Finding the right size tyre for your bike can be more tricky than it at first appears. That’s a result of different ways to define a wheel and tyre’s size and the need for the tyre to fit in your bike’s frame without rubbing.
There are, however, universal standards to which wheels and tyres conform, so if you know how to read these numbers, it’s a lot more straightforward.
Here, we’ll go through how bike tyre sizes are defined and how to choose a tyre that will fit your bike and meet your riding needs.
What dimensions define a tyre’s size?
Two fundamental dimensions define a tyre’s size: its diameter (specifically, the diameter of the tyre's bead which connects it to the wheel rim) and its width.
The bead diameter needs to precisely match the size of your wheel rim, and there are a limited number of discrete values.
In contrast, there’s a lot more latitude in tyre width, meaning the actual width of a tyre might differ slightly from the number on the sidewall. The wheel rim’s internal width and how much pressure there is inside the tyre can both affect how wide a tyre is when inflated.
Ultimately, the clearance designed into the bike’s frame and fork will limit how wide a tyre you can fit, with the rear wheel clearance usually being the limiting factor. Rim brake calipers also limit the size of the tyre you can fit.
The maximum recommended tyre width for a given bike model will usually be stated by the bike manufacturer. The international standards described below specify a 6mm clearance around the tyre. This should prevent the tyre from rubbing on the frame and potentially dangerous wear to both. For tyres used off-road, it will also provide some space in case of mud build-up – a common occurrence.
How is bike tyre size defined?
Confusingly, there are multiple methods used to define tyre size. Road bike tyre sizes are usually measured using the older French system - where road cycling has its strongest roots - while mountain bike tyres are usually measured in inches, borne out of the Americas where mountain biking originated. Commuter bike tyres and gravel bike tyres may be measured either way, although they usually use the French measurements too.
The ultimate basis to define a tyre’s width is the ETRTO number. Regardless of which units are used to designate the tyre size, all tyres will have an ETRTO number embossed on their side wall.
Standing for European Tyre and Rim Technical Organisation, ETRTO is a Brussels-based body that specifies standards for tyres for all-wheeled vehicles, not just bikes. It also works with the International Organization for Standardization (ISO) to ensure that the two bodies’ standards are aligned. Though some ETRTO standards particularly for tubeless tyres were updated recently.
The ETRTO number consists of two numeric values separated by a dash. The first designates the nominal tyre width in millimetres, and the second the bead diameter. A 28mm wide 700c road bike tyre, for example, would have an ETRTO number of 28-622.
Why don’t my tyre and wheel use the ETRTO number?
For historical reasons, wheel and tyre diameters and widths, although defined by the ETRTO number, are usually expressed in one of two other ways. This is the pair of numbers such as 700x28c or 29x2.2 inches that you’ll usually see printed more prominently than the ETRTO number on the tyre’s sidewall.
For road wheels, gravel wheels and most commuter bike wheels, the French system is used, in which the principal wheel size found is 700c, but some bikes will have 650b (ETRTO diameter 584mm) wheels.
The French system is a carry over from the historical way of measuring tyres, and while the majority of road bikes use 700c wheels, the name doesn't actually describe anything with any real accuracy.
700 was originally used to describe the diameter across the tyre (outside to outside), but of course, nowadays with varying tyre sizes and rim widths available, the actual diameter of the tyre can vary greatly.
The 'C' is also pretty unhelpful. It previously denoted a choice of four tyre sizes: a, b, c and d, decreasing in width from a to d. Due to wheel size complications, sizes a, b and d were all thrown out as wheel and tyre brands settled on c as the best standard.
Naturally, a 23mm tyre and a 30mm tyre will have a different diameter when mounted onto the same rim. What's more, 700c wheels used to be made to similar standards with a narrow internal rim bed, but these days you can commonly find them anywhere from 17mm to 25mm wide, with outliers available too.
Mountain bike wheel sizes are usually expressed in inches. The main sizes used are 29 inches and 27.5 inches. For a long time, MTBs exclusively used 26-inch wheels and you may still find a 26-inch wheeled mountain bike out there. Children’s bike wheel diameters are usually measured in inches too.
Luckily - or perhaps adding to the complication - the two measuring systems are actually cross-compatible: 29-inch MTB wheels are the same diameter as 700c road bike wheels, while 27.5-inch MTB wheels are the same diameter as 650b wheels.
