How to Measure Bike Helmet Size

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A helmet is one of the most critical pieces of biking equipment. It helps protect your head from injuries, but only if you wear an appropriate model. Besides shopping for high-quality materials, you must pick the right helmet size to ensure a proper fit and the best protection. Read on to find out how to measure bike helmet size.

How to Measure Bike Helmet Size: In-Depth Guide

Bike helmets that are too tight or small are uncomfortable and unsafe. Likewise, loosely-fitting products may not stay on your head during accidents, increasing the risk of severe injury.

Here’s what you should do to select the correct size:

Measure Your Head

Wrap a measuring tape around your head an inch above your ears and eyebrows, ensuring the tape is level. Most brands express the size of their helmets in centimeters. If possible, take your measurement in centimeters to ensure accuracy; one inch equals about 2.54 centimeters if your tape is in inches.

A tape isn’t the only tool you can measure your head with. You can also wrap some string around the head and measure the string.

Find the Adequate Helmet

The next step is to find the appropriate helmet based on your head measurement. Most brands use the following designations:

  • Extra small – under 51 centimeters (20 inches)
  • Small – between 51 and 55 centimeters (20-21.75 inches)
  • Medium – between 55 and 59 centimeters (21.75-23.25 inches)
  • Large – between 59 and 63 centimeters (23.25-24.75 inches)
  • Extra large – over 63 centimeters (24.75 inches)
  • One size fits all – adjustable fit systems

Try the Helmet

After receiving the helmet, try it on. It should sit perfectly flat on your head (about one inch above the eyebrows) to protect your forehead. You may be able to adjust your fit with a retention system. The feature usually sits at the back and comes in various forms, from ratchets to click wheels.

Make sure your helmet feels snug. There shouldn’t be any space between the head and the product. Also, adjust the product if there’s any pressure, which indicates it’s too small. If you can’t modify it any further, switch to a larger one.

Strap Up

Connect your chin strap until it’s snug but still comfortable beneath the chin. You shouldn’t be able to squeeze more than a finger between the chin and strap.

Another thing to look for is a V shape on the straps after buckling up your helmet. Most straps form this shape around the ears while viewed from the side.

Once you’ve fastened your chinstrap, open your mouth. The model should press against your head. The product might be too large if you can’t feel any pressure.

Perform a Shake Test

Push your helmet from front to back and side to side. If it moves noticeably, adjust it with your retention system. It shouldn’t shift more than an inch in all directions during the test after dialing in your perfect fit.

Additional Tips

In addition to measuring your head correctly and adjusting the helmet, you can do many other things to ensure you get the right-sized model:

Go to the Brick-and-Mortar Store

The sample measurements provided above can differ from manufacturer to manufacturer. Therefore, always double-check the circumference of the product you’re considering.

More importantly, remember that brands have proprietary shapes that result in different molds. Therefore, you should try your model in-store to determine if it fits your head perfectly.

Go Smaller if You’re In-Between

Many people are torn between slightly larger helmets and others that are somewhat smaller than their heads. In this case, you should opt for the smaller model. This is because it provides better protection than loosely-fitting products.

Alternatively, look for a different helmet. If the smallest model is a bit big, you might be able to find a suitable one in the kids’ department. Also, consider wearing a cycling cap. This applies only if the helmet is slightly larger since the cap adds only a little to your head’s circumference.

Consider the Ventilation

Staying cool is essential for comfortable riding. Most modern helmets have a ventilation system that draws cool air through front ports and removes warm air.

A suitable helmet should wick sweat away naturally. Most ventilation problems arise from a tighter helmet. It may prevent air from circulating by closing off air exits, which decreases comfort.

Two men riding a bike near the sea with helmets

Does Multi-Directional Impact Protection Affect Sizing?

Multi-directional Impact Protection (MIPS) is one of the most impressive advancements in helmet technology. It features an outer shell that rotates independently of your cradle. This way, it enhances the safety of your model. It typically comes in the form of a liner.

When this technology was introduced, it was retrofitted into models that had been available for years. Unfortunately, this affected the sizing since it added a 0.5-0.8mm layer and reduced the headspace available.

Fortunately, this is no longer a problem. The developers of MIPS work with helmet brands throughout the manufacturing process to integrate it into the design more seamlessly.

MIPS can introduce some movement to the helmet (about 10-15mm), which might be disconcerting when adjusting your model. However, this shouldn’t worry you if the shifts are limited and only occur in the outer shell. But if the cradle moves, too, consider buying another product.

Can You Wear Your Helmet with a Hat or Ponytail?

Many things can impair your helmet fit. A ponytail is one of the most common obstacles.

Although some models can accommodate your hairstyle with large exits at the beck, some aren’t. For this reason, you may need to switch to a different helmet or wear your hair accordingly to ensure safety.

As previously discussed, thin cycling caps and hats are fine, provided they don’t interfere with the fit. However, if they make the helmet too tight, avoid wearing them.

Regarding hoods or beanies, few garments can fit under your helmet without compromising the fit. In addition, they add unnecessary thickness, making the product uncomfortable.

What Are Helmet Adjustment Systems?

Fit adjustment equipment varies, but most manufacturers opt for one of two solutions.

Removable Pads

Models that feature removable pads typically have one or two sets that vary in thickness. This enables you to customize the fit.

If you prefer this adjustment, look for pads with appropriate thickness. The units should touch the head evenly without making your helmet too tight.


Helmets with ratchets usually feature a ring, crank, or dial in the rear section that lets you customize your fit. The most significant advantage of this system is its high resistance to sweating. As a result, the materials usually don’t deteriorate due to exposure to moisture.

Strive for Perfection

You shouldn’t gamble with ill-fitting helmets, especially when you know how to measure bike helmet size. They can dramatically reduce safety and comfort, preventing you from enjoying your experience.

Expand your search with web browsing if you can’t find the right model in-store. Be sure the product matches your head, and you should be ready to ride.