How Should A Bike Helmet Fit? Helmet Sizes Explained [With Bike Helmet Size Chart]

Professional bike fitter Robbie Ferri walks you through how to find the perfect bike helmet size and fit

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reviewed by Rory McAllister

No piece of cycling protective equipment is more important than the humble bike helmet.

Most cyclists know very well the importance of quality when it comes to buying a bike helmet – but fewer seem to understand how vital it is to get the right helmet size and fit too.

In my experience as a professional bike fitter, you’d be amazed at how often I have clients (who are often very experienced cyclists themselves) come in with poorly sized or adjusted helmets, significantly reducing the protection they’ll provide in a crash.

Your bike helmet should sit comfortably and level on top of your head, with the front around one to two finger-widths above your eyebrows. There should be no gaps between your head and the helmet interior, and it should stay in place when you tilt or shake your head.

However, there’s a little more to it than that – and I’ll be diving into the details and providing you with a rough bike helmet size chart to get you up and running.

How should a bike helmet fit? This cyclist in a yellow jersey demonstrates it on a road bike.

Why Is It Crucial To Have A Helmet That Fits Properly?

When I first started cycling, I was given a helmet from a friend who had just upgraded his. It wasn’t the right size for me, and I didn’t enjoy wearing it, which meant I was reluctant to wear it – and if I’d had a crash, it might not have protected my head properly either.


A well-fitting helmet is going to be much more comfortable.

When wearing my current (properly sized) helmet, I often get to a cafe, forget it’s on, and wear it while eating and drinking. A comfort helmet feels like a part of you, and that’s the way it should feel!

Better Protection

If a helmet doesn’t fit you properly, it can’t protect you properly.

When a helmet is too loose or tight, it doesn’t cover your head as it should. This means if the helmet takes an impact, it can’t properly distribute the force, or it could leave areas completely uncovered.


If you don’t have a helmet that fits, it doesn’t rest very well on your head.

If it’s too small, it can’t grip properly and sits a little too proud. If it’s too large and loose, it moves around while you’re riding.

I demonstrate how a bike helmet should fit on my mountain bike.
Your helmet should be level on your head. © Robbie Ferri/BikeTips

How Should A Bike Helmet Fit?

How should a helmet fit? Well, here’s what you can expect from a properly fitted helmet.

Snug And Secure

A helmet should fit snugly and securely on your head. It should feel like a part of you.

It should also be tight enough not to move when you tilt or shake your head, but not so tight that it’s uncomfortable.

I tilt my head forwards to show how a bike helmet shouldn't move.
Your helmet shouldn’t slip forward or back when you tilt your head. © Robbie Ferri/BikeTips

On The Level

A helmet should feel completely level.

It should not obstruct your view at the front by slipping forward, and shouldn’t be rolling off the back of your head. There should be a gap of one or two finger widths between your eyebrows and the front edge of the helmet.

If you wear glasses, you should also make sure you can fit them without the helmet interfering.

Strapped Without Constricting

A helmet, when being worn, should be securely strapped on. When clipped in, you should be able to slide a finger between your chin and the chin strap.

Any less and the helmet will become uncomfortable over time as the strap begins to choke you, but any more and the helmet won’t be attached securely enough and could slip around on your head.

How To Measure Your Head for a Bike Helmet

The first step to getting the correct-sized bike helmet is to measure your own head.

This is a fairly simple process, and all you need is a tape measure. It helps to either be in front of a mirror or have someone help you.

Take the tape measure and start by putting it 1 cm above your eyebrows. Wrap it around your head as level as you can. Where it completes a full loop around your head back to where it started, make a note of the length.

This number is the diameter of your head, and the measurement we need to use when it comes to finding the perfect size helmet for you. 

Demonstrating the right bike helmet fit on my road bike.
© Robbie Ferri/BikeTips

What Helmet Size Do You Need? Our Bike Helmet Size Chart

When it comes to picking the correct bike helmet sizes, it can be very challenging.

Firstly, bike helmets are not sized universally. A medium for some companies could be a large for others. It can be tough to know what you need without actually trying a helmet on. 

Not many people know what size head they have. It doesn’t always work in a similar proportion to your body. You could be 6 feet tall, but that doesn’t necessarily mean you need a large helmet.

To make it easy for you, here’s our step-by-step guide to finding the right bike helmet size.

Step #1. Measure Your Head

This bit’s simple – just follow the process described above!

Step #2. Use The BikeTips Sizing Chart

This bike helmet size chart was compiled with data from many different helmet manufacturers’ sizing guides to provide a rough average, but bear in mind that sizes will still vary between different brands.

Adult Bike Helmet Size Chart

Adult Bike Helmet Size Chart
Adult Bike Helmet Size Chart

Children’s Bike Helmet Size Chart

Children's Bike Helmet Size Chart
Children’s Bike Helmet Size Chart

Using these charts, you can roughly gauge what size helmet you are looking for. Then, you can start looking, knowing you are heading in the right direction regarding sizing. 

Step #3. Check the Manufacturer’s Guidelines

Once you have found the helmet you like the look of and it fits in with our general sizing chart, it’s time to check the helmet manufacturer’s guidelines.

The reason we do this is to ensure their sizing is going to work for you. As we mentioned before, sometimes a manufacturer’s sizing will slightly differ, and it’s good to double-check that it can work for your head size. 

Nearly all helmets come with a certain amount of adjustability to fine-tune the fit. This means you don’t have to allow extra size for caps worn under your helmet, or if you have very long hair, which can expand your head size. 

Some cyclists use sizes fairly close to if they can’t get the perfect size. We don’t recommend this.

If a particular helmet model isn’t made in the perfect size, choose a different model that is.

And with that, you should be ready to find the perfectly sized helmet for you!

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Robbie has traveled the globe as an endurance athlete and bikepacker, breaking world records and competing in international ultra-cycling events such as the BikingMan series and the Transcontinental Race. He's also worked as an ambassador for some of the industry's leading names, including Shimano and Ritchey. If Robbie's not on a bike, he's either fixing them or out walking with his dog!

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