Helmet length-to-width ratios #askanerd

This is part tech question and part general request - it’s perhaps too long for the tech mailbag but I assume others have the same problem.

James wrote a great article years ago about the issues with helmet sizing, where he calculated the length-to-width ratio for different models in the same size. As someone with a similarly oblong head, I really appreciated that thinking. A lot of helmets leave large gaps on the side of my head, where for other people it’s the opposite problem. Apart from comfort, I assume a poor fit can have safety implications too.

I’ve been using a Kask Protone for a few years and it’s a decent fit for me. A medium is perfect in front-to-back length, though still a bit wider than my head on the sides. Unfortunately it’s also not ventilated enough for truly hot summer days.

CT has a nicely written helmet guide but even that sends some mixed signals. One Giro helmet is praised for fitting a narrow head well, yet Giro was the roundest brand in that old article by James. That matches my experience with them, though I’ve only tried a few models.

Fit is a personal thing and obviously there’s no substitute for trying a helmet on. That said, would CT consider including length and width measurements (or ratios) in helmet reviews? Like publishing stack and reach instead of t-shirt sizing, it seems like an easy way for people to filter out the models that definitely won’t work – and to maybe nudge the industry in a positive direction.

Personally, I’m looking at the Abus Airbreaker, MET Trenta and Limar Air Pro as highly vented but still aero-ish lids, though finding models to try locally is challenging where I am. If anyone has thoughts on which does or doesn’t fit an oblong noggin, I’d be grateful to hear them. (Other options welcome too!)


Can’t say what my head shape is but I have found POC are so far off my head shape it’s silly, had to go for a mega large one and I looked like a 90s era mushroom. MET on the other hand fit wonderfully, so much so I’m happy to order them online.

So I guess if you’re looking at MET Trenta don’t bother with POC!

I ended up sizing up to a size L Spec Evade because the M pinched my sides, but it’s so huge that I look like a goof. So I ordered a size M MET Rivale (seems illegal in the US?) from Italy. Praying it fits my head. Internet seems to say that MET is rounder.

I’ve found that most Giro are too round for me (have tried several models) as well as Kask. While I usually wore a medium in those brands, I found that Specialized/S Works Prevail, Prospero and Evade all provide a better, narrower fit, and for me, in the Small size. While I own all three models, I greatly prefer the Prevail (I or II), for both fit, and weight–I own 7 or 8 Prevails, and all are at or under 200 grams, without MIPS, 40 or 50 more grams with.

I know that measurements alone wouldn’t help much more than what the stated size range would tell you (such as 52 - 56 cm), as all helmets allow some degree of dialed in sizing, and still wouldn’t really help regarding whether it fit my own head snugly or not–the measurements would have to be based on the inner “cage” which may or may not be close to the actual helmet shell, once dialed in to your head size.

If it can help: I always rode Giro. But now I have a Scott Arx and a MET Trenta. Specialized helmets also seem to suit me well. So all these seem to lie in the same line of shape.

Btw I can confirm that a size M Met Rivale MIPS is juuust barely wide enough, while a size M evade 2 is too narrow and a size L evade 2 is too long. So glad that someone caters to round heads.

As an east asian person with a very round head, the most comfortable helmet by far has been the Kabuto Izanagi. Unfortunately don’t think it’s available in the US. Not sure about EU, but I get the sense it’s only sold in Asia.

Mavic Comete wiggles side to side on my head. Bell is a near perfect match. Mavic = rounder.

FYI, I believe that Giro and Specialized offer Asian fit versions of their helmets that are also rounder, but they’re only certified for sale in Asia (i.e. they don’t meet the US CPSC standard).

Great question. If there is a narrower brand than Specialized I’m interested.

My head is very long and narrow and 59cm, so a medium Evade II just fits (really only just) whereas Giro I’m a large and it fits nicely font to back but wiggles side to side. Other rounder brands often look like kids helmets and don‘t fit on my head at all.

I fit the Lazer G1 and Bell z20 very well. Specialized lids squeeze the sides of my head too tight, POC has straps that dig into my ears, and Giro have several tight areas that are uncomfortable (odd, since my ski helmet is Giro and it fits beautifully). The Bell is by far the most comfortable but it’s the worst looking in my opinion so I’ve settled on the Lazer G1 series.

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I ride Met helmets road (Trenta) and mtn (Terranova) and they fit me great, I have a rounder head.
Just as a reminiscence, I remember the old Spec. Sub 6 being the narrowest helmet I ever tried on. Sorry I don’t have any helpful feedback, but I’d cross Met off the list. : (

I’m just here to say I want this article written so bad. I’ve been having a hard time finding a helmet that doesn’t wiggle side to side on my head. I have a rather long and narrow head which forces me in to a large in most brands. I can fit four fingers up into the side of a large Giro aether, but the medium won’t fit on period. I was told this was a pretty oval form, so I’m a bit at a loss…

Still a bad-ass helmet…

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