Hong Life CalculatorsUnderstanding the bicycle tire size chart — concept and principle
Bicycle tyres carry several size labels at once: an inch size such as 29 × 2.2, a French size such as 700 × 25C, and an ETRTO/ISO code such as 25-622. Only the ETRTO code tells you for certain whether a tyre fits a rim. This chart starts from the common wheel name and shows the ETRTO bead seat diameter, an example tyre code and the kinds of bikes that use it.
How it is calculated
The chart is a lookup table. The ETRTO (ISO 5775) code has two numbers:
- The first is the approximate inflated tyre width in mm (25 in 25-622).
- The second is the bead seat diameter (BSD) in mm, the diameter of the rim where the tyre bead sits (622 in 25-622).
- A tyre fits a rim only if the BSD matches exactly; width only has to fall within the range the rim allows.
- Rough outer diameter ≈ BSD + 2 × width, because a tyre's height is close to its width.
The principle
Inch and French names describe the approximate outside diameter of an inflated tyre, which depends on the tyre as much as the rim. That is why different rims ended up with the same nominal name and the same rim ended up with several names. In the French system the number is a nominal outside diameter in mm and the letter marks a rim size: 700A, 700B and 700C sit on rims of 642, 635 and 622 mm. Likewise 650B is 584 mm and 650C is 571 mm. The ETRTO system avoids all this by naming the one dimension that must match, the bead seat, so 700C, "28 inch" and "29er" are all simply 622.
Worked example
Default selection: 700C / 29″ / 28″.
- ETRTO bead seat diameter: 622 mm.
- Example tyre: 25-622, sold as 700 × 25C, about 25 mm wide.
- Outer diameter ≈ 622 + 2 × 25 = 672 mm; circumference ≈ π × 672 ≈ 2,111 mm, a useful starting value for a bike computer.
- Used on road, gravel and 29-inch mountain bikes.
How to read the result
Match the BSD first, then the width. A 622 rim can take a narrow 23-622 road tyre or a wide 60-622 mountain-bike tyre only if the rim's internal width suits that tyre, so check the tyre and rim makers' width charts. 27.5″ and 650B are the same 584 mm size; old 26″ mountain bikes use 559 mm, which is not interchangeable with 584 or 571.
Common mistakes
- Buying a "26 inch" tyre for a rim that is actually 571, 584 or 590 mm.
- Assuming 27″ (630 mm) vintage road tyres fit 700C (622 mm) rims.
- Treating "29 × 2.2" and "700 × 25C" as different rim sizes; both are 622.
- Using the nominal 700 mm as the real wheel diameter when calibrating a bike computer.
Frequently asked questions
Is a 29er the same as 700C?
Yes. Both use a 622 mm bead seat diameter; 29er tyres are just wider.
What does 700 × 25C mean?
A tyre about 25 mm wide for a 622 mm (700C) rim, written 25-622 in ETRTO.
Are 27.5″ and 650B the same?
Yes, both are 584 mm.
Will a 26″ tyre fit a 27.5″ rim?
No. 26″ MTB rims are 559 mm and 27.5″ rims are 584 mm.
Sources: ISO 5775-1, Bicycle tyres and rims — Part 1: Tyre designations and dimensions · ETRTO (European Tyre and Rim Technical Organisation), Standards Manual