One K Helmet Sizing Guide
One K Helmet Sizing: Size Chart, Round vs Long Oval and Liner Fit
Pick your head shape, then fine-tune the fit with a liner.
Measure about 1 inch above your eyebrows in centimeters and match it to One K's chart, from XS at 51 to 52 cm up to XXL at 61 to 62 cm. Then choose Round or Long Oval; every adult line comes in both.
How to measure your head for a One K helmet size
Measure your head with a soft tape about 1 inch (2.5 cm) above your eyebrows, around the widest part of your head and over your ears, and read the number in centimeters to pick a One K helmet size.
Then check your head shape. Seen from above, a Round head is about as long front to back as it is wide, and a Long Oval head is clearly longer front to back than ear to ear. One K sizes are the same in both shapes, so your measurement picks the size and the shape check picks the version.
- Use a soft tape measure and keep your hair the way you wear it under a helmet.
- Place the tape about 1 inch (2.5 cm) above your eyebrows.
- Wrap it around the widest part of your head, over your ears and around the back.
- Read the number in centimeters, which is the unit One K recommends, and repeat a few times to confirm it.
- Match the number to the chart below, then decide between Round and Long Oval.
One K helmet size chart: cm, inches and hat size
One K adult helmets run from XS at 51 to 52 cm (20.1 to 20.5 inches) to XXL at 61 to 62 cm (24.0 to 24.4 inches), and junior helmets start at XS Jr for 47 to 48 cm.
Go by the centimeter column. The hat-size column is there for riders who know their hat size, and if it ever disagrees with your tape at a boundary, trust the centimeters. Every adult size is made in both Round and Long Oval.
| Helmet size | Head circumference (cm) | Head circumference (in) | Hat size |
|---|---|---|---|
| XS | 51 to 52 | 20.1 to 20.5 | 6 3/8 to 6 1/2 |
| S | 53 to 54 | 20.9 to 21.3 | 6 5/8 to 6 3/4 |
| M | 55 to 56 | 21.7 to 22.0 | 6 7/8 to 7 |
| L | 57 to 58 | 22.4 to 22.8 | 7 1/8 to 7 1/4 |
| XL | 59 to 60 | 23.2 to 23.6 | 7 3/8 to 7 1/2 |
| XXL | 61 to 62 | 24.0 to 24.4 | 7 5/8 to 7 3/4 |
| XS Jr | 47 to 48 | 18.5 to 18.9 | 5 7/8 to 6 |
| S Jr | 49 to 50 | 19.3 to 19.7 | 6 1/8 to 6 1/4 |
| M Jr | 51 to 52 | 20.1 to 20.5 | 6 3/8 to 6 1/2 |
One K long oval vs round: every helmet comes in both
Every adult One K helmets line comes in Round and Long Oval, so "will it fit my round head?" stops being a question and becomes a choice: pick the shape that matches your head.
A Round helmet suits a head whose front-to-back and side-to-side measurements are nearly equal. A Long Oval helmet suits a head that is longer front to back than from ear to ear. The sizes, centimeter ranges and refit liners are the same for both, so changing shape never changes your size. Adult sizes run from XS at 51 to 52 cm up to XXL at 61 to 62 cm in both shapes, and junior sizes start at 47 cm, so the chart above applies whichever shape you choose.
On an adult helmet's product page, the Long Oval sizes carry "Long Oval" after the size in the size menu. Riders tell us shape is what makes the difference: one calls her One K the only helmet that fits her oval-shaped head, and another says One K is one of the only brands that fits her round head.
RoundFront to back is about the same as ear to ear.
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Long OvalFront to back is longer than ear to ear.
Shop Long Oval helmetsSecond one I am buying (to get a clean one for shows). It's the only one that fits my oval-shaped head! Love it.
I have a OneK without MIPS since they're one of the only brands that seems to fit my round head. The liner is removable and machine washable, I have two so I can always have a clean one. I tried the Trauma Void EQ3 but unfortunately their round version was a little too oval-y for my head.
One K liner sizing: three shells and refit liners
One K builds every adult size from 3 base shells, Medium, Large and X-Large, and a refit liner changes the fit inside a shell: a Medium helmet with a Small or X-Small liner becomes that size, and an X-Large helmet with an XXL liner becomes an XXL.
The Large shell takes only a Large liner, so L is the one adult size that does not convert, though a spare liner is still handy. Junior helmets follow the same idea: XS Jr, S Jr and M Jr are re-sized with the junior refit liner. Round and Long Oval helmets use the same liners.
Liners also handle hair, sweat and growth. Swap in a fresh liner when yours has packed down or gotten sweaty, a larger liner to make room for a winter cap or a different hairstyle, or the next junior size as a rider grows. Choose the liner for your line: the CCS Mips Refit Liner for Mips CCS helmets, the Defender Refit Liner for Defender helmets, and the CCS Mips JR Refit Liner for junior Mips CCS helmets. Riders tell us the removable liner is what keeps a snug fit over time.
Medium shell: XS, S, MOne refit liner converts between XS, S and M.
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Large shell: LL stays in its own shell with no liner conversion.
