How To Determine Bicycle Tire Size? (Road and Mountain Bike)

If you are new to cycling and shopping around for new tires for your bicycle, then you have been faced with the shop assistant politely saying, “sure, I can help you.  What tire size are you looking for?” “Wait? There are different sizes?” you think to yourself. However, determining what tire you need is easy when you understand it.  

The size of your tire is normally stamped on the sidewall.  Typically tire sizes are denoted with two numbers, such as “27.5×2.5” or “700x25c.”  The first number indicates the diameter of the wheel rim, whereas the second number refers to the width of the tire.  

It can be intimidating to understand tire sizes and pick the correct tire size for you.  Yes, you always have the option to replace the tire with the same type your bike came with, but if you want to open new avenues of customization and riding, you need to learn to understand tire sizes.  

How To Determine Bicycle Tire Size

Road Bike Tire Sizes

As a mountain biker myself, I find road bike tire sizes a little strange in the way they are measured. But, at the same time, unlike the mountain bike industry, it’s refreshing that they don’t have the same ridiculous number of options.  

Apart from the rabbit hole of alternative options used by a handful of pro-riders, road bike wheel sizes have become pretty standardized.  For example, most riders use an outer wheel diameter of around 700 millimeters.  In addition, most road bike tires have a “C” letter in the tire sizing, such as 700x 25c.  

The “C’s” history is a little complex, but to keep it simple, it is an outdated French system assigning letters “A” through to “D” to different tire width and wheel size combinations. So “A” would be a bigger rim coupled with a thinner tire, and “D” would be the exact opposite.  

Since then, the industry has standardized at the “C” level, a slightly thicker tire and smaller rim, but not the same extreme as “D.”   

As I mentioned earlier, the first number in the size stamp is the diameter of the wheel.  Road bike wheels are normally 700mm, which is the rough outer diameter of the tire on the rim.  

The second number (25, in this case) is the outer width of the tire measured in millimeters at its widest point.  There are three primary options in the racing road bike industry, namely 23, 25, and 28mm.  

The industry has also reached a preferred width, with the 25mm tires being the most commonly suitable race tire as a balance of rolling resistance and comfort. However, if you are riding on a track or are a light rider, you can switch to 23mm tires.  

On the other hand, if you ride rougher roads or are a heavier build, you can use the 28mm tires.  If you are not a racer at all and just want to ride, there are tire width options that go up 50mm in the case of commuters, tourers, and gravel bikes.  

Mountain Bike Tire Sizes

Before I get into this section, I apologize on behalf of the entire mountain community and industry. Unfortunately, we got carried away somewhat regarding tire size options. 

As with a road tire, the sizing stamp has two numbers, of which the first is the outer diameter of the tire on the rim, and the second number is the width of the tire at is the widest inflated point.  

The difference with mountain bike tires is that the measurements are in inches.  

And then there are the variations and options.  There are so many options. 

1. Mountain Bike Wheel Size / Diameter

Currently, you can find three primary tire diameters at your local bike shop, namely, 26, 27.5, and 29 inches.  26er wheels are the smallest and possibly the most agile.  However, they don’t roll over obstacles well and are harder to peddle.  

The 26-wheel size had its last solid fan base in the extreme downhill market, which has moved to 27.5-inch wheels a few years ago, securing the slow death of the 26er.  If you’re buying a bike on the used market, do not buy a 26-inch bike because tire stock is gradually becoming quite rare. 

The 27.5 wheel (or 650b) is the balance between the agility of the 26er and the rolling capability of the 29er.  Because of this balance, it has been dominating the downhill sector for a few years now.  It is a popular size and for a good reason.  

Lately, however, 29er wheels, previously reserved for the cross-country scene, have been spotted on bikes competing in the world cup downhill series.  With the likes of Greg Minnaar, the greatest of all time, preferring the bigger wheel.  

The 29er roles brilliantly over just about anything.  They say it is harder to corner, but Minnaar’s gold medals aren’t evidence of that.

2. Mountain Bike Tire Width

Once you know your wheel diameter size, you are locked into buying tires for those rims, obviously, but you still have a whole world of possibilities when it comes to tire widths.  

Modern mountain bike tires range from a now less-common width of 2 inches up to 2.7 or even 2.8 inches.  Narrower tires have become less common over the last few years 

as the industry has seen a gradual shift toward thicker tires.  When I got my new tires, I upscaled the width from 2.2 to 2.4 inches and was quite shocked at the difference.

One thing to keep in mind is that your rim’s inner width needs to be wide enough to accommodate wider tires, or you be left with the tire climbing off the rim by itself. 

With tubeless setups, wider tires can be run at low pressures, providing ridiculous amounts of grip.  The downside is that bigger tires offer more peddling resistance, but you can overcome this with smoother tread patterns.

Here is a video explaining tire width and rim width 

3. Plus Sized Tires

Road bikes have recently introduced the gravel bike category that uses a broader tire, sometimes even a different rim size of 650b.  Likewise, the mountain bike industry has been pushing the wider-tire market.  

Enter plus-sized tires.  No, not fat bikes; those are something else entirely.  Plus-sized bikes are commonly 27.5 plus wheel size and come with rims wide enough to accommodate beefy 3-inch-wide tires.  

The marketed benefit is that the wider tire offers superior grip because of the increased contact surface and the tires ‘suspension’.  Both may be true, but what is fantastic about these bikes is that a 27.5 plus bike can also accommodate standard 29er wheels, giving you two bikes in one.  

How To Chose Your Tire Size

The tire size that you settle on will depend on what type of riding you primarily do. For example, if you are a road racer aiming for top speeds and built like Chris Froome, you should get 23mm or thinner.  

However, if you ride road bikes on rough and unkept roads, you will need to look at changing to thicker tires and possibly changing your vote for repaired roads.  

If you’re a mountain biker firmly into the cross country or downhill scene, you will need to choose tires made for those jobs, respectively.  

But if you are a weekend-warrior rider who likes to race, shred, and drop in, then my advice is to choose the widest tire you can fit on your rim can fit, up to about 2.7 inch

es.  You will quickly adapt to the added rolling resistance and will very quickly be thankful for the added grip. 

How To Determine Bicycle Tire Size Conclusion

Although bike tire sizes can be confusing, it is easy to establish the tire’s size by looking at the stamped sizing on the sidewall.  Remember that, barring a few odd cases, the first number is the wheel diameter, and the second is the tire’s width. 

If you need to replace your tires, look at some potential options in tire widths that may best suit your needs, weight, and riding style to give you the best possible experience.  Wider tires offer more grip, while narrower tires are preferred for sheer racing speed. 


About Martin

I am Martin, I am the author of this blog. My main interests include cycling. That's why this blog was created. I have always been interested in technology and how bicycles are made, so I built one myself.

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