How to Measure Bike Helmet Size | One Tape, Three Steps

Getting the right bike helmet size starts with one number: your head circumference in centimeters. Almost every helmet brand publishes its sizes as ranges—like 55–59 cm for a medium—so once you have that measurement, the right size is simply the range your number falls inside. The entire process takes under a minute and works for both adult and kids’ helmets.

How Do I Measure My Head For A Helmet?

Wrap a flexible tape measure around the largest part of your head, keeping it level across your brow and over your ears. If you don’t have a tape, wrap a piece of string around your head the same way, mark where it overlaps, and lay it flat against a ruler.

Two details matter more than people expect:

• Keep the tape level—running it at an angle or too high on the forehead adds inches and points you at a size too big.
• Write the number down in centimeters; most size charts list centimeter ranges first, with inches in parentheses.

Once you have that number, pull up the size chart for the specific helmet you want. Find the range that contains your measurement and order that size.

Which Helmet Size Chart Should I Trust?

Use the chart published by the helmet’s own manufacturer, not a generic one. The label alone isn’t standardized.

Brand-specific quirks also exist. When in doubt, follow the chart on the product page you’re buying from.

How Should A Bike Helmet Fit Once It’s On?

Put the helmet on and adjust the fit ring or sizing pads until it sits level on your head—roughly two finger-widths above your eyebrows, with the front edge low enough to protect your forehead. The helmet should feel snug all the way around, not tight at one point and loose at another. Shake your head gently; if the helmet rocks or shifts, tighten the fit system or move down a size.

The straps deserve equal attention: the front strap should meet just below your ear, and the chin strap should sit tight enough that you can fit only one finger between it and your chin. When you open your mouth wide, the helmet should pull down slightly against your head.

What Mistakes Ruin A Helmet Fit?

Choosing by hat size is the most common error. A hat size tells you roughly how big your head is, but helmet sizing runs in tighter bands and varies by brand, so you need the exact centimeter measurement. Buying a helmet a size up because “it feels roomier” is just as bad—a loose helmet can shift in a crash and leave your head unprotected.

The other frequent mistake happens before the helmet is even on: measuring wrong. A tape angled upward or pulled tight can add a full centimeter, pushing you into a size too large. If you’re shopping in person, try before you buy—fit varies by head shape and model, and the published chart is only a starting point.

When you’re ready to shop, our roundup of the best cycling helmets with visors covers models that suit different head shapes and budgets.

Finally, remember that size selection is only half the job. A correctly sized helmet that isn’t adjusted properly—loose straps, a tilted shell, an open chin buckle—offers far less protection. The NHTSA’s helmet fitting guide walks through the full process and is worth a read before your first ride.

FAQs

Do I measure my head with a tape in centimeters or inches?

Either works, but centimeters are the safer bet. Most helmet size charts, including Giro’s and Trek’s, list their ranges in centimeters first and inches second, so measuring in centimeters means less converting and fewer rounding errors. If you measure in inches, write down the decimal rather than rounding to the nearest half-inch.

What if my head measurement falls between two helmet sizes?

Choose the smaller size and use the fit ring or sizing pads to adjust. Going up a size instead often leaves the helmet too loose even at its tightest setting.

Can I use my hat size to pick a bike helmet?

Not reliably. Hat sizing and helmet sizing use different scales and tolerances; a hat size might point you at a large helmet that’s actually too big. Always measure your head circumference and check it against the helmet brand’s specific chart, since a “medium” in one brand can differ from a “medium” in another.