How To Choose The Right Size Bike Tire

Time for a new tire? Worn out tires increase the chance of getting flats, decrease performance, and can be a safety hazard. When it’s time to replace your tires, the choices can be overwhelming – with many sizes to choose from along with confusing standards and misleading naming conventions. This guide is your map to navigate the maze of tires and find the one that’s right for you. 

The single most important tire measurement is the rim diameter. On road bikes, this is usually named using the old French system (700c, 650b, 650c) and, on mountain bikes, rim size is almost always named in inch measurements (29”, 27.5”, 26”). Unfortunately, these naming conventions are, at best, arbitrary and other times very confusing (for example, there are at least four different sizes of 26” rims). To combat this confusion, tire sizes can also be referred to by their ISO size. 

ISO, the International Organization for Standardization, developed a universal sizing system which provides a reliable metric for measuring tire and wheel diameters. These numbers represent the diameter measurement of the tire bead and are a surefire way to size-match a tire. On many tires, ISO numbers can be found on the sidewall similar to where you might find tire pressure recommendations. At Performance Bicycle, our tire specs include ISO information, so it’s easy to confirm that you’ve found the right match. 

Many tires are available in multiple widths. In general, narrower tires are faster and wider tires provide more comfort and traction. On most bikes, the frame presents a physical limit to how wide your tires can be. The inner width of your rims only needs to be close to the right width for the tire. For example, a 19mm inner width road bike rim will work with nearly all road tires and even some gravel tires, but will not be suitable for wider mountain bike tires. Likewise, a 30mm inner width mountain bike rim will not work for road tires.   

It should also be noted that the width designated by a tire manufacturer is usually an estimate based on a common rim width. Depending on the rim, these measurements could be off by as much as a few millimeters.  

Most road bikes use 700c (622 ISO) rims and tires. Some road bikes for smaller riders use 650c (571 ISO) or 650b (584 ISO) and it’s not uncommon to see 27” (630 ISO)* rims on older road bikes. As for road tire width, the most popular size range among both professions and amateurs is 25 to 28mm. Research has found that running a narrower tire doesn’t actually make you any faster so it’s better to have a tire that provides a little more comfort over rougher roads. Be aware that some road frames won’t fit tires larger than 27 or 28mm, while many new road frames can clear 32 or even 35mm tires.   

*It is worth noting that 27” (630 ISO) is not the same as 27.5” (584 ISO) mountain bike tires.  

Mountain bike rims mostly come in three standard sizes – 26” (559 ISO) , 27.5” (584 ISO), and 29” (622 ISO). All three are very common so it is important to double-check which size you have. Additionally, kids mountain bikes have smaller rim sizes such as 20” (406 ISO) and 24” (507 ISO). Mountain bike tire widths tend to be based around the type of riding the bike is designed for. Cross country (XC) bikes and older mountain bikes often have limited clearance, especially for the rear tire, and may only be able fit tires around 2.1” and narrower. On modern trail bikes, 2.3” to 2.6” tires are the norm. And on downhill bikes, ‘plus-sized’ tires can be as large as 3.0”. Going wider than that moves into the realm of fat bikes, which are specially designed to handle extra-wide tires.   

Stradling the line between road and mountain tires, most gravel tires are either 700c (622 ISO) or 650b (584 ISO).  These are sometimes used interchangeably with the inch measurement names for the same rim diameters: 29” (622 ISO) and 27.5” (584 ISO) respectively. Since 700c is the same as 29” and 650b is the same as 27.5”, they can be used interchangeably so long as rim width and frame clearance are suitable. The range of frame clearance in this category is wide – from older cross bikes that max out around 32mm to some modern gravel bikes that can run 2.4” mountain bike tires. It is best to look up tire clearance on the bike manufacturer’s website or measure it yourself before purchasing a wider tire than you are currently using. 

700c/29” (622 ISO) and 26” (559 ISO) are, by far, the most common sizes for city bikes and cruisers. Older town bikes may use a 27” (630 ISO) tire and more recent models may also use a 650b/27.5” (584 ISO) tire. Look out for older Schwinn rims as they follow their own sizing standards.  

20” (ISO 406) is the most common size of BMX tire. Though, it’s not uncommon for BMX bikes to have both larger and smaller sizes of rims. With BMX and kid’s bikes, pay attention to decimal vs. fractional sizes – decimal sizes are more common, but fractional sizes are not rare. Tire clearance on kid’s bikes is often limited so it is usually best to replace with a similar width tire to what was originally used. 

With this information, you should be all set to find a tire that fits your bike. If you’re having trouble wading through the specs and deciding what kind of features you want from a mountain, road, or gravel tire, check out our Tire Recommendations article. If you’re looking to take your tire performance up a notch, check out our guide on Tubeless Tires. And, of course, if you have any questions, doubts, or concerns about buying a tire, please reach out to our customer service team who will be happy to help guide you to the right tire.   

If you want to go down a bike nerd rabbit hole and read about obscure tire sizes and how we ended up with the ISO system, check out Sheldon Brown’s article on Tire Sizing Systems.