A Guide To Bike Tire Size: How It’s Measured and Why It Matters

Published on Dec 29, 2021

Your bicycle’s tires might not be something you think about very often, but they have crucial implications for every aspect of your ride. Better understanding how your mountain, gravel, and road bike tire size is measured can help you fine-tune your equipment, and learning how tire size relates to performance can make you faster when it counts. What are the basics of bike tire size?

How Do You Measure Bicycle Tire Size?

Bike tires are typically measured in two dimensions— diameter and width. The diameter measurement is an approximation of the tire’s total outside diameter including treads, and the width is a measurement of the approximate total width of the tire when mounted and inflated. For mountain bike tires these dimensions are expressed in inches, while a millimeter-based system called French sizing is used for road, gravel, and track. For example, a 29 x 2.25 mountain bike tire is about 29” in diameter and about 2.25” wide, while a 700c x 25 road tire is approximately 700mm in diameter and 25mm wide.

This makes it pretty straightforward to fit a modern tire to a modern rim—a 700c tire will almost definitely fit a 700c road rim (we’ll explain that “c” later), and a 29” tire will likely fit a 29” mountain bike rim. But some obsolete or unusual sizes can be misleadingly labeled, and any tire’s nominal measurements (especially width) are really just approximations. Rim width and tire pressure can significantly influence the size of a tire when mounted and inflated, and tires often measure a bit larger or smaller when installed than the printed dimension would suggest.

To reduce confusion, most tires are also labeled with a second system of measurements called ISO (formerly known as ETRTO). The ISO measurement displays the tire’s nominal width in millimeters, followed by the diameter of the tire’s bead (the surface that actually attaches to the rim) in millimeters (ex: 25 x 622 is a common road tire). This measurement can help resolve any ambiguity about whether a tire will fit a particular rim, but as with other systems, the ISO measurement of a tire’s width is an approximation and may be impacted by pressure and rim width.

Nearly all modern road bikes use 700c wheels and tires. It used to be widely accepted that narrower tires were faster and 23mm was the standard width. But recent research has proven wider tires to be faster and more comfortable in most situations. As a result, 700c x 25mm and 700c x 28mm are now the most common road tire sizes; many riders prefer even wider widths of 30mm or 32mm. The limiting factor is usually the bike itself, with some frames unable to accommodate tires beyond a certain width. Most new road frames can at least fit up to 28mm tires but double-check your frame’s allowance before sizing up.

A few other less common wheel and tire sizes exist for road bikes. 650b (ISO 584) and the rarer 650c (ISO 571) are two examples, both sometimes used on bikes for smaller riders. The letter that follows the diameter measurement in French tire sizes originally delineated width, but it’s now mostly just useful to differentiate between similarly-named but incompatible sizes. For instance, a 650b tire will not fit on a 650c rim.

It’s also important to understand the different types of mutually-incompatible road tires. Clinchers are most common; these are the familiar tires that seat into a walled rim around an inner tube. Certain clincher rims can also be used with tubeless tires, which use a liquid sealant in place of an inner tube. Finally, tubular tires are permanently sewn closed around an internal tube and are glued into a specially-made rim. All 3 of these tire types use the same sizing standards and terminology but are generally not interchangeable.

Tires are printed with a manufacturer’s recommended pressure range, and road riders used to think inflating their tires to the highest possible pressure was fastest. But with the move to wider tires has also come a trend towards lower pressure. The science of tire pressure is complicated, but wider tires require less pressure for the same volume of air than narrower tires, allowing for a more comfortable ride. Additionally, wider tires at lower pressures reduce bouncing and are actually faster on most surfaces than smaller, harder tires.

It’s tough to make a generalized recommendation for pressure—riding conditions, your weight, and the tire’s size all play a part. But generally, the larger the tire and the rougher the surface, the lower the optimal pressure. With each 3mm increase in tire width, you can usually reduce pressure by 1 Bar (~14 psi). Also, tubeless tires can generally be ridden at lower pressures than tubed tires of the same size. Some tire and rim manufacturers have calculators on their websites that make personalized recommendations for pressure; these are a great starting resource to make your ride faster and more efficient.

Mountain Bike Tire Sizes

Mountain bike tires are measured in inches and are offered in 3 non-interchangeable diameters corresponding to common mountain bike wheel sizes. Most popular for high-end mountain bikes are 29” tires and wheels. Next come 27.5” setups, preferred by some riders who like smaller,  slightly more maneuverable wheels. And finally, 26” wheels and tires used to be the standard, but are now found mostly on entry-level and kids’ bikes.

