Read ETRTO bicycle tire size markings
How to decode an ETRTO bicycle tire size such as 40-622: tire width and inner diameter in millimeters, how it differs from inch, French and car tire labels, and why it matters.
Contents
The short answer
An ETRTO bicycle tire size is two numbers in millimeters separated by a hyphen, printed on the tire sidewall. The first number is the tire width. The second is the tire's inner diameter. On a tire marked 40-622, the tire is 40 mm wide and has an inner diameter of 622 mm. ETRTO stands for the European Tyre and Rim Technical Organisation, the body behind this notation.
How to decode a marking step by step
- 1. Find the size on the tire sidewall. Tire maker Schwalbe identifies the sidewall as the place where size markings appear.
- 2. Look for a pair of numbers joined by a hyphen, such as 40-622. The hyphen only separates the two dimensions.
- 3. Read the first number as the tire width in millimeters (40 mm in this example).
- 4. Read the second number as the tire's inner diameter in millimeters (622 mm in this example).
- 5. Write down the complete pair, not just one number or a nearby inch label.
Don't read it like a car tire size
Motor-vehicle tire sizes often include a number that expresses sidewall height as a percentage of width. Bicycle ETRTO markings contain no such percentage. In 40-622, the 622 is a diameter measured in millimeters, and both numbers are actual dimensions in the same unit. Applying car-tire logic to a bicycle marking produces a wrong reading.
Other labels that may appear on the same tire
Bicycle tires often carry an inch size or a French size alongside the ETRTO size. These formats measure different things. Schwalbe lists the following three labels together for the same size.
| Label | Format | What the numbers describe |
| 40-622 | ETRTO | Tire width and tire inner diameter, both in millimeters |
| 28 × 1.50 | Inch | Approximate outer diameter and width, in inches |
| 700 × 38C | French | Approximate outer diameter and width, in millimeters |
The inch and French labels describe approximate outer dimensions. The ETRTO label gives width and inner diameter. That is why the numbers don't convert neatly from one format to another, and why the ETRTO pair is the more specific reference.
Why an inch label alone can mislead
The same inch designation can cover different ETRTO sizes. Schwalbe lists 20 × 1.00 with both 25-406 and 25-451. Both tires are 25 mm wide, but their inner diameters differ by 45 mm. A shopping note that says only "20 × 1.00" cannot tell those two apart. The full ETRTO pair can.
Illustrative comparison worksheet
The following worksheet is a hypothetical example. It shows one way to compare markings when you are checking a replacement tire or tube against the one you have. Split each ETRTO marking into its two numbers, then compare each column separately.
| Marking | Width (mm) | Inner diameter (mm) | Note |
| 25-406 | 25 | 406 | Same width as the row below |
| 25-451 | 25 | 451 | Different inner diameter: not the same size |
If the inner diameter differs, the sizes are different even when the width and the inch label match. Comparing the second number first catches the mismatch that inch labels can hide.
What a matching marking does and doesn't tell you
Matching ETRTO numbers confirm that two tires share the same stated width and inner diameter. They do not confirm that a tire suits a particular rim or frame. Before choosing a different width, check the rim and bicycle manufacturers' published specifications.
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