Quick conversion table
The number that decides whether a tire fits a rim is the ISO/ETRTO bead seat diameter in millimeters. Inch and French names are looser. These are common pairings, but always confirm against the marking on your own tire.
| Common name | Typical ISO bead seat diameter | Notes |
|---|---|---|
| 700c, 28”, 29” | 622 mm | Road, gravel, city bikes and 29er mountain bikes |
| 650b, 27.5” | 584 mm | Mountain and some gravel bikes |
| 26” (mountain) | 559 mm | Older mountain bikes, many fat bikes |
| 27” | 630 mm | Older road bikes, not the same as 700c |
| 28 x 1 1/2 (700B) | 635 mm | Older roadsters |
| 24” (kids’ mountain) | 507 mm | Other 24” sizes exist |
| 20” (BMX style) | 406 mm | Other 20” sizes exist |
Several 24” and 26” sizes exist with different diameters, which is why the ISO number matters.
Why inch sizes are unreliable
Inch sizes grew out of different historic systems and marketing choices. “29er” is largely a name for wide tires on the same 622 mm rim as 700c, and “27.5” is a name for tires on the 584 mm rim also called 650b. Because of this, the same label can cover different rims. When buying, match the ISO/ETRTO number and treat the inch size as a hint.
How to read the sidewall
Most tires carry an ISO/ETRTO marking such as 47-622. The first number is the tire’s designed width in millimeters, and the second is the bead seat diameter. A tire marked 47-622 fits a 622 mm rim. Many tires also show an inch or French size, such as 700x45c or 29x2.0. For a fuller walkthrough, see converting between inch, metric and ETRTO tire sizes.
If the ISO number matches the rim, the tire will fit it, and you can usually vary width within what the rim and frame allow.
Is 700C the same as 28 inch?
For most modern bikes, yes in practice: 700c and 28” both usually refer to a 622 mm rim, and so does 29”. The exception is that some 28” sizes, mainly older roadster tires, use 635 mm, which is not interchangeable. The 27” size at 630 mm is also different from 700c. Check the ISO number rather than assuming. Likewise, 622 and 700c refer to the same rim size.
Measuring if the markings are gone
If the tire and rim are unmarked, you can measure the rim.
- Remove the tire if possible and measure across the rim, between two opposite edges.
- Take several readings and use the largest or an average.
- The outside rim diameter is slightly larger than the bead seat diameter, often by around 10 mm, so compare the result against a published table. Sheldon Brown’s rim measuring article is a well-known reference.
- If the wheel is built up, you can measure the circumference instead and divide by pi for the diameter.
Other options are checking the owner’s manual or asking the bike’s maker or a shop. Measure carefully, because a few millimeters can separate two different standards.
Width conversions and rim compatibility
Width converts directly between systems: 25.4 mm equals one inch, so 2.0” is about 51 mm and 2.3” is about 58 mm. Tires have a recommended range of rim internal widths, so check that your rim suits the new width. For more on fitting different wheels to a frame, see can you change the wheel size on a bike.
Other formats readers ask about
- 3.00-21 vs 90/90-21: these are motorcycle tire formats. They are similar in nominal size, but the systems differ, so check the manufacturer’s chart or a motorcycle tire specialist. They do not apply to bicycles.
- 35 inch tires: this size is usually for vehicles rather than bicycles. For metric equivalents, consult the tire maker’s chart.
- Measuring a bike size: frame size is separate from wheel size. Frame sizing depends on the maker’s chart and rider height, so check their sizing guide.
For fat bike specifics, see fat bike wheel sizes.
Practical steps
- Read the ISO/ETRTO number on the tire.
- Match it to the replacement tire or rim.
- Confirm the width fits your rim and frame.
- If unreadable, measure the rim and compare against a table.