To find your motocross helmet size, wrap a soft tape measure around the widest part of your head, about one inch above your eyebrows, and read the circumference in centimeters. Take three measurements and use the largest number. Then find that number on the BTO helmet sizing guide for the specific brand you're buying. That single centimeter measurement is the only number that matters. Hat size, S/M/L labels, and past helmet sizes from other brands are unreliable starting points.
If you've never bought a dirt bike helmet before, the size chart can look confusing fast: centimeters, inches, hat sizes, and letter sizes all appear on the same row, and every brand formats their chart slightly differently. This guide walks through exactly how to measure, how to interpret what you're reading, and where each major brand's sizing tends to run so you don't end up with a helmet that's too loose to protect you or too tight to wear for more than ten minutes.
How to Measure Your Head for a Motocross Helmet
You need a soft fabric tape measure, the kind used for sewing. A metal tape measure or a piece of string held up to a ruler both introduce error. Start by placing the tape one inch above your eyebrows. This is the reference point every helmet brand uses, and it corresponds to where the helmet's forehead pad will make contact. Pull the tape around the widest point of your skull, which is typically found just above the ears and across the back of the head at the occipital bone.
Take the measurement three times without moving the tape between reads. Your muscles will subtly shift the tape on the first attempt, and a second or third pass gives you a more stable number. Use the largest of the three readings. A helmet that is slightly snug at first will break in as the comfort liner compresses; a helmet that is slightly loose will only get looser. When you have your number, bring it to the BTO sizing page and match it to the brand-specific chart for whatever helmet you're considering.
Measure on bare skin or with your hair pulled flat. Thick or long hair can add one to two centimeters to the reading, which is enough to push you into the wrong size bracket. If you typically wear your hair up while riding, measure with your hair in that position instead.
How to Read the Size Chart
Most motocross helmet size charts list four columns: centimeters, inches, hat size, and S/M/L/XL label. Centimeters is the most precise column. Inches rounds to the nearest quarter inch and introduces more room for error. Hat size is an older US system that many riders no longer know offhand. The S/M/L label covers a range of centimeter measurements, so two riders who both measure 58 cm might wear a medium in one brand and a large in another depending on how that brand defines its medium range.
Always use the centimeter column. If a brand lists only inches, convert your measurement directly (1 inch equals 2.54 cm) rather than using the letter size as an intermediary.
Head shape is the other variable the chart doesn't show you. The two most common shapes in helmet design are intermediate oval and round oval. Intermediate oval is the most common head shape in North America: the skull is slightly longer front-to-back than it is wide. Round oval heads are nearly equal in both dimensions. A helmet designed for intermediate oval will feel tight at the temples on a round oval head, even if the circumference measurement is identical. If a helmet creates sharp pressure at the sides of your head but not across the forehead, head shape is the likely cause, and no size change within that brand will fix it. Trying a brand that builds to a round oval last, or one with a more neutral shell geometry, is the right move.
When you're ready to compare options across brands, start at the full motocross helmet collection and filter by brand to pull up brand-specific fit notes.
Brand-Specific Sizing Notes
Bell helmets tend to run slightly large relative to other brands at the same centimeter size. Bell builds to a round oval shape, which makes them a natural choice for riders with rounder heads but can feel wide for narrower intermediate oval shapes. If your measurement falls on the upper end of a Bell size range, try the size down first. Bell's size overlap is generous, and the smaller shell will often fit better once the liner breaks in.
Fox helmets are consistent and sit in the intermediate oval category. Their size charts are reliable and translate closely to your raw centimeter measurement. Use their chart exactly as published and don't adjust up or down unless a physical try-on tells you otherwise. The Fox V3 RS is a good reference point: it fits true to Fox's chart and is widely available, making it easy to establish your Fox size before ordering other models in the lineup.
Alpinestars takes a different approach with the Supertech M10. The M10 uses Alpinestars' A-Head Fitment System, which adjusts both the angle and height of the helmet's internal fit frame independently of shell size. Four shell sizes cover the full XS through XXL range, and the internal adjustment system handles fine-tuning within each shell. This means two riders with the same head circumference but different head proportions can both get a precise fit in the same shell size. When sizing the M10, use the Alpinestars chart to find your shell size, then use the A-Head dial to dial in the fit once the helmet is on your head. The M10 Aeon uses the same system in a different colorway and graphic package, so sizing is identical across M10 variants.