Tires at each of these diameters are available in a wide variety of widths, which riders select for the specifics of their discipline and terrain. Cross-country racers usually choose comparatively narrow tires ranging from 1.9” to 2.25” wide. Trail, all-mountain, and enduro bikes are normally equipped with wider tires between 2.25” and 2.4”, and downhill tires are even wider at 2.4” to 2.6”. Finally, fat bike tires are mounted on purpose-built rims and push the boundaries even further, sometimes measuring as wide as 5”. The specifics of tire choice are carefully considered by mountain bikers, with racers often choosing different widths and tread patterns depending on the course and conditions.

Interestingly, most mountain bike wheel sizes are actually the same diameter as road wheels—29” wheels are equivalent to 700c, while 27.5” are the same as 650b. But you wouldn’t want to put tires intended for one on a wheel intended for the other, as the rim’s width is dramatically different for road and mountain bikes and would interfere with the tire’s performance.

Mountain Bike Tire Pressure

Tire pressure is a crucial concern in mountain biking. Small changes in pressure can dramatically impact performance and handling on the trail, and experienced mountain bikers regularly adjust pressure depending on terrain, conditions, riding style, and tire choice.

Because all of these variables are factors to consider, it’s nearly impossible to make a general recommendation for mountain bike tire pressure. Online calculators can help suggest a starting pressure based on equipment, weight, and conditions, but in the end, it’s ultimately a matter of personal preference and learning from experience. A good strategy is to treat the first few rides on a new setup or in new terrain as experiments. Carry a digital gauge, start with pressure on the higher side, and gradually let a few psi out/ add some pressure back in as you ride to experiment with what works and feels best. Check and record your pressure when you find the sweet spot and use this as your starting point for future rides.

There are a few general principles to keep in mind when finding the right pressure. Typically, the larger your tire, the lower the optimal pressure. Tires with thinner casings require higher pressure, as do heavier riders. Rocky terrain may also necessitate higher pressure to avoid flats, while lower pressures can be used in smooth, grassy, or muddy conditions. Finally, some riders like to use tire inserts, which provide more flat protection and allow a few psi reduction in pressure.

Tire Sizes for Other Cycling Disciplines

Gravel, cyclocross, and track cyclists also choose specific tire sizes and pressures to optimize performance.

Cyclocross Tire Sizes

Cyclocross bikes use 700c road wheels, so cyclocross tires are all designed for this standard diameter. In the past, most serious cyclocross racers used tubular tires, but tubeless tires have become increasingly popular over the last few seasons. Tires at CX events have traditionally been allowed up to a maximum width of 33mm, and UCI-governed events still impose this limit. Non-UCI races often allow larger tires, such as the 38mm maximum width allowed at USA Cycling masters, collegiate, and single speed national championships. Many local events impose no size restrictions at all—check your race’s rules to know for sure.

Gravel Tire Sizes

Gravel bikes used to be repurposed cyclocross bikes, but with dedicated gravel equipment introduced over the last few years tire options have greatly expanded. Most gravel bikes use 700c wheels, but 650b wheels are occasionally used for especially technical trail riding and bikepacking. Most new gravel bikes have clearance for tires ranging up to at least 42 or 45mm width, and some allow for even wider tires. Virtually all gravel riders use tubeless tires.

Gravel tires all balance speed and efficiency with offroad traction. Narrower tires with minimal treads are fastest on hardpack and paved surfaces but offer poor grip in loose corners. Wider tires with more aggressive tread patterns are more capable on loose terrain but roll much more slowly on smooth or paved roads. Gravel riders choose the width and tread pattern that offers the best balance for their local terrain, but may significantly adjust their tire choice and pressure for different conditions.

Track Tire Sizes

Like road bikes, track bikes use 700c wheels. But unlike on the road where slightly wider and softer tires are usually faster, on a smooth track harder and narrower tires have an advantage. For this reason track racers still prefer 21mm – 23mm wide tubular tires inflated to very high pressures—usually 150 psi or more on indoor tracks. Racers on rougher outdoor tracks don’t inflate their tires quite this high, but they still use much more pressure than they would on the road, with relatively narrow tires offering little in the way of puncture protection.