What a bike tire size is telling you
A bike tire size describes two things: roughly how big the tire is across, and how wide it is. Unfortunately, bikes have been built for well over a century by different countries and industries, so several naming systems are still printed on tires today. Some are precise and some are loose historical labels, which is why two tires that look like they should match sometimes do not.
The good news is that you only need to understand one number to know whether a tire will physically fit your wheel. The rest is detail about width and frame clearance.
The three labeling systems
Most tires carry one or more of these markings on the sidewall.
ETRTO (also written ISO)
ETRTO is the European Tyre and Rim Technical Organisation, and its marking is the most reliable. It is written as width, a dash, then bead seat diameter, both in millimeters. For example, 37-622 is a 37 mm wide tire for a rim with a 622 mm bead seat diameter.
Inch sizes
Inch markings such as 26 x 1.95 or 29 x 2.10 are common on mountain bikes, cruisers and kids’ bikes. The first number is a nominal outside diameter in inches, and the second is the width in inches. Because the first number is nominal, it is not exact, and different tire designs have used the same inch label for different rims.
French metric sizes
Markings such as 700x28c come from an older French system. The first number is a rough outside diameter in millimeters, the second is the width in millimeters, and the letter was originally a code for a rim diameter category. Today the letter c is generally associated with the 622 mm rim, but you should confirm with the ETRTO number whenever it is printed.
Bead seat diameter: the number that decides fit
The bead seat diameter (BSD) is the diameter of the rim where the tire’s bead sits, in millimeters. A tire fits a rim when their BSDs match. Width is more flexible, within limits set by the rim and your frame.
| Common name | Bead seat diameter |
|---|---|
| 700c, 29 inch, modern 28 inch | 622 mm |
| 650b, 27.5 inch | 584 mm |
| 26 inch (mountain) | 559 mm |
| 20 inch (BMX) | 406 mm |
If you want the full list, see our bike tire size chart.
Is 29 inch the same as 700c?
For tire fitting purposes, yes. A 29 inch mountain bike tire and a 700c road tire both use a 622 mm bead seat diameter, so they are mounted on the same size of rim. The difference is mostly width and tread: 29er tires are generally much wider. Whether a given tire fits your frame is a separate question of clearance, covered below. For label details on those wider tires, see mountain bike tire dimensions. For more detail see what 700c really means.
What 24 inch, 26 inch and 20 inch mean
These are traditional nominal diameters. A “26 inch” mountain bike wheel typically has a 559 mm bead seat, but older 26 inch labels exist with other rims (for example some older 26 x 1 3/8 tires use a different, incompatible rim size). A “24 inch” kids’ wheel is commonly 507 mm and a “20 inch” BMX wheel 406 mm, though some 20 inch tires, like 20 x 1 3/8, use a different 451 mm rim. This is exactly why checking the ETRTO number is safer than trusting the inch label. For converting between systems, see converting between inch, metric and ETRTO sizes.
Reading a full sidewall
Suppose a tire says 700x32c and 32-622. Here is how to read it:
- 700: approximate outside diameter in mm (a legacy label)
- 32: tire width in mm
- c: legacy letter code, associated with a 622 mm rim
- 622: the bead seat diameter that must match your rim
If the sidewall shows only an inch size, such as 27.5 x 2.4, it is about a 27.5 inch (650b-class, 584 mm) tire 2.4 inches wide. Our guide to 650b tires explains that size in more detail.
A note on the “3% rule”
You may see a rule of thumb that overall tire diameter should stay within about 3% when swapping sizes. It is most often applied to things that depend on overall diameter, such as speedometer accuracy or gearing feel. It is not a fit rule, and it does not override bead seat diameter or frame clearance. Treat it as a rough guide only.
Width and frame clearance
Even with matching BSD, a tire has to fit between the fork and stays and clear brakes and fenders. Manufacturer specs for your frame are the best source for maximum width. Rim width also matters because each rim has a recommended range of tire widths. Measuring is covered in how to find and measure your bike tire size.
Where to go next
- Look up a size: bike tire size chart
- Find your own size: how to measure bike tire size
- Kids and rider height: choosing a tire size for your height