Wrap a soft tape around your head about an inch above your eyebrows, just above your ears, around the widest point at the back. Pull it snug, not tight. Do it twice. If the two readings differ, use the larger one.
That is the whole method, and it comes straight from Harley-Davidson’s fit guide. The reason it needs an article is that almost every wrong-size helmet starts here, not at the size chart.
The four ways riders get the number wrong
1. The tape sits too high or too low
An inch above the eyebrows is not decoration. Go higher and you measure the narrowing crown; go lower and you catch the brow ridge. Either way you are measuring a circle that is not the one the helmet has to clear. A centimetre of error is enough to move you a full size.
2. A rigid tape measure instead of a soft one
A builder’s tape cannot follow the curve of a skull. It bridges across the back of the head and reads short. If you do not own a soft tape, wrap a length of string or a phone charging cable around your head, mark where it overlaps, and lay that flat against a ruler.
3. Pulling it tight
Snug means resting against the skin with no slack. Tight means compressing it. You are not measuring how small your head can be made — the helmet has to fit the head you actually have, with the padding it comes with.
4. Measuring once
Harley’s guide says to measure at least twice and take the larger reading, and that instruction exists because a single pass is unreliable. The tape drifts. Take three passes if two disagree.
What to do with hair
Measure with your hair the way you wear it when you ride. If you tie it back under the helmet, tie it back to measure. Thick hair worn loose can add enough to the reading to push you into a size you do not need — and a helmet chosen that way will be loose on every ride where you tie it up.
Now turn the number into a size
Once you have the measurement, run it through our motorcycle helmet size chart tool. It converts to centimetres, inches and hat size, all of which are exact, and shows what one manufacturer’s published chart calls that head.
Only the measurement travels. The size letter does not: Bell’s own website lists 34 separate size charts, one per model, and its general and HP/Carbon/Pro charts disagree about what a 56 cm head is. That is why the useful output of measuring is a number, not a letter.
The measurement is the start, not the answer
A good measurement narrows you to one or two sizes. It cannot tell you whether a given helmet fits, because that also depends on the shape of your head — two riders with identical 58 cm measurements can need different helmets. See motorcycle helmet head shapes.
And once the helmet is on, there is a specific set of checks that tells you whether it is right: how tight a motorcycle helmet should be.
Fit is separate from protection. What the certifications on the shell actually mean is covered in helmet ratings explained.
Your measurement, converted
Two numbers travel between brands: the circumference itself, and hat size. Both are arithmetic, so they mean the same thing on any manufacturer’s chart. Size letters do not.
| Centimetres | Inches | Hat size |
|---|---|---|
| 50 cm | 19.69″ | 6 1/4 |
| 51 cm | 20.08″ | 6 3/8 |
| 52 cm | 20.47″ | 6 1/2 |
| 53 cm | 20.87″ | 6 5/8 |
| 54 cm | 21.26″ | 6 3/4 |
| 55 cm | 21.65″ | 6 7/8 |
| 56 cm | 22.05″ | 7 |
| 57 cm | 22.44″ | 7 1/8 |
| 58 cm | 22.83″ | 7 1/4 |
| 59 cm | 23.23″ | 7 3/8 |
| 60 cm | 23.62″ | 7 1/2 |
| 61 cm | 24.02″ | 7 5/8 |
| 62 cm | 24.41″ | 7 3/4 |
| 63 cm | 24.80″ | 7 7/8 |
| 64 cm | 25.20″ | 8 |
| 65 cm | 25.59″ | 8 1/8 |
Hat size is simply the circumference in inches divided by pi. Run that formula against the eleven points Bell Racing publishes on its own chart and it lands on their figure at ten of them, so it is a conversion you can rely on rather than a rule of thumb.
When the two readings disagree
Harley-Davidson’s guidance is to measure at least twice and take the larger reading, and it is worth understanding why the instruction is phrased that way rather than “take the average”.
A tape that reads short has almost always slipped off the widest point — ridden up at the back, dropped at the front, or been pulled tight enough to compress. A tape that reads long is much rarer. So the larger of two readings is the more likely to be correct, not merely the more cautious.
If two passes differ by more than about a centimetre, something is wrong with the technique rather than with your head. Reset and do it again slowly, watching the tape at the back in a mirror.
Measuring someone else
Easier than measuring yourself, and worth doing for a pillion or a child before buying anything.
- Stand behind them so you can see the tape across the back of the head, which is where it slips.
- Ask them to look straight ahead. Tipping the chin changes the reading.
- Children’s heads grow, and a helmet bought with room to grow into is a helmet that does not work now. Measure again before each season rather than assuming last year’s size.
What the number can’t tell you
A measurement narrows you to one or two candidate sizes and no further. Two riders who both measure 58 cm can need completely different helmets, because circumference says nothing about the shape of the skull it is wrapped around.
That is why the sequence is measure, then try. Take the number to our size chart tool to convert it and see how one manufacturer’s published chart reads it, work out your shape in motorcycle helmet head shapes, then run the fit checks in how tight a motorcycle helmet should be.
Frequently asked questions
Where exactly do you measure for a motorcycle helmet?
Around the largest part of your head: roughly one inch above your eyebrows at the front, just above your ears at the sides, and around the widest point at the back. The tape should form a level ring, not a diagonal.
How do I measure my head without a tape measure?
Wrap string, ribbon or a charging cable around your head in the same position, mark the overlap, then measure that length against a ruler. It is as accurate as a soft tape provided the material does not stretch.
Should I measure my head with hair up or down?
However you wear it when riding. If you tie your hair back under a helmet, measure with it tied back, or you will size up unnecessarily.
What if my measurement falls between two sizes?
Take the smaller size if it produces no pain or pressure points, because the padding compresses with use and a loose helmet only gets looser. If the smaller size hurts anywhere, that is a shape problem, not a break-in problem.
Measuring method follows Harley-Davidson’s published motorcycle helmet fit and size guide. Size letters referenced are from Bell Racing’s published chart and are not universal. Always check the chart for the specific model you intend to buy.