If you've shopped for a bike recently, or even if you're an experienced rider who's ever shopped for new tires or inner tubes, you've come up against the confusing arrary of sizes available. What do all those numbers mean? And why have so many choices in the first place? Why don't they come up with a standard?
As we used to say in the computer industry, the great thing about standards is that there are so many to pick from. It may sound surprising to you that there actually used to be many times more varieties of tire than are common today. In the early years from the invention of the bicycle through the early 1900's, many bike manufacturers had their own set of sizing standards that only applied to their own bikes. It is a testament to how far we've come that today you can buy tires, tubes, and wheels from three different manufacturers and be sure that they will all work fine together, as long as you select the proper sizes of each.
There are four groups of tire sizes that are commonly used on standard adult bikes today. Each group is defined (roughly, as we'll get into shortly) by the diameter of the tire, with varying widths available within the same diameter.
The most common tire (and wheel) