There's a nice, cheap used helmet on sale that is described as being 'large'. My head is 57cm, which is 'medium'. How well would this helmet fit (I guess it does vary between manufacturers), and how can I improve the fit if it doesn't?

  • We can't answer this for you. You will have to try it on.
    – Criggie
    Apr 10, 2018 at 0:56
  • I don't know where you are but here in the UK you can buy helmets that comply with the relevant safety standards for £10 (€12 or $14) in a big supermarket, and you can try them on. Of course they're not the best, but I'd trust them more than a helmet of unknown provenance. The situation is similar in the bits of western Europe I know, I'd assume it's the same in the US/Canada etc.
    – Chris H
    Apr 10, 2018 at 8:28

1 Answer 1


I'd strongly advise against buying used helmets because you just don't know how well they've been looked after. If it's been dropped, it might be structurally compromised without your being able to see any damage. And, if you think about it, why would anybody sell a cycling helmet? I guess maybe if they've upgraded to something lighter and/or more aero but, in most cases, people will wear a helmet until they think it's worn enough that it needs to be replaced. If it's worn enough that the original owner doesn't want to wear it, you don't want to wear it either.

In any case, this one is too big for you. A circle with circumference 62cm has radius 9.9 cm and a circle with circumference 57cm has radius 9.1 cm. So, with the helmet centred on your head, you'd be able to move it roughly a centimetre in any direction.

  • 3
    Even assuming it was the right size, it's advisable to try a helmet on before buying in. It's not something soft and flexible like a baseball hat. Last time I bought a helmet I had to try on at least 4 or 5 different models of the correct size before I found something comformtable. They all fit around my head properly, but had other issues that cause them to fit incorrectly.
    – Kibbee
    Apr 9, 2018 at 20:44

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