To make things even more complex, 29-inch tyres are sometimes labelled 28 inches, as this is a traditional European tyre size which also equates to a 622mm ETRTO tyre diameter.
Despite sharing diameters, it's worth knowing that mountain bike wheels have a much wider rim than road bike wheels - often between 25 and 35mm - so in practice, it’s unlikely that you will be able to fit an MTB wheel into a road bike or vice versa. Even if you could make it work, there are also differences in the axle width, which will cause complications, as well as freehub standards used which may further limit compatibility.
There is a grey area in the middle, which applies to some gravel bikes and commuter wheels and tyres. However, proceed with caution if you want to try mixing tyres, wheels and framesets for different bike types.
Stated tyre width versus actual tyre width
Typical tyre and rim widths have increased over the last several years, in the quest for greater comfort and grip and reduced rolling resistance.
For clincher tyres, the bead-to-bead internal rim width has a significant effect on a tyre’s actual width. This is why our reviews might include lines such as “on the wide rims the 28mm tyres measured 30mm”.
This is something of a win-win as you can run a lower tyre pressure for a more comfortable ride without any additional tyre weight. The tyre sidewalls are also likely to take on a less lightbulb-like profile, which improves the tyre support, particularly when cornering, and can provide aerodynamic gains. Do bear in mind how wide a tyre your frame can handle though.
Road bike tyre sizes
As mentioned above, the majority of road bikes will be fitted with 700c wheels and so need 700c tyres. Some manufacturers, such as Canyon, fit 650b wheels to smaller sizes of some of their road bikes though.
Tyre width can be as narrow as 18mm and a few years ago 23mm was a typical road bike tyre width. But most road bikes are now fitted with 25mm or 28mm nominal width tyres and 32mm and 35mm tyres are not uncommon on new comfort-focused road bikes.
Many road bikes are now fitted with tubeless-ready wheels. If you want to run them tubeless, you’ll need tubeless clincher tyres rather than standard clinchers. Make sure that you choose tubeless tyres, or you risk not being able to get the tyre to mount and stay inflated or possibly a blow-out when inflating or riding on the tyre.
Commuter bike tyre sizes
Tyre sizes for commuter bikes are more complex and tyres are typically wider than those fitted to road bikes. Since performance is a less significant consideration, tyres may be heavier and have a more pronounced tread pattern than found on road bike tyres.
Many commuter bikes have 700c wheels fitted, but typical tyre widths start at around 28mm and increase to 40mm or more, allowing them to be run at lower pressure.
You’ll also find commuter bikes with 650b/27.5-inch wheels, which may have even wider tyres fitted.
As with road bike tyres, you may well be able to run commuter bike tyres tubeless, if you have tubeless-ready wheels fitted. In this case, always choose tyres that are designed for tubeless use; gravel and MTB tyres are almost invariably designed to run tubeless these days. But it's sensible to check with the manufacturer, retailer or your local shop if you aren't sure.
Gravel bike tyre sizes
Many gravel bikes use 700c wheels, as found on road bikes, however, you’ll find some gravel bikes fitted with 650b wheels.
- 700c vs 650b: The differences explained
700c gravel bike tyres start at around 32mm width and balloon out to 45mm or more. 650b tyres may be as narrow as 30mm but are typically much wider to provide greater grip and ride comfort. Confusingly, once you get past around 50mm wide, tyres are typically expressed in inches and marketed more towards mountain bikes.
While choosing a road bike tyre is mainly a question of width, weight and how much you want to pay, there’s more to consider when choosing a gravel bike tyre. A key decision is the tread pattern, which needs to match where you plan to ride and how wet and muddy conditions are likely to be.
Width is also important, as a narrower tyre may bite deeper into the mud and so provide extra grip, while a wider tyre will float more over dry, bumpy terrain. You also need to consider frame clearance; even the best gravel riders can be caught out, as witnessed by the clogged wheels at the 2023 Unbound Gravel race.
Mountain bike tyre sizes
Mountain bike tyres, and the rims they fit to, are wider still and most mountain bike tyres are heavier and have a more aggressive tread as well as multiple compound and sidewall options.
As with road bike tyres, width has increased significantly recently and you can expect tyres to start at around 2 inches, while some are as wide as 2.6 inches. Again, consider your MTB frame’s clearance when choosing.
Finally, some fat bike tyres can head out even wider, to as much as 5 inches wide.
