A helmet that's the wrong size doesn't protect you the way it's supposed to, no matter how good the helmet is. Here's how to measure your head, read a size chart correctly, and know when a helmet has actually reached the end of its life.
How to Measure Your Head for a Helmet
Wrap a soft measuring tape around your head about one inch above your eyebrows, at the widest point around the back of your skull. Keep the tape snug but not tight. Note the measurement in centimetres; nearly every helmet brand sizes by head circumference in cm, not hat size or age.
If you don't have a soft tape measure, a piece of string works too, just measure the string against a ruler afterward.
General Size Ranges
Use this as a starting point, then check the specific brand's chart before buying, since ranges shift slightly between manufacturers.
| Size | Head Circumference |
|---|---|
| XS | 51–54cm |
| S | 54–57cm |
| M | 57–59cm |
| L | 59–62cm |
| XL | 62–65cm |
Why Brand Charts Differ
Bell, Giro, Smith, and Troy Lee Designs all build around slightly different head shapes, some rounder, some more oval, so identical circumference measurements can land in different size categories or fit differently even within the same labeled size. This is the biggest reason online helmet purchases go wrong: two "medium" helmets from different brands can fit noticeably differently on the same head.
If you're between sizes on a chart, or buying a brand you haven't worn before, try it on in person before committing. We size every helmet in-store at both Dunbar Cycles and Corsa Cycles.
Getting the Fit Right Once You Have the Size
Getting the right size on paper is step one. A properly adjusted helmet should pass this quick check, sometimes called the 2-2-2 rule:
- Two fingers above the eyebrows: the front edge of the helmet should sit roughly two finger-widths above your eyebrows, not tilted back on your head.
- Two fingers to form a V below the ear: the side straps should meet just below your earlobe.
- Two fingers between your chin and the strap: the chin strap should be snug enough to fit two fingers but not gape open.
If you can't get all three right even at the correct size, try a different shape from another brand rather than forcing a fit with padding alone.
When a Helmet No Longer Fits
Growing kids obviously need resizing, but adult fit changes too. Most helmets include swappable pad kits (thinner and thicker foam pads) specifically to fine-tune fit within a size, so if a helmet feels slightly loose after a season, check whether thicker pads are available before assuming you need a different size entirely.
When to Replace Your Helmet: Beyond the Basic Rule
Most advice stops at "replace every 3 to 5 years." That's a reasonable default, but it's not the full picture. Here's what to actually check:
After any crash involving head impact. Replace it immediately, even if you can't see damage. The EPS foam liner is designed to crush on impact to absorb force, and once it's crushed, it doesn't spring back, it just looks intact while offering less protection on the next hit.
Visible cracks in the foam or shell. Not always visible from outside. If you've had a hard hit, check the foam liner where you can see it around vents, not just the outer shell.
MIPS or rotational liner integrity. If your helmet has a MIPS liner or similar system, it should move slightly and independently from the outer shell when you push on it gently. If it feels seized or doesn't move at all, the system may be compromised.
Strap and buckle wear. Frayed webbing or a buckle that doesn't click securely is a straightforward replacement trigger, no ambiguity needed.
Age, even without a crash. UV exposure and sweat degrade EPS foam over time, even if the helmet looks fine. Three to five years is the standard window; riders who ride year-round in BC's wetter climate should lean toward the shorter end of that range.
Check for a crash replacement program. Several brands we carry, including Bell, Giro, and Troy Lee Designs, offer discounted crash replacement programs if you register your helmet or bring in proof of purchase after a crash. It's worth asking before paying full price for a replacement.
FAQ
How do I know my bike helmet size?
Measure your head circumference about an inch above your eyebrows with a soft tape measure, then match that number in centimetres to the specific brand's size chart, since ranges vary slightly by brand.
What is the 2-2-2 rule for bike helmets?
A quick fit check: two finger-widths between your eyebrows and the helmet's front edge, a V-shape of two fingers where the side straps meet below your ear, and two fingers of space under the chin strap.
Does a helmet loosen over time?
Straps and pads can compress slightly with use. Most helmets include replaceable pad kits to restore a snug fit without needing a new helmet, so check that option before assuming you've outgrown the size.
Can I use a helmet that doesn't fit perfectly if it's the safety features I want?
No. Fit is the single biggest factor in whether a helmet protects you in a crash. A perfectly fitted mid-tier helmet outperforms a premium helmet that's the wrong size or improperly adjusted.
Get Sized Properly at Dunbar Cycles
Charts get you close, but nothing replaces trying a helmet on. Come into Dunbar Cycles in Vancouver or Corsa Cycles in Squamish and we'll size you properly across brands, check your current helmet if you're not sure it still fits or needs replacing, and point you toward the right shape for your head. If you're deciding between a full-face and convertible helmet at the same time, our full-face vs convertible helmet guide covers that decision, and our best full-face helmets for 2026 roundup has our current picks once you know your size.
Browse our full-face helmet collection or our full helmet lineup for every riding style.
