Standard Tire Sizes
Most tire sizes are specified with two numbers. The first number refers to the outside diameter of the tire, and the second number refers to the width. For example, a standard road tire is specified as 700C x 23, which indicates a tire that is approximately 700mm in diameter and 23mm wide. In actuality, neither of these number is an exact measurement.
There are five tire diameter sizes that are in common use on modern standard adult bicycles.
26" is the most commonly used size for modern mountain bikes and is also used by many hybrid bikes.
27" is an older size that was used for road bikes, but since so many bikes were built with this size, it is still readily available.
27.5" is actually the same rim diameter as 650B, but is used on mountain bikes. The terms 650B and 27.5" are used somewhat interchangeably; they describe more or less the same thing.
650B is used for some randonneur bikes and other specialized bikes.
650C is used for smaller road bikes and triathlon bikes.
700C is the most commonly used size for modern road bikes.
The second number in the tire size refers to the width of the tire. However, these numbers are often slightly exaggerated, so a 700c x 28mm tire may actually be only 26mm wide. This is probably the result of competitive pressures, since the narrower tire will be lighter, and light weight is the driving force behind many buying decisions in the cycling industry.
This would all seem relatively straightforward except that older bicycles often used some of these same size designations (particularly 26" and 27") for tires that are not compatible with these sizes. If you have a bike that is more than about 15-20 years old, please read our article on older tire sizes.
There are a wide variety of tire widths available for the "standard" tire sizes. Which width is best for you depends on your applications. The most common width for road riding is 23mm. This is a good compromise between aerodynamics, weight, rolling resistance, and comfort.
A narrower tire will have lower aerodynamic drag and lighter weight. At the same inflation pressure, a wider tire will actually have lower rolling resistance on most road surfaces because the majority of the rolling resistance comes from the heat loss of tire deformation. Since a wider tire (at the same inflation pressure) will deform less, it will lose less energy while rolling.
Generally, wider tires can be run at lower inflation pressure. The added volume allows lower inflation pressures to be used without the risk of pinch flats and rim damage. The lower inflation pressure will provide a more comfortable ride.
The tire widths that you can use on your bike are determined by the rim width and the frame clearance. The cart above shows the range of tire widths that can be used for a given rim width. The rim width measurement is the inside width of the rim (i.e., the width of the bead seat in the rim). This is a fairly conservative range - i.e., you can probably get a way with using a tire that is narrower or wider than the range indicated. If you use a tire that is too narrow for the rim, you're more likely to get pinch flats and risk damaging the rim if you hit pot holes or other road hazards. If you use a tire that is too wide for the rim, you risk damaging the rim and tire, and are also likely to have handling problems.
A bike frame designed for 23mm tires is unlikely to have the clearance between the tire and frame to support a 42mm tire, even if the rim could accommodate such a wide tire. Most road bike frames can accommodate a tire as wide as about 28mm. Cyclocross and touring bikes are generally designed to accommodate wider tires.
We recommend 23mm and 25mm wide tires for recreational road cyclists. The 25mm width is nice for long distance riding since it will provide a more comfortable ride. Narrower widths are worth considering for racers that are looking for every advantage.
For self-supported touring, a wider tire is desirable since the added load can be distributed over a larger contact patch. This will improve handling and reduce flatting. If your bike can accommodate it, use a tire that is at least 28mm.
Many touring and hybrid bikes will be fitted with even wider tires - up to 47mm wide. These wider tires will definitely provide a cushier ride, so if comfort is your main priority, sticking with these wider tire widths is a good idea. The main disadvantage to the wider tires is weight. Switching to a slightly narrower tire will give you a little better acceleration performance and provide a zippier ride.
For mountain biking, a wider tire (2.0 to 2.5") will provide more air volume which is beneficial for riding on loose surfaces. It will also prevent pinch flats on very rough terrain. If your riding is primarily on hard-pack dirt roads, a slightly narrower tire (1.5 to 2.2") will reduce weight and provide better performance.
Most modern mountain bikes have rims that are fairly narrow in the interest of saving weight. The cross-country tires that are usually installed when you buy the bike are actually on the wide end of the range that can be accommodated by the rims. While you probably should measure the rims to be sure, most mountain bike rims can easily accommodate a tire that is as narrow as 1.5" or even 1.3" without difficulty.