What Does “700C” Mean in Bicycle Tyre Sizes?

The term 700C appears on countless bicycle tyres, yet it rarely means what people assume it does. While it looks like a precise measurement, 700C is actually a legacy label from an older sizing system and does not describe the true diameter or width of a modern tyre. This article explains what 700C originally meant, why it is still used today, and how tyre sizes should be properly understood using the ETRTO standard.

The term 700C comes from an old French tyre sizing system that is no longer used in any technical sense.

In that system:

• • 700 referred to the approximate outer diameter of the tyre in millimetres
  • In the original French system, the number indicated approximate outer diameter and the letter indicated the inner diameter of the tyre. For 700C tyres, the letter C corresponds to a 622 mm bead seat diameter. Over time, this became the standard for road tyres, and the letter is now mainly a historical label.

So a 700C tyre was one of several tyres that all had a similar overall diameter, but different widths.

This system relied on the outer diameter of the inflated tyre, which varies depending on tyre width, pressure, and rim width. For that reason, it was never precise enough to be reliable long term and has since been replaced.

Even though the French sizing system is obsolete, the label 700C remained in common use.

Manufacturers, retailers, and cyclists continued using the term as shorthand for a specific family of tyres that fit the same rim diameter. Over time, 700C stopped being a measurement and became a name.

Today, when you see 700C on a tyre:

• • It does not describe the tyre’s true outer diameter
  • It does not describe tyre width
  • It simply identifies which rim diameter the tyre is designed to fit

For anything technical, the industry no longer relies on 700C alone.

The ETRTO standard is the gold standard for bicycle tyre sizing, and it is what we always use as the final reference point.

Under ETRTO, tyres are defined by:

• • Tyre width (mm)
  • Bead seat diameter (mm)

A 700C tyre always fits a rim with a 622 mm bead seat diameter.

For example:

• • A 700 x 25c tyre is an ETRTO 25-622
  • A 700 x 32c tyre is an ETRTO 32-622

The 622 mm bead seat diameter is the critical measurement. If this number does not match between tyre and rim, the tyre will not fit, regardless of what the sidewall says.

When you see a size written as 700 x 25c or 700 x 32c, both numbers matter, but they mean different things.

• • The first number (e.g. 700) is expressed in millimetres (mm) and represents the approximate overall diameter of the tyre when inflated to a specific pressure on a particular rim width.
  • The second number (e.g. 25) is expressed in millimetres (mm) and represents the approximate width of the tyre when inflated to a specific pressure on a particular rim width.

Because these measurements depend on rim width and pressure, they are only approximations. A 25 mm tyre may measure wider or narrower in real use.

This is why ETRTO sizing is preferred. The ETRTO width and bead seat diameter provide a consistent reference that does not change between brands or setups.

From an ETRTO perspective, 700C, 28 inch, and 29 inch tyres can all use the same 622 mm bead seat diameter.

The difference is not the rim diameter, but the tyre width and intended use.

• • 700C is typically associated with road, gravel, and hybrid tyres
  • 29 inch is typically associated with wider mountain bike tyres
  • 28 inch is a legacy term still seen in some regions (particularly Europe), often referring to road tyres

Despite the different names, tyres labelled 700C and 29 inch often fit the same rims, provided the ETRTO bead seat diameter is 622 mm and there is sufficient frame clearance for the tyre width.

Some common examples you will see:

• • 700 x 25c: Narrow road tyre. Approximate measurements depend on rim width and pressure. ETRTO reference: 25-622.
  • 700 x 28c: Common modern road size offering more comfort. ETRTO reference: 28-622.
  • 700 x 32c: Popular for endurance road, commuting, and light gravel. ETRTO reference: 32-622.
  • 700 x 40c: Wider gravel tyre. Same bead seat diameter, very different overall diameter when inflated. ETRTO reference: 40-622.

In all cases, the tyre fits the same 622 mm rim. Only the width and resulting outer diameter change.