One of the most important and often overlooked aspects in buying a helmet is sizing and fitment.
If your helmet is uncomfortable to wear that's going to impact your attention (which can be dangerous) and how long you can ride (which sucks), making you less inclined to wear it in the first place (which is ill-advised). Likewise your helmet can't do it's job properly unless it sits correctly on your head and stays in place when it counts.
Finding a good fit can be broken down in to these steps.
1. Identifying what you need in a helmet
2. Getting the measurement correct
3. Understanding head shapes & helmet internal shapes
4. Testing fitment
5. Confirm the fit
1. Identifying what you need in a helmet
I'm talking less about the style of riding you do than the how, where, and when you intend to use this helmet along with other factors specific to you. Just about any style of helmet can be used for any type of riding, yet some may be better suited for a particular riding style. However, your unique use cases and personal needs will inform what you need in any style of helmet.
For example:
Say I ride rain or shine, warm to cool weather, day or night. I typically wear contacts, but occasionally may use glasses. My typical riding sees me on and off the bike frequently for short stops, and needing to talk with someone, snack, or drink something intermittently.
Multiple seasons and wet weather riding are a lot more comfortable if that helmet has vents that are adjustable to match the venting you need for the temp and closable to seal out wind and rain. If those are operable easily with a gloved hand you can adjust them on the fly, which is especially useful.
If you live in climates that are particularly hot, the venting may not be sufficient without opening the face-shield. In which case you'll likely need a drop-down sun-visor or a pair of googles or glasses that are impact rated to keep wind and debris out of your eyes.
With helmet face-shields, a dark or mirrored shield is going to impair your vision at night, so you'll likely need to keep a clear shield available to swap.
Speaking of vision; when you wear glasses you should probably make sure:
- You can put them on and take them off with the helmet on.
- That they sit correctly with the helmet on so you can see effectively.
- It doesn't create any pressure points which would make you uncomfortable.
Even little things, like how your ear can get folded uncomfortably when putting on the helmet were you have to re-adjust can go from mild annoyance to frustration depending on how often it happens and your temperament.
A helmet that is frequently off and back on to accommodate activities while out on rides... That ease of entry/exit may matter more vs. a modular helmet where I can flip up the chin-bar, or an an open face 3/4 helmet I can leave on for most things.
2. Getting the correct measurement
The tried and true method is to use a sewing/tailor's tape measure wrapped around the circumference of your head just above the eyebrows (~1" or 2.5cm), and record the measurement based on where the zero end overlaps the tape.
If you don't have flexible tape measure like that lying around, but you do have a construction type recoil-able tape measure or long straight rule. You can try to use a string/cord/etc. wrapped around your head similarly; pitching where the ends overlap, and transfer that straightened out on to your available measurement reference.
Regardless of which option you have available; I'd recommend taking the measurement 3 times, and then averaging between the results to ensure the most accurate result.
3. Head shape vs. helmet internal shape
If your head shape leans one way and the helmet's internal shape the other, even if the measurement says it should fit it might not. It may even hurt, causing 'hot-spots' and headaches, as if it was too small and/or not stay in place properly given it doesn't contact your head evenly.
Human head shapes are a spectrum; at one end of that are people where it's more rounded than oval, and at the other end are people where it's more an elongated oval than round. People in the North American market the typical head shape is a blend between those extremes, which is appropriately referred to as 'intermediate-oval'.
As genetics play into that more than geography, you may find yourself leaning more toward a 'round-oval' or 'long-oval' than typical for where you live. So your best option is to figure out what your head shape actually looks like.
To get your head shape you can have a friend look at or take a photo of your head from above to judge the shape, just make sure your hair doesn't obscure the natural shape of your head.
Alternately a trick I like to use and can also be used to determine your rough head measurement, is to take a snap-back adjustable baseball cap and adjust it to sit fully down on the top of the head and the band comfortably snug. Then gently hold the sides of the cap as you take it off your head, and note the shape the cap took while on your head.
Since there's no universal head shape helmet, a variety of exterior shell shapes that may lend toward one interior shape or another, and with material limitations effecting thickness of materials used you can't please everyone.
Typically manufactures pick a place on that head shape spectrum to optimize for when introducing a model to a specific market, but model to model within that market internal shapes may be different. Some manufactures like Shoei are pretty much exclusively 'intermediate-oval' for models they market in the US, but for most just because one helmet in their line-up fits you well isn't necessarily a guarantee their others will.
4. Testing fitment
Try the helmet on, pulling the straps on each side outward to compress the neck-roll as you are pulling it on to your head. Make sure the top of the helmet's interior is in direct contact with the top of your head, and if not try to pull it down further until the helmet rest against the top of your head.
The helmet's liner should fit snugly around your head, there should be no noticeable voids or gaps between your head and the helmet interior. It's normal to feel some compression, as most helmet liners will 'break-in' to conform to the shape of your head during the first 10-20 hours of wear becoming less tight and more comfortable. However, you shouldn't feel hot-spots/pressure-points or like it might start giving you a headache. If you do try going a size up or try a different shape of helmet.
If you can get a finger in between your brow and the lining in the front it's too loose. With your head held forward try rotating the helmet slightly to the left and right, if your head wants to move with the helmet vs. the helmet sliding over your head that's what we are looking for.
Next fasten the chin strap/retention system so that there's no slack between it and your neck; check that if you push up on the back of the helmet and/or down on the front, that it doesn't want to slide and rotate forward on your head. Then do the opposite and try to push up on the front and/or down on the back. The helmet should stay in place on your head and cause your head to try move with the helmet.
Should the helmet move independently of your head in those tests, it's too loose, and you should try a size down or a different shape helmet.
5. Confirming fitment
How a helmet feels after a minute or two can be very different than it feels after a whole day in the saddle, so before you commit to riding in that helmet you should spend a little more time to confirm your initial impressions.
Just keep in mind this confirmation should be done off the bike. One you don't want to be in the middle of a ride to find it's distractingly uncomfortable, and two most stores (including ours) will not allow you to return a helmet once you've ridden in it. Maybe walk inside, sit and watch some TV, play a video game, or just doom-scroll on your phone for 15 to 30 minutes to see if anything starts to feel uncomfortable.
Snug is good and tightness is OK, but pressure-points or discomfort that makes you want to take the helmet off for relief is not. Two common places that you may find discomfort are the forehead or around the temples; if you find it's the first you may need something more toward 'long-oval', while the latter suggests it's not 'round-oval' enough for you.
Likewise, if you either need glasses to see or ride with sunglasses it's a good idea to pop those on to see if that changes your opinion of the fit over some time.
If you've made it this far you know that it generally fits, and that there's likely no surprises about how it will feel during a short ride. So what are you waiting for? Go out and ride!
Your helmet will likely only start to get more comfortable over time as it wears in. However, not a bad idea to repeat those fitment checks at the start of each riding season just to make sure that fitment hasn't changed too much for the helmet to do it's job effectively.