The most common road bike wheel size with an ETRTO of 622mm, the same as a 29" wheel.
Where the difference lies is in the width/volume of the tire. 700C tires used on the road are generally skinnier in the 23mm - 38mm width range while 29" tires are used for mountain bikes and can range from 50mm - 71mm (2.0" - 2.8").

The "700" used to refer to the outside diameter of the tire in millameters when inflated while the "C" in 700C refers to the Bead Seat Diameter (BSD). Since the outside diameter of a tire will vary depending on the width , 700mm is not an accurate representation of a tire size, it is simply used as a legacy term. The BSD or ETRTO is a consistent measurement that doesn't vary from a 23mm tire to a 32mm tire width.

For example:
700 x 23C has a BSD of 622mm and a width of 23mm
700 x 25C has a BSD of 622mm and a width of 25mm
700 x 32C has a BSD of 622mm and a width of 32mm

The outside diameter of the inflated tire in each of the examples above will vary hence the 700mm term is inaccurate. A 32mm tire width may have an outside diameter of 708mm while a smaller 23mm width tire has a outside diamter of 698mm.


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