Spin Doctor Tech Tip – Tire Sizing

Many new riders are confused by bike tire sizing.  Hopefully the following tutorial by our Spin Doctor Tech Tip team will clear up some of the confusion.

Tire Sizing:

Bike tires are sized by a simple combination of diameter and width.  Modern road tires are sized in millimeters.  The most common road tires are 700C’s, meaning they are about 700mm in outside diameter (O.D.).  If the tire is 700 x 23 then it is ~700mm in diameter and the inflated tire is 23mm wide.  MTB tires are sized in inches, so a mounted and inflated 26 x 1.95 tire is about 26” in O.D. and 1.95” wide.

Common road tire diameters:

700C is by far the most common modern size

650C is a smaller size used on some time trial and triathlon bikes

27 inch is a less common, older American size

27” & 700Cs are not interchangeable [see ISO Sizing below]

Common mountain bike tire diameters:

26″ is the most common size

29” is based on the 700C road size (see ISO Sizing below) and is essentially a wider and knobbier 700C tire fitted to a wider and stronger 700C rim

What’s the largest and smallest tire I can put on a rim?

John Barnett of Barnett’s Bicycle Institute is very conservative when he recommends that:

A road tire can be no larger than 2 times the inside width of the rim nor smaller than 1.4 times that width

A MTB tire can be no larger than 3 times the inside width of the rim nor smaller than 1.4 times that width

A less conservative guide is: The width of the tire must be larger than the width of the rim from braking surface to braking surface. For example, a Mavic Open Pro 700C rim is 19.4 mm wide (braking surface to braking surface).  With this rim a 700 x 19 mm tire would be too narrow but a 700 x 20 would work fine.

ISO Tire measurement:

The International Standards Organization (ISO) has adopted more precise metric standards for tires and rims. Unlike traditional sizing, e.g. 26 x 1.95 or 700C, ISO sizing is based on accurate measurable dimensions.

For instance, the ISO equivalent of a 700 x 25 tire is 25-622 where 25 is the width of the inflated tire in mm and 622 is the inside diameter of the tire’s bead in mm. The bead is one of two steel or Kevlar cables that run along the inside edges of the tire casing. They keep the tire from blowing off the rim. The 622 is called the Bead Seat Diameter (BSD) because it is also the diameter of the place along the inside of the rim where the inflated tire beads will seat.

The ISO equivalent of a 26 x 1.95 mountain tire is 50-559 where 50 is the width (50mm = about 1.95”) and 559 (22”) is the BSD. Most modern tires have both measurements on their sidewalls.

Here’s a chart of common tire sizing equivalents: