Bicycle tyre sizes explained: ETRTO, 700C, 650B and 29 inch

Schwalbe explains the ETRTO designation using 37-622: the first number describes tyre width, the second its inner diameter. In a tyre/rim comparison, that diameter belongs to the bead-seat interface. Write it as a named quantity rather than simply wheel diameter. The rubber’s outer surface is elsewhere. A tyre marked 37-622 is not a 622 mm outside circle and its 37 mm label is not an observation of the installed casing.

700C commonly denotes the 622 mm bead-seat family. The commonly used 29-inch mountain-bike family also uses 622 mm, while 650B/27.5 corresponds to 584 mm. These are interface identifications, not statements that every tyre in a family has the same exterior dimensions. Check the actual marking on the particular product, especially for older equipment or a listing that abbreviates its size.

The letter in a traditional French designation is part of the designation: do not discard it as decoration. Likewise, an inch label does not uniquely resolve every historical size. Schwalbe lists multiple inner diameters under some inch names. A shopping page that says only 26 inch therefore leaves a question unanswered. Preserve the complete millimetre designation instead of selecting whichever result has the closest-looking name or picture.

Imagine an ideal fictional 622 mm bead-seat circle with a tyre extending 35 mm radially above that reference on each side. Its outside diameter in that simplified construction is 622 + 2 × 35 = 692 mm; outside radius is 346 mm. The 35 mm radial extension was declared separately. It was not extracted from the width part of a tyre label.

This is geometry, not a method of predicting real tyre height from nominal width. A mounted casing has its own profile and conditions. Writing 700C into a field for measured 700 mm diameter confuses catalogue language with an observation. Writing half the bead-seat diameter into a front-geometry wheel-radius field instead omits the tyre altogether. Preserve the original label, then obtain the particular quantity the next task actually needs.

Write nominal tyre width, bead-seat diameter, mounted outside dimensions and rolling circumference independently. Add the source and measurement condition to each. A manufacturer label can supply an identity dimension while a physical reading supplies an installed dimension. A model-derived radius is a third kind of value. Do not remove those origin labels just because all the numbers use millimetres.

If you are preserving a setup rather than changing it, use a blank note and record what is accessible. The next width guide explains the installed envelope; the compatibility guide keeps permissions separate; the final guide distinguishes rolling distance from a static radius. Reading any of them creates no bike profile. The free worksheet is a paper-first next step, not a recommendation to copy these fictional dimensions onto a bicycle.