How Many Numbers Does It Take To Change A Tire?
One piece of cycling knowledge that is supremely frustrating is tire sizing. The late Sheldon Brown, was, undoubtedly, the foremost authority on bike tire sizing, and even he had to admit that "bicycle tires come in a bewildering variety of sizes."
For tire sizes, we primarily use the The European Tyre and Rim Technical Organisation (ETRTO) method of measurement. The ETRTO format has been adopted by the International Organization for Standardization (ISO) and is recognized in the industry as a reliable and uniform method for tire sizing.
An ETRTO size is comprised of two millimeter measurements, separated by a hyphen: "Tire Width - Rim Diameter." A tire reading "50-406" is 50mm wide and 406mm in diameter. For convenience, we've included traditional units beside the ETRTO listings on our website.
To find your size, simply search your tire until you find your ETRTO measurement (## - ###). Remember, if you want to change the width to a skinnier or fatter tire, then you want to get a tire with a smaller or larger first number, but be sure to keep the second number the same. For example, a 50-406 and a 40-406 will fit the same rim on a bike; the difference is that the 50-406 will be a fatter tire. But a 50-406 and a 50-355 will not fit the same rim on a bike, even though they're the same width.
Click here for a good explanation of the difference between ETRTO, Inch and French readings.
Click here for Sheldon Brown's own website about the trials and tribulations of tire sizing.