Loading page
About this tool
Find your helmet size from head circumference, flag borderline fits and identify your head shape from the cephalic index.
A helmet size finder converts a head circumference measured about 2.5 cm above the eyebrows into a helmet size letter, using the two-centimetre bands most full-face helmets follow — 55–56 cm for S, 57–58 cm for M, 59–60 cm for L and so on. It flags measurements sitting within half a centimetre of a band edge, where two sizes should be tried side by side, and adjusts for brands that run a size small or large. Head length and breadth also give the cephalic index, the anthropometric ratio that tells you whether you need a long oval, intermediate oval or round oval internal shape.
Open Helmet Size Finder on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Half a centimetre from a band edge is flagged instead of silently rounded.
Uses the cephalic index to name the internal shape that fits your head.
Compares your tape reading with the circumference implied by head length and breadth.
Run a soft tape around the widest part of your head — roughly 2.5 cm above the eyebrows and above the ears — keeping it level all the way round, snug but not pulled tight. Measure three times and take the largest reading; two millimetres is enough to move you between sizes, so the tape must not ride up at the back.
Yes — firm, evenly pressed all round, with the cheek pads squashing your cheeks and no gap you can slide a finger into at the forehead. The comfort liner compresses over the first few rides, so a helmet that feels merely comfortable in the shop will be loose in a month. It should not rock forward off your head when fastened and pushed from behind.
Divide your head's side-to-side breadth by its front-to-back length and multiply by 100. Under about 76 is a long oval, 76 to 81 is intermediate oval — the most common shape and the default for most helmets — and above 81 is round oval. Buying the right shape is what removes pressure points that no amount of size change will fix.
Immediately after any impact, even one that leaves no visible damage, because the EPS liner crushes once and does not recover. Otherwise most makers advise about five years from first use or seven years from the manufacturing date moulded into the shell, whichever comes first — glues, resins and the liner degrade with sweat, sunlight and time.