Tire Sizing[]
This page provides a quick guide to buying the right size tire For complete guides see the Park tool page and the Sheldon Brown page.
Rim and tire diameter must match.[]
A given rim fits a wide range of tire widths, but the diameter of the rim/tire interface must match, so that's essential to get right. It can be confusing because each size is designated differently in different systems:
- French metric sizes, such as 700c, 650b, 650c, etc. The number is the nominal outside diameter, and the letter (b, c, etc.) originally indicated the width of the tire. But now you can get "700c" tires in many widths, and the outside diameter is rarely 700 mm. So a code like "700c" is best considered the name of a size category, rather than an actual measurement.
- Inch sizes, such as 26", 28", 29", etc. These are nominal outside diameter of the tire, but depending on the tire width, it might not match the nominal size. Again, these are best viewed as the names of size categories rather than actual measurements. The inch sizes in particular are a confused mess, since there can be many different "standard" rim interfaces for the same nominal size. The standard being used is sometimes signaled in obscure ways, such as the use of decimal or fractional inch widths: 26 x 1.5 indicates a different size from 26 x 1-1/2.
- ISO/ETRTO size system. This is an international standard system, designed to alleviate the confusion in the other systems. A tire size is listed as XX-YYY where XX is the width in mm and YYY is the actual diameter or the rim/tire interface: the bead seat diameter. For example, 32-622 is a 32 mm wide tire to fit a rim with a 622 mm bead seat diameter. A corresponding rim would be designated as 622 x 17c, where 17 mm is the interior width of the rim. The 622 should match exactly between the two; the tire width will be wider than the rim width and there's more flexibility there (see below).
Fortunately, if you have a modern, fairly standard, adult-size bike, it most likely matches one of these three sizes:
| Description | French name | inch name | ISO (bead seat diameter) |
|---|---|---|---|
| Standard road bike and large MTB "29er" | 700C | 28 (Europe), 29 (MTB) | 622 mm |
| Mid-size MTB and gravel/etc. | 650B | 27.5 (MTB) | 584 mm |
| 26" MTB, with decimal inch widths | none | 26 x <decimal inch width> | 559 mm |
Other notable sizes include 27 x <anything>, which is a mostly obsolete road-bike size distinct from the 27.5 MTB size, which has a 630 mm ISO bead seat diameter, and other 26 inch sizes which are listed with fractional inch widths. For more on these and other sizes see the Park tool and Sheldon Brown page pages.
Tire width[]
You can use a range of different tire widths on a given bike, with two constraints: rim width and frame clearance.
Rim width and tire width[]
To have the tire sit nicely on the rim, the tire should be wider than the rim, but not too much wider. A general conservative rule is that the tire width should be 1.5 to 2.5 × the internal rim width. The internal width of the rim should be listed as part of the ISO (ETRTO) rim size designation as something like 622 × 19C, where the C indicates a "crotched" type rim that has a hooked profile, and the 19 is the internal width.
The range of 1.5 to 2.5 is often expanded to 1.4 to 2.8 or even 1.3 to 3.0. The ISO/ETRTO standard includes a chart of compatible sizes, and WTB provides a less conservative chart.
The reasons for considering the match between tire width and rim width are that:
- A tire that is too wide for the rim can flop sideways too easily.
- A tire that is too narrow can lead to problems where the tire pressure pulls it off the rim, and cornering can end up running the tire on the sidewall instead of the tread, leading to a dangerous loss of traction and/or damage to the sidewall.
Tire clearance[]
Usually the limit on fitting a wider tire is clearance for the tire, not rim compatibility. Ideally, there should be 4-6 mm clearance between the tire and the frame, fork, or brake arms to allow for flex or mud and grit without rubbing on the frame, or, worse, jamming up and locking the wheel. ISO standards require at least 6 mm for most bike types, but allow that to drop to 4 mm for road bikes. Depending on your risk tolerance and riding conditions, it might be reasonable to push it down to ~3 mm clearance. Tight clearance on the rear is less of a safety hazard than tight clearance at the front, as locking up a rear wheel is less dangerous than locking up a front wheel.
A good way to measure clearance is by inserting a hex key between the tire and frame, as they come in calibrated sizes. Places to check include:
- Sides and top through the fork.
- Sides and top at the seat stays and brake bridge.
- Sides and top where the chain stays attach to the bottom bracket. This is often the limiting factor for the rear wheel. If you have horizontal dropouts, you can gain tire space by sliding the wheel back all the way in the dropouts.
In theory, increasing tire width by, for example, 4 mm, will make each side grow by 2 mm and make the top grow by 4 mm. However, actual tire widths vary widely by brand and model, partly because the width will be different on different rims, and the rim width used for the measurement is not standardized and is rarely specified. So ordering new tires based on calculating what size will fit is something of a gamble.
Tube sizing[]
Tube sizing is much less critical than tire sizing. There's nothing that needs to be an exact fit. Most tubes will be labeled for the diameter and for a range of widths. For example, a tube might say 700c x 28-35. That means it will work for 700c (aka 622 or 28") tires with widths anywhere from 28 mm to 35 mm. In a pinch, you can use a tube for applications beyond that range, but:
- Too wide a tube will mean you have excess material that is hard to get tucked in to the tire without getting pinched in the bead when you mount the tire.
- Too narrow a tube will mean that the rubber is stretched thin when it's inflated. It will then puncture more readily with minor damage.
For best robustness, pick the largest tube that is rated for your tire width: for a 28 mm wide tire, a 28-35 mm rated tube is more robust than a 23-28 mm rated tube.
Valve[]
Common bicycle valve types are Schrader (the valve used on car tires and on older, lower-end bikes) and the skinnier Presta valve. Match your current valves to fit the hole in the rim. Presta valve tubes are also available in different lengths; long ones are need to reach out of a deep-cross-section rim.