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Rim and Tire Sizes - ISO and ETRTO
There are a LOT of tire sizes in the world of bicycles. This is by no means a thorough explanation of all the different sizes - it's just meant as a quick guide. Here's a run-down of the common rim and tire diameters that we run into here at the bike shop.
Rim and tire diameter...
Which is larger, a 27" rim or a 29" rim? Well if you haven't happened to have worked with bikes a whole lot you could easily think that the 29" rim is bigger than a 27", when in fact it's the other way around. You'll normally see rims advertised with a measurement such as 26 inch, 700c, 650b, 29 inch, and others. Those numbers do refer to a specific measurement - but unfortunately those names don't all refer to the same measurement so there is no way to compare them just by name.
The standardized way to measure the diameter of rims and tires is with ISO (same as ETRTO - The European Tyre and Rim Technical Organisation ). The ISO or ETRTO designation of rims uses a fixed measuring point. It measures the Bead Seat Diameter (BSD) in millimeters, the bead seat being the part of the rim that the tire hooks into (for clincher tires). Here are the ISO measurements, which you will sometimes see in tiny print on the stickers of the rim or molded onto the tire, along with the size that you are probably used to hearing.
Common
Name
| ISO Size
(in mm)-
| Common
Use
| 27"
| 630
| Old School Road/Touring Bikes
| | 29" | 622 | Mountain Bikes
| | 700c | 622 | Road Bikes
| 650b
| 584 | Mountain Bikes (also called 27.5" for a short time)
| 650c
| 571 | Small Road or Triathlon/Time Trial Bikes
| 26"
| 559 | Mountain Bikes
| 24 x 1-1/8"
or 1-3/8"
| 520 | Jr BMX Cruiser
| 24"
| 507 | Kids Bikes
| 20 x 1-1/8"
or 1-3/8"
| 451 | Jr/Mini BMX, Some Road Recumbent
| 20"
| 406 | BMX | | 19" | 381 | Trials Bikes
| | 16" | 305 | BMX |
Note that there are two different diameters for 20 and 24". When a tire has a fractional width measurement on it (1-1/8", 1-3/8"), it has a different diameter than a tire with a decimal width measurement (1.95, 2.1, 2.35). For example, a 24x1.95 tire will not fit on a wheel that has a 24x1-3/8" tire on it.
Also, a 700c road rim is the exact same diameter as a 29er rim, though a 29er rim is typically wider to accept a wider tire.
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