Measure the widest circumference of your head about two finger widths above the eyebrows, over the ears, and around the fullest part of the back of the skull, then match that number to the helmet maker’s own size chart.
An extra-large label means nothing until you know what it covers. One brand’s XL starts at 61 cm; another’s ends there. The tape measure is the only number that travels between them, and getting it wrong by a centimeter lands you in a return queue with a helmet that pinches at the temples.
The process takes about two minutes and one flexible tape. Here’s how to run it so the result actually matches a size chart, plus what a circumference number can’t tell you about fit.
What You Need Before You Start
A flexible tailor’s tape is the right tool, and nonstretch string works as a backup if you lay it against a ruler afterward. Stretchy string, a phone cable, or a fabric tape that has gone limp will all read long and push you into the wrong size.
Have a pen and paper ready, or your phone’s notes app open. You’ll record two measurements, and the second one is the check on the first.
How Do You Measure Head Circumference Step by Step?
Wrap the tape level around your head roughly two finger widths above your eyebrows, pass it over both ears, and bring it around the fullest point at the back of the skull. Keep the tape flat against the head without compressing hair or skin, then read the number where the tape meets itself.
The federal Consumer Product Safety Commission requires bicycle helmets to ship with fitting instructions and a label naming the model, and its guidance describes the same landmarks for wearing one: level on the head, low on the forehead, about two finger widths above the eyebrows, evenly spaced between the ears. Measuring at those same points keeps your number consistent with how the helmet is designed to sit.
- Wrap the tape around the head at the forehead and occipital landmarks above.
- Level it — front and back at the same height, no tilt.
- Snug it until it touches the head everywhere but doesn’t dent the skin.
- Read and record the circumference in both centimeters and inches, since size charts use one or the other.
- Repeat the whole wrap a second time, then use the larger of the two consistent results.
When the tape meets itself and stays put while you exhale normally, you’ve got a clean read. If it slipped or rode up over your hair, start over rather than splitting the difference.
Why “Extra-Large” Isn’t a Standard Size
No universal body sets the centimeter range for an XL helmet, so the label is a product category each manufacturer defines for itself. A helmet one brand calls XL may span a range that another brand splits between L and XL.
That makes the model’s current size chart the only authority. Find the chart for the exact helmet you’re considering, locate the row your measurement falls into, and follow that maker’s instruction when you land between two sizes — some say size up, some say size down, and neither rule is universal.
| Measurement Result | What To Do Next | What To Watch For |
|---|---|---|
| Falls inside one chart row | Order that size | Confirm the chart is for that exact model |
| Falls between two sizes | Follow that maker’s tie-break instruction | Don’t assume a universal up-or-down rule |
| Sits at the top of XL | Check whether the maker offers a larger shell | A dial retention system can only add so much |
| Two readings differ by a lot | Re-measure with nonstretch tape or string | Stretchy tape reads long |
| Reads far outside every row | Contact the manufacturer before buying | Shell shape may matter more than circumference |
| Number matches but temples ache | Try a different model in the same size | Head shape vs. shell shape |
| Number matches but helmet shifts | Adjust pads and chin strap per instructions | Never add aftermarket padding |
What a Tape Measure Can’t Tell You
Circumference alone doesn’t establish fit, because head shape, shell shape, the retention system, hair volume, eyewear, and the helmet’s adjustment range all change how a given size feels. A round head and an oval head can share a measurement and need different shells.
The CPSC’s fit standard is the test that matters: the helmet sits level and low on the forehead, feels snug and comfortable, and stays put front-to-back and side-to-side once the chin strap is tightened and the interior pads are set. If the shell presses painfully, leaves big gaps, or won’t sit level while covering your forehead, that model is wrong for you regardless of the number.
Two more rules from the same guidance. Don’t add extra padding or parts, and don’t remove protective components — the helmet was certified as shipped. And stop using a helmet after a crash or impact whenever the manufacturer says to, even if you can’t see damage.
Before you buy, check three things on the helmet: the certification label, the model designation, and the maker’s fitting instructions. Bicycle helmets made after 1999 must meet the federal bicycle-helmet standard, and a helmet built for biking is not automatically right for motorized riding — the CPSC lists separate categories and standards for motorcycles, mopeds, and powered scooters. BMX can call for its own standard as well.
Once your number is settled, comparing shells side by side is the next step — our roundup of tested x-large bike helmets lays out the size ranges each one covers.
The CPSC’s guidance on bicycle helmet fit and use covers the wearing standard in full.
Getting the Number Right the First Time
Two finger widths above the eyebrows, over the ears, around the back of the skull, level and snug, measured twice and matched to one specific model’s chart. That sequence is the whole method.
The measurement narrows the field; the fit check decides. Order the size your chart points to, then test it against the CPSC standard the moment it arrives — level, snug, stable, with no added padding and no removed parts. If it fails any of those, the size is wrong even if the centimeter count matched.
Common Questions
Should I measure in centimeters or inches?
Record both. Most helmet size charts list centimeters, but some manufacturers publish inches, and having both on hand means you can check any chart without re-measuring. If your tape only shows inches, multiply by 2.54 for the centimeter figure, or read the conversion off a ruler.
Can I measure with a piece of string?
Yes, as long as the string doesn’t stretch. Wrap it around your head at the same landmarks, pinch the point where the ends meet, then lay the string flat against a ruler to read the length. Elastic, yarn, and phone cables all stretch under tension and will read longer than your actual circumference.
My measurement is right at the top of the XL range — what then?
Check whether the manufacturer offers a larger shell size above XL before you order. Dial retention systems adjust the fit band, not the shell, so they can only compensate so far. If no larger option exists, contact the maker directly and describe your measurement and head shape.
Sources
- U.S. Consumer Product Safety Commission. “Bicycle Helmets: Use Your Head” Supports the measuring landmarks, fit standard, and the warning against added padding.
- U.S. Consumer Product Safety Commission. “Which Helmet for Which Activity?” Supports the separation of bicycle, BMX, and motorized helmet standards.
- U.S. Consumer Product Safety Commission. “Bicycles – Helmets” Supports the federal bicycle-helmet standard for helmets made after 1999.