Shop refit liners
X-Large shell: XL, XXLOne refit liner converts between XL and XXL.
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Junior shell: XS Jr, S Jr, M JrA junior liner moves up as the rider grows.
Shop junior linersI particularly liked being able to switch out for a larger liner to make room for my thermal therapy liner cap in the winter.
I also love having the option of removing the liner to wash or to replace, allowing me to retain a snug fit when the padding eventually packs down.
One K CCS panel size guide: top panel, rails, shield
One K CCS top panels and vent stripe rails come in three sizes: Medium fits XS, S and M helmets, Large fits L, and X-Large fits XL and XXL. The front shield is one size and fits every CCS helmet.
Order by your helmet's size group, not your head measurement. The groups match the shells, so a liner swap inside a shell never changes your panel size: an M helmet re-fit to S still takes Medium parts. In short, 3 panel sizes cover all 6 adult helmet sizes from XS through XXL, so the quickest check is your helmet size, then the matching panel size.
CCS parts fit CCS helmets only, which means the Mips CCS Helmet, Defender Mips CCS, Avance Mips CCS, Mips CCS Avance Wide Brim Helmet and Mips Jr CCS Helmet, and not the classic Defender Helmet series. They swap with clips and a One K key or a quarter, and One K describes the system as made to be customized on occasions, not on a daily basis. For the look of each line, read One K Mips CCS vs Defender vs Avance helmets compared.
Between sizes? One K fit by Mips CCS, Defender, Avance
If you land between two One K sizes, such as 54 cm at the top of S or 55 cm at the bottom of M, check the shells first: XS, S and M share one shell, so a rider between S and M can choose either and fine-tune with a refit liner, while a rider between M and L is choosing between two shells and should repeat the centimeter measurement and confirm her shape before ordering.
Fit by line works the same way across the range. The Mips CCS Helmet, the Defender Helmet and the Mips CCS Avance Wide Brim Helmet use one size chart and the same Round or Long Oval choice, so you size once and then choose the look. What separates the lines is style and features, which One K Mips CCS vs Defender vs Avance helmets compared lays out side by side.
Hair and season change fit too. Riders tell us the interchangeable liners are what make One K sizing easy: one says a spare liner ends the different fit that comes with hair up or down, and another notes that the liners helped with sizing even though her Large fit perfectly.
Shopping for a growing rider? Choose by centimeters with the junior sizes, XS Jr to M Jr, and re-size with the junior liner as she grows. Browse all One K helmets when you have your size.
One K Defender Chrome Stripe Helmet
I had not hear of the One K brand and tried on the more recognized name brands first. This was by far the most comfortable--and it has interchangeable inserts to help with sizing (although the L fit perfectly). I like the no fussing around with many adjustments to straps (because it doesn't have a lot of straps and buckles).
I personally find this perfect as I can buy more than one liner and no longer have to deal with the different fit that is caused by wearing my hair up or in a pony…
What riders say about One K helmet fit
One K Defender Chrome Stripe Helmet
Finally! A helmet that fits well AND looks good on. I've been riding for over 40 years and this is the prefect schooling helmet. Well done OneK!
I CANNOT fit in a CO. I tried both Samshield and One K (was prepared to buy whatever fit better), and it was the One K.
The fit is great and it’s a very comfortable helmet. Helmet designs with adjustments in the back cause me occipital headaches. This is the only one that doesn’t. This is my second One K, and I wouldn’t choose any other brand.
The fit of the One K is excellent, and is definitely one of the best helmets I’ve owned.
Shop One K riding helmets in every size
Shop One K Riding Helmets
Shop all ›Shop One K helmets for heads from 47 to 62 cm, in Round or Long Oval, with the refit liners that fine-tune the fit
One K helmet sizing FAQ
How do I measure for a One K helmet and what is the size chart?
Wrap a soft tape around your head about 1 inch (2.5 cm) above your eyebrows and over your ears, and read centimeters. Adult sizes run XS 51 to 52, S 53 to 54, M 55 to 56, L 57 to 58, XL 59 to 60 and XXL 61 to 62 cm; junior XS Jr to M Jr cover 47 to 52 cm. Check the table above.
CloseDoes One K come in round and long oval?
Yes. Every adult One K helmet line comes in Round and Long Oval, in the same sizes and with the same liners. Round is about as long front to back as side to side, and Long Oval is longer front to back than ear to ear. Pick the shape that matches your head in the size menu.
CloseHow do One K refit liners change helmet size?
One K builds adult sizes from three base shells, and a liner converts a helmet within its shell: XS, S and M share the Medium shell, L has no conversion, and XL and XXL share the X-Large shell. Between sizes, a neighboring size plus a liner fine-tunes the fit. Choose the liner that matches your helmet line.
CloseWhat size CCS panel fits my One K helmet?
CCS top panels and vent stripe rails come in Medium for XS, S and M helmets, Large for L, and X-Large for XL and XXL; the front shield is one size. Order by your helmet's size group, and remember that CCS parts fit CCS helmets only, not the classic Defender series. Check your helmet size before you shop panels.
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