Bicycle tyre sizes: ETRTO, 700C and inch labels

A listing says “700C”, another says “29-inch”, and your tyre has “37-622” on its sidewall. Those labels are not three ways to write the same measurement. Before comparing replacements, separate the size system from the individual dimensions it describes.

This guide helps you read the markings and prepare a useful question for a seller. It does not establish whether a particular tyre fits your bicycle. Diameter, width, rim construction and available space still need checking against the exact equipment instructions.

Read the two ETRTO numbers separately

In an ETRTO tyre marking, the numbers on either side of the hyphen have different jobs. Michelin's size guide explains that the first gives the tyre's width under specified rim and pressure conditions; the second gives its bead-seat diameter. Both are expressed in millimetres.

For 37-622, read 37 mm as the nominal width and 622 mm as the bead-seat diameter. The latter concerns where the tyre meets the rim, not the outside diameter of the inflated tyre. Do not read the hyphen as a subtraction sign or combine the two numbers into one model code.

Hypothetical markings: separate width from diameter

Marking	Nominal width	Bead-seat diameter
32-622	32 mm	622 mm
37-622	37 mm	622 mm
37-590	37 mm	590 mm

The first two examples share a diameter but differ in width. The last two share a width but differ in diameter. Neither comparison is a fit recommendation. The table is a reading exercise, not a compatibility chart or a list of products we have measured.

Keep inch and French labels in context

Schwalbe's size-designation guide explains why inch labels can be ambiguous: “26-inch” can refer to bead-seat diameters including 559, 571 or 590 mm. “28-inch” can refer to 622 or 635 mm. A matching large number on two listings is therefore insufficient.

Michelin also explains that 700C road tyres and 29-inch mountain-bike tyres can share a 622 mm bead-seat diameter while having different widths. The traditional outer-size labels do not promise an exact measured outside diameter. Do not multiply an inch label by 25.4 and treat the result as the required bead-seat diameter.

When taking notes, retain every designation shown. Put the ETRTO marking in its own field and the inch or French wording alongside it. If a listing supplies only “28-inch”, leave the missing metric detail unresolved rather than filling it in from a familiar bicycle.

Matching diameter is only one check

Schwalbe's dimension guidance says tyre and rim diameters must match, and their widths must also be compatible. Mounted tyre width can vary with the rim and pressure, so a nominal width is not a guaranteed tape-measure reading on every setup.

Frame clearance and the permitted tyre/rim combination remain separate checks. Hookless rims, in particular, have manufacturer requirements about compatible tyres and operating limits. Use the exact rim, tyre and bicycle instructions; this guide supplies no mounting method, pressure setting or universal clearance allowance.

Our racket head-size guide makes a related distinction between a published specification and a calculation. Here, too, writing down a size correctly does not mean you have physically measured or approved the equipment.

Send a question that preserves the missing detail

Make a short comparison note with four entries: the current tyre's complete marking, its brand and model, the proposed replacement's full marking, and the rim/bicycle model. A clear sidewall photograph helps preserve characters you cannot confidently transcribe. Mark unreadable text as unreadable.

For an incomplete listing, ask: “Please confirm the tyre's full ETRTO marking and exact model. Where can I find the manufacturer's compatibility information for my rim and bicycle?” Keep the reply with the listing rather than replacing the original wording in your notes.

A product reference and a size marking answer different questions. Our product-code guide explains why keeping field names matters. For tyres, record the model reference separately from the dimensions instead of treating either as a substitute for the other.

Practical takeaway

Read both ETRTO fields, preserve the other labels, and ask about missing information. A shared diameter narrows the comparison; it does not settle width, clearance or rim-type compatibility. Resolve those questions using the exact manufacturers' instructions before choosing a replacement.