Troy Lee Designs helmets run true to size across their lineup and are built for intermediate oval heads. Their size chart translates cleanly from centimeter measurements, and there is no consistent need to adjust up or down. If you're between sizes in TLD, the standard rule applies: size down.
Leatt helmets use a numeric size system that does not translate directly to other brands' letter or numeric sizing. A size 60 in Leatt means a 60 cm head circumference, which is precise. But don't assume a "Large" in another brand equals the same shell as a Leatt 59-60. Always use the Leatt-specific chart on the BTO sizing page rather than converting from a size you know in another brand.
Common Sizing Mistakes to Avoid
The most common mistake is measuring over hair. Even a centimeter of error from hair volume can push a measurement across a size boundary. Measure on bare skin at the correct reference point, one inch above the eyebrows, and you eliminate the most frequent source of wrong-size orders.
Choosing by feel in a store without riding is the second most common problem. A helmet that feels comfortable the moment you pull it on is often too large. Helmet liners are designed to break in, which means a correctly sized helmet will feel slightly firm at first. Comfortable from the first second usually means there's already too much space, and that space will increase as the liner compresses over the first few rides.
Sizing up "for comfort" extends from the same instinct and leads to the same result: a helmet that is loose, moves on impact, and does not protect the way it was designed to. The size chart exists precisely so you don't have to guess. Use your centimeter measurement and trust the chart.
Ignoring head shape is the fourth mistake, and it's the one that confuses riders who have measured correctly but still feel like the size is wrong. If pressure is concentrated at the temples rather than distributed evenly across the forehead and sides, the shell shape doesn't match your head shape. The fix is a different brand, not a different size. The Fit + Function FAQs section has guides on head shape and brand fit profiles if you need help narrowing down the right shell geometry for your skull shape.
The Cheek Pad Test
Once a helmet is on your head, the cheek pad test is the fastest way to confirm fit before you commit to a purchase. Press your fingers against your cheeks through the helmet's face opening. You should feel firm resistance from the cheek pads immediately. If you can easily slide your fingers behind the pads, the helmet is too large regardless of what the size chart says.
With the helmet buckled, check for forehead gaps by looking in a mirror. The front rim should sit one to two finger-widths above your eyebrows with no visible space between the rim and your forehead. Any gap means the helmet is riding high, which reduces the coverage zone in a crash.
Finally, grip the back of the helmet with both hands and try to roll it forward off your head. It should resist firmly, pulling your skin with it rather than sliding. If it rolls forward without significant resistance, the fit is loose. A helmet that passes the cheek pad check, has no forehead gap, and fails the roll test is typically the wrong head shape for your skull, not simply the wrong size.
Browse the full range of dirt bike helmets at BTO Sports, and use the BTO sizing guide to pull up the chart for any brand before you add to cart.
How do I measure my head for a motocross helmet?
Use a soft fabric tape measure placed one inch above your eyebrows, running around the widest point of your skull just above the ears. Take three measurements and use the largest reading in centimeters. Measure on bare skin or with your hair laid flat to avoid adding artificial volume to the number.
What head shape am I? Round oval or intermediate oval?
Stand in front of a mirror and look down at the top of your head, or have someone take a photo from directly above. If your skull is noticeably longer front-to-back than it is wide side-to-side, you have an intermediate oval shape, which is the most common profile in North America. If the front-to-back and side-to-side dimensions are roughly equal, you have a round oval shape. Pressure concentrated at the temples in a new helmet is a reliable indicator that the helmet's shell geometry does not match your head shape.
Should I size up or down if I'm between helmet sizes?
Size down. Helmet liners are designed to compress and conform to your head shape over the first several rides. A helmet that starts slightly snug will break in to a precise fit. A helmet that starts slightly loose will only get looser, which reduces its ability to manage impact forces correctly in a crash. Confirm by running the cheek pad check and the roll test after pulling the smaller size on.
Why does my helmet feel tight when I first put it on?
A tight feeling when the helmet is new is normal and expected in a correctly sized helmet. The EPS liner and comfort foam are at their maximum thickness before use. Even pressure across the forehead and sides of your head, without sharp localized hot spots, means the fit is right. If the pressure is concentrated at a single point, that indicates a shape mismatch rather than a size issue. The tightness from correct sizing will ease within five to ten rides as the liner breaks in.
