32 Inch wheels are here!
Making Sense of Bicycle Tire Sizing
What does the “C” on a tire mean? (Or the “B”?)
By Lennard Zinn
Have you ever wondered what the “C” in “700C” or “700 × 28C” means (or the “B” in 650B)? You would not be alone in not knowing or in thinking it is an abbreviation of “cm” any other word at all. Rather, it is an arbitrary designation stemming from an arcane French sizing system.
ISO/ETRTO size designation
In bicycle tire sizes, the first number in the traditional French labeling tradition is the nominal tire diameter in millimeters, the second number is the nominal tire width in millimeters, and the letter is used to designate the width of the rim and the tire and the rim diameter. However, nowadays, the letter only specifies with accuracy the rim diameter.
Tires today not only carry the traditional French style of size labeling (like 700 × 25C) but also carry their more precise ETRTO (European Tyre and Rim Technical Organization) designation, which is shared by ISO (International Organization for Standardization, a non-governmental global organization developing voluntary international standards to ensure safety, quality, and efficiency of products, services, and systems). ISO/ETRTO specifies tire size in an XX-XXX format with only two numbers and no letters.
The first number (XX) is the nominal tire width in millimeters, and the second number (XXX) is the Bead Seat Diameter (BSD): the internal diameter of the rim that the tire bead sits on (see above illustration). So, 700 × 25C is 25-622 in ISO/ETRTO parlance.
Importantly, “nominal tire width” is the tire’s width when inflated on an ISO/ETRTO-specified “design rim”; see the below chart of design rim widths for tire widths. The actual tire width changes slightly, by around ±0.4mm per 1mm change in rim width, when mounted on rims with different internal widths.
The internal width of a hooked rim is measured between the faces of the hooks, as you can see in this illustration from Zinn and the Art of Road Bike Maintenance. It can vary a little for the same rim; internal width increases under tire inflation pressure and can also change under rim-brake forces and spoke tension.
ISO’s “nominal” tire width allows a tolerance range of width. ISO/ETRTO bases the tolerances on the section width of the tire as shown in the below table.
From the table, you can see that, for instance a tire with a 28mm section width would have a tolerance of +/-2mm, resulting in an acceptable width range from 26mm up to 30mm. So, a 700 × 28C tire can have a mounted width of anywhere between 26mm and 30mm and be considered in spec.
Rim internal width must fall within a narrow range for a given section width of tire; you can’t safely use any 700C rim with any 700C tire. I will discuss tire fitting based on rim and tire width in a future blog.
The “C”, “B”, “A”, etc. on a tire size label
Contrary to popular belief, the “C” on tire size was never an abbreviation of any kind, for instance of “centimeters” or “clincher” (or “crotchet”), as some think, referring to the type of rim; it was always an arbitrary designation as part of a letter series. Originally, the A, B, C, D designations in the old French sizing system referred to the tire and rim width, A being the narrowest and D being the widest. Now those letters have largely lost their tire width meaning and refer only to a diameter, namely the Bead Seat Diameter (BSD).
In the case of 700C tires, the “C” would have denoted a medium-width tire in the old French system (around 38-40mm historically) with a 700mm nominal outer diameter. 700C now refers to any tire, from very narrow to very wide, that fits on a rim with a 622mm BSD. In addition to these letter designations, tires can also be sized in decimal and fraction inch sizes, which carry different meaning.
In the old French sizing system, since the first number was a nominal tire diameter and the letter was a nominal tire width, the letter sizes of fatter tires resulted in smaller rim diameters to achieve the same nominal outside tire diameter, and vice versa. For example, 700A (narrow) tires fit on rims with a 642mm BSD, 700B (medium narrow) tires fit on 635mm BSD rims, 700C (medium wide) tires fit on 622mm BSD rims, and 700D (wide) tires fit on 583mm BSD rims. There’s no need to remember 700A, 700B, or 700D, since the A, B, and D have largely been abandoned in “700” sizes.
In contrast, in “650” sizes, only the 650B (584m BSD) designation remains on tire labels, although 650A (590mm BSD) and 650C (571mm BSD) tire sizes still very much exist. You won’t find 650A or 650C printed on tire labels; tires for those rim sizes are instead labeled in inch designations; 26 X 1-3/8” is 650A, and 26 X 1” is 650C. (Yes, tires labeled in fractional inch sizes fit different rims than those labeled with decimal inch sizes, even though the first number, denoting the diameter, is the same!)
As examples of road and MTB tires sharing the same 622mm bead seat diameter (BSD), a 700 × 28C road tire is ISO size 28–622, a 29 × 2.35” mountain bike tire is ISO size 61–622, and a 28 × 1” road tire is denoted by ISO as 25–622.
In these arcane labeling traditions, tires bearing equivalent magnitude decimals and fractions do not denote the same tire size. For instance, 26 × 1.0” through 26 × 2.5” decimal sizes (with two ignorable exceptions) are tire sizes that fit on 26-inch mountain bike rims (559mm BSD). Fairly logically, the ISO designation of a 26 × 1.0” tire is ISO 25–559. However, 26 × 1” tires (no decimal or fraction), which you might logically assume would be the same size as a 26 × 1.0” tire, instead fit a 650C rim (571mm BSD; their ISO size is 25-571)! 26 × 1” was formerly popular in triathlon and called 26”. Adding yet more confusion, 26 × 1¼” and 26 × 1-3⁄8”(fractions) are both ISO 597 (597mm BSD), while 26 × 1½” (different fraction) tires are 650B (584mm BSD) and are also called 27.5”!! And 26 × 1¾” (different fraction yet) is an ISO 571 Schwinn cruiser size (yes, same BSD as a 26-inch triathlon tire that has no decimal or fraction)! Clear as mud?
Until the ISO/ETRTO tire and rim standards were finally approved by all ISO member countries in early 2023, some rim brands intentionally used a slightly increased BSD and lowered the height of the rim wall above the bead seat to change the fit and shape of the mounted tire. Some tire manufacturers tweaked the BSD of their tires accordingly, making tire fit a crapshoot dependent on brand. Now we should finally have confidence that bike tires and rims are made to the same standards, leading to more consistent tire fit, reduced mounting problems, and increased safety.
Since bigger wheels are becoming more popular, I want to mention two others. 32” is 686mm BSD. And 750D (660mm BSD) is a size between 700C and 32” that I like to call 30.5” when talking about MTB tires; it was introduced by WTB in 2023.
In the old French tradition, the “D” in 750D would have indicated a wide (~45mm) tire. However, WTB offers only 750x34D and 750x42D. 750D was chosen as the size reference mostly to add a letter to call attention to the bead seat diameter. As the tire cross sections WTB produces are more road than mountain, a metric number size rather than an inch size was chosen. The 38mm jump in bead seat dimension from 700C to 750D is the same as between 700C and 650B; 650B=584mm, 700C=622mm, and 750D=660mm. Also, 26” MTB tires are 559mm BSD, which also defines the 650D size; if you add 101 (almost 100!) to the 559mm BSD for 650D/26” MTB tires, you get the 660mm bead seat size for 750D.
Sheldon Brown has complete charts of tire sizing here. You won’t find 750D on them, as that size didn’t come along until 15 years after his death.
Thanks for reading.
― Lennard
Lennard Zinn has been designing and building custom bicycles for over 45 years; he founded Zinn Cycles in 1982 and co-founded Clydesdale Bicycles in 2017. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.
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