Convert your head measurement
Measure once, then take the numbers to whichever brand you are buying. This converts your measurement exactly and shows you what one manufacturer’s published chart calls it — because no chart is universal.
The centimetre, inch and hat-size figures are exact conversions. The size letter is Bell Racing’s published chart only, shown as a worked example — it is not a universal size and other brands will label the same head differently. Bell’s own site lists 34 separate size charts, one per model.
There is no universal helmet size chart
Every page that promises one is overpromising, and the manufacturers say so themselves.
Bell’s size chart page opens by telling you to “browse Bell moto and bike helmet size charts by model” and states that 34 helmet size charts are available. That is one brand, 34 charts. Harley-Davidson’s fit guide says plainly that helmet sizes vary by brand and tells you to compare your measurement against the manufacturer’s own chart.
It goes further than brand-to-brand. Bell Racing publishes two charts side by side, and they disagree with each other:
| Head circumference | Bell Racing, general chart | Bell Racing, HP / Carbon / Pro |
|---|---|---|
| 55 cm | XS | XS |
| 56 cm | — | SML |
| 57 cm | SML | SML |
| 58 cm | MED | MED |
| 59 cm | — | LRG |
| 60 cm | LRG | LRG |
| 61 cm | XLG | XLG |
A 56 cm head is a small in one Bell range and unlisted in the other. Same brand, same year.
The reason is simple: the letters are commercial, not regulatory. DOT and ECE certify the helmet’s protective performance, not what a manufacturer prints on the size tag. Nothing obliges two companies to mean the same thing by “medium”.
The number that does travel
Your head circumference in centimetres is the only figure that means the same thing everywhere. Get it right once, write it down, and use it against whatever chart the brand publishes.
Hat size is the other portable number, and it is pure arithmetic: circumference in inches divided by pi. That is not an approximation someone invented. Run it against the eleven points Bell Racing publishes and it lands on their figure at ten of them — the exception is 55 cm, where the formula gives 6 7/8 and Bell rounds to 7:
| Centimetres | Inches | Formula gives | Bell Racing publishes |
|---|---|---|---|
| 51 | 20.08 | 6.39 | 6 3/8 |
| 55 | 21.65 | 6.89 (6 7/8) | 7 ← rounding differs |
| 58 | 22.83 | 7.27 | 7 1/4 – 7 3/8 |
| 60 | 23.62 | 7.52 | 7 1/2 |
| 63 | 24.80 | 7.89 | 7 7/8 – 8 |
| 67 | 26.38 | 8.40 | 8 3/8 – 8 1/2 |
This is why the calculator above gives you centimetres, inches and hat size rather than a single letter. Those three travel. The letter does not.
How to measure so the number is worth something
Harley-Davidson’s fit guide sets out the method, and it is the one every manufacturer effectively repeats:
- Use a soft tape. A rigid tape measure will not follow the skull.
- Sit it about an inch above your eyebrows, just above your ears, running evenly around the back at the widest point.
- Pull it snug but not tight.
- Measure at least twice. If the two readings differ, use the larger one.
Most sizing mistakes are measuring mistakes, not chart mistakes. The full walkthrough, including what to do with hair and what to use when you have no soft tape, is in how to measure your head for a motorcycle helmet.
The measurement gets you close, the fit decides
A correct measurement narrows you to one or two sizes. It cannot tell you whether the helmet fits, because that depends on the shape of your head as much as its circumference.
Harley’s guidance on what a correct fit feels like:
- Snug, but no pain and no pressure points.
- Cheek pads should press lightly, with no gap between cheek and padding.
- The helmet should not shift when you shake your head.
- Padding breaks in over time, which matters when you are between sizes.
And the part most size charts skip entirely: if the helmet’s internal shape does not match your head’s shape, it will feel tight in some places and loose in others at the correct size. That is covered in motorcycle helmet head shapes, and the full fit checklist is in how tight a motorcycle helmet should be.
What a size chart cannot tell you
Fit is not safety certification. A helmet that fits perfectly and carries no meaningful certification is still a bad helmet, and the labels are easier to misread than people assume — helmet ratings explained covers what DOT, ECE, SNELL and SHARP actually test.
If you are shopping rather than measuring, our picks are in best motorcycle helmets for women. And the rest of the kit follows the same rule that fit beats specification: riding boots and riding pants.
Frequently asked questions
How do I know my motorcycle helmet size?
Measure your head circumference with a soft tape about an inch above your eyebrows and around the widest point at the back, twice, taking the larger reading. Then compare that number against the size chart published by the specific brand and model you are buying, because size letters are not standardised.
What is 22 inches in helmet size?
22 inches is 55.9 centimetres, which is a hat size of about 7. On Bell Racing’s published chart a 55 cm head is XS and 57 cm is SML, so 22 inches sits between the two — which is exactly why you check the chart for the model you are buying rather than relying on a letter.
What size helmet is a 23 inch head?
23 inches is 58.4 centimetres and a hat size of about 7 3/8. Bell Racing lists 58 cm as MED. Another manufacturer may call the same head large or small, so use the centimetre figure against their chart.
Is there a standard motorcycle helmet size chart?
No. Bell alone publishes 34 model-specific charts, and its own general and HP/Carbon/Pro charts disagree about what a 56 cm head is. DOT and ECE certify protective performance, not size labelling, so nothing requires two brands to mean the same thing by medium.
Should I size up or down if I am between sizes?
Go with the smaller size if it causes no pain or pressure points, because padding breaks in over time and a helmet that is already loose will only get looser. If the smaller size hurts anywhere, it is the wrong size or the wrong head shape, not a helmet to break in.
Does hair affect helmet size?
Measure with your hair as you normally wear it when riding. Thick hair worn loose can change the reading, so if you tie it back to ride, tie it back to measure.
Conversions are exact. Size letters shown are from Bell Racing’s published chart and are reproduced as a worked example, not as a universal standard. Measuring method and fit guidance follow Harley-Davidson’s published fit guide. Always check the size chart for the specific helmet model you intend to buy. Last reviewed August 2026.