Helmet Size Chart: Head Circumference & Fit Guide 2026

The standard helmet sizes

Helmet sizes are typical adult bike-helmet sizes, and they vary by brand and model — measure your head and check the brand’s own chart before buying. The table below lists the standard sizes: the head circumference range in centimetres and inches, the midpoint, and who each size fits. The ranges touch: 51 cm is the top of XS and the bottom of S, and the same at 55 cm and 59 cm. That is deliberate, and the rule below says which side of the boundary you take.

How the pick works

The pick is the smallest size whose range top is strictly above your measurement, and on a boundary you take the larger size: a slightly loose helmet adjusts down with the fit system, and a tight one does not. Below 47 cm is below the chart — a kids helmet. Above 66 cm is above the chart — look for XXL or extended sizes. The pick also shows how far into the range you are (your measurement minus the range low), your head in inches, and the next size up with how many centimetres to it — or “largest standard size” for XL, which has nothing above it.

At the default — a 57 cm head — the pick is M (55–59 cm), 2 cm into the range, which is 22.44 in. The next size up is L, starting at 59 cm, which is 2 cm away.

The worked example, line by line

Setup: head circumference 57 cm.

Line 1 — the pick. 57 cm is above 55 and below 59, so the smallest size whose range top is strictly above 57 cm is M (55–59 cm).

Line 2 — how far into the range. 57 − 55 = 2 cm into the range. The midpoint of M is (55 + 59) ÷ 2 = 57 cm, so a 57 cm head sits in the middle of the size.

Line 3 — the inches. 57 × 0.393701 = 22.44 in.

Line 4 — the next size up. The next size is L, starting at 59 cm, which is 59 − 57 = 2 cm away.

Reverse: 22.00 in is 55.88 cm, size M; 23.23 in is 59.00 cm, size L — a boundary takes the larger size; 57 cm is 22.44 in and 59 cm is 23.23 in; the midpoint of M is 57 cm (22.44 in) and the midpoint of L is 61 cm (24.02 in); and from 57 cm it is 2 cm to L.

Every head size, checked

47 cm: the pick is XS (47–51 cm), 0 cm into the range, which is 18.50 in. The next size is S at 51 cm, 4 cm away. 50 cm: the pick is XS, 3 cm into the range, which is 19.69 in. The next size is S at 51 cm, 1 cm away — close to sizing up.

51 cm: the pick is S (51–55 cm), 0 cm into the range, which is 20.08 in — the boundary takes the larger size, so 51 cm is S, not XS. The next size is M at 55 cm, 4 cm away. 52 cm: the pick is S, 1 cm into the range, which is 20.47 in. The next size is M at 55 cm, 3 cm away.

55 cm: the pick is M (55–59 cm), 0 cm into the range, which is 21.65 in — again the boundary takes the larger size. The next size is L at 59 cm, 4 cm away. 58 cm: the pick is M, 3 cm into the range, which is 22.83 in. The next size is L at 59 cm, 1 cm away — close to sizing up.

59 cm: the pick is L (59–63 cm), 0 cm into the range, which is 23.23 in — the boundary takes the larger size again. The next size is XL at 63 cm, 4 cm away. 60 cm: the pick is L, 1 cm into the range, which is 23.62 in. The next size is XL at 63 cm, 3 cm away.

63 cm: the pick is XL (63–66 cm), 0 cm into the range, which is 24.80 in — the largest standard size. 64 cm: the pick is XL, 1 cm into the range, which is 25.20 in. 66 cm: the pick is XL, 3 cm into the range, which is 25.98 in — the top of the chart.

46 cm: BELOW CHART — under 47 cm, a kids helmet. 66.5 cm and 67 cm: ABOVE CHART — over 66 cm, look for XXL or extended sizes.

Reverse — the other way round: 22.00 in is 55.88 cm, size M; 23.23 in is 59.00 cm, size L; 57 cm is 22.44 in and 59 cm is 23.23 in; the midpoint of M is 57 cm (22.44 in) and the midpoint of L is 61 cm (24.02 in); and from 57 cm it is 2 cm to L.

What the chart assumes

The chart assumes the standard adult bike-helmet sizes above, with the ranges as listed. It assumes you measured with a soft tape, level around your head, about 1 cm above your eyebrows and ears. It does not know the brand: sizes vary by brand and model, so check the brand’s own chart before buying, and try the helmet on if you can. It does not score fit beyond the size: the helmet should sit level and not rock when you shake your head, and the fit system should take up the slack. With 0 cm or a negative measurement the calculator asks for a real head size, and fractional centimetres are fine to one decimal. Rounding convention: all figures are computed from unrounded values and rounded only for display — inches to two decimals, so a figure worked out from the rounded numbers can differ by a few hundredths from the figure shown.

Output · formula · meaning

Who uses this

• The rider with a 57 cm head buying online: they learn the size is M — so they know 57 cm is 2 cm into the 55–59 cm range and the midpoint of M, and they check that L starts 2 cm away before ordering the middle of the size.
• The rider who measures exactly 51 cm: they learn the size is S, not XS — so they know a boundary takes the larger size, and they check that M starts 4 cm away before squeezing into the smaller size.
• The rider between sizes at 58 cm: they learn the size is M, 3 cm into the range — so they know L starts 1 cm away, and they check the fit system before choosing between a snug M and a loose L.
• The rider with a 66.5 cm head: they learn they are above the chart — so they know no standard size fits, and they check XXL or extended sizes instead of ordering an XL that will not close.

Accuracy & limitations

The picks are exact for the numbers you enter: the pick is the smallest size whose range top is strictly above your measurement, the centimetres into the range are your measurement minus the range low, the inches are centimetres times 0.393701, and the next size is one size up. What the chart cannot do is know your real helmet: check the brand’s own chart, because sizes vary by brand and model and a 57 cm head may be an M in one brand and an L in another. It assumes the standard sizes above; some brands run an XXS or an XXL outside this chart. It leaves out the shape of your head, the padding, and the fit system, which all change how a size feels. For other standard sizes, see hat size chart, glove size chart, and shoe size chart.