The S, M or L on the box is a hint, not an answer. The Snell Memorial Foundation, which certifies helmets, puts it plainly: the size label is only a rough guide because sizing varies between manufacturers. What actually matters is whether the helmet stays put on your head. This covers measuring, reading a size chart, and testing the fit — including on a helmet that comes with a rental bike.
How to measure your head
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Use a soft tape and measure the largest circumference of your head:
- Run the tape across the forehead about a finger’s width above the eyebrows and around the most prominent point at the back of the skull.
- Keep it firm against the scalp but not tight, and keep it level — letting the tape drop at the back gives you a wrong number.
- Measure twice and take the larger figure. With no tape, use a length of string and measure the string against a ruler.
- If you have thick hair or usually tie it back, measure with the hairstyle you will actually ride in.
That number is where you start looking at charts — not where the decision ends.
Reading a helmet size chart
We are not publishing a generic “S equals this many centimetres” table, because any such table is wrong for somebody’s helmets. Instead:
- Use the chart from the brand of that specific helmet — usually printed on the box, on the inner label, or on the manufacturer’s own site.
- If your measurement falls between two sizes, try both. Many riders end up in the smaller one because the lining softens with use, but that only applies when the helmet does not hurt from the start.
- Sizes do not transfer between brands. One brand’s M can be another brand’s L.
Why sizes differ between brands
Beyond circumference, each helmet line has a different internal shape. Snell notes that people with broad or narrow foreheads may fit better in a different brand or model line — so two people with identical measurements can need completely different helmets. That is also why buying on the number alone is unreliable.
Head shape: round or longer oval
Manufacturers commonly divide their shells by head shape — rounder, longer front-to-back, or somewhere between. That classification comes from the brands rather than from a shared standard, so the terminology varies.
You do not need the labels. You need to recognise a shape mismatch while the helmet is on your head:
- Pressure at the temples while the forehead and back stay loose: the shell is longer front-to-back than your head.
- Pressure on the forehead and back of the head while the sides stay loose: the shell is rounder than your head.
In either case, changing size will not fix it — you need a different model line.
The fit test, step by step
Snell’s criterion is short: a well-fitting helmet is snug all the way around the crown of the head and does not shift on the head. These steps are how you check that.
- Step 1 — put it on. A correctly sized helmet is usually a little tight going past the ears. If it slips on easily, it is probably too big.
- Step 2 — check the contact. The lining should touch evenly all round, with no gap at the forehead or the sides.
- Step 3 — move your head. Shake gently side to side and nod. Your scalp should move with the helmet. If the helmet slides while your head stays still, it is too big.
- Step 4 — leave it on for a few minutes. A helmet that feels fine for ten seconds can press painfully on the forehead after five minutes. A concentrated pressure point means wrong shape or wrong size.
Cheek pads and chin strap
- Cheek pads should press lightly against the cheeks. A new helmet hugs the cheeks more than people expect, and this is the part that softens fastest with use.
- The chin strap should buckle snugly under the chin. Snell gives an easy check: do not allow more than two fingers to slide under the strap. A loose strap can let the helmet move excessively or come off in a crash.
A certified helmet in the wrong size does not protect the way it was designed to. Snell links better-fitting helmets with fewer head injuries, and recommends trying a helmet on before buying rather than ordering by measurement.
Checking the fit of a rental helmet
With a helmet that comes with the bike you cannot choose the size in advance — but you can check it in a minute, at pickup:
- Put it on, fasten the strap, shake your head side to side. If it shifts on your head, say so and ask for a different one.
- Check the gap under the strap with the two-finger test above.
- Glance at the lining: if it is flattened or compressed, the helmet is old — and an old helmet usually fits looser than its label suggests.
Checking the condition of a rental helmet — shell, buckle, visor, age — has its own guide: checking a rental helmet before riding. Choosing a helmet type and reading the safety standards is covered in motorcycle helmet types and standards. This article is only about size and fit.
When you collect your bike from Mr Tú – Old Quarter Motorbike Rental, try the helmet on there and then, and say straight away if it does not fit — that is the easiest moment to swap it.
FAQ
How tight is correctly tight?
Snug evenly around the crown, no movement when you shake your head, and no pain at any single point. The right sensation is even pressure, not a pinch in one place.
Will it loosen up once I have worn it a while?
The cheek pads do soften, more than the crown lining does. That is not a reason to accept a helmet that hurts from the first minute, and the hard shell does not stretch at all.
My measurement is between two sizes. Which one?
Try both in that particular model. The head-shake test decides it, not the number.
Does tied-up hair change anything?
It changes the fit, so measure and try the helmet with the hairstyle you will ride in. A high bun at the back of the head tends to tip the helmet and let it shift.
Can I just order online from my measurement?
Snell recommends trying a helmet on before buying and advises caution with online purchases — for the two reasons above: sizes vary between brands, and head shape matters as much as circumference.
The rental helmet does not fit. What should I do?
Say so at pickup rather than accepting it for the trip. For a long tour some riders bring their own helmet, but for a short trip a properly fitting one from the shop is enough.
The right size still does not help if the helmet is not a real one. See how to spot a fake or unsafe helmet to rule out shells with no impact liner first.
Thinking of fitting speakers in the helmet you just sized? See helmet communication systems: half helmets are the hard case, and Vietnam has its own rule on this.
Shared helmets bring one more problem: flattened, damp padding makes a helmet looser than its stated size. See wearing a shared or rental helmet.
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