unrounded inch measurement by 2.54 and compare the original centimeter limits. For example, 22.5 inches equals 57.15 cm.
Does a matching size mean a helmet is safe for me?
No. The chart neither verifies certification nor tests individual fit. Follow the specific helmet’s instructions and use equipment intended for the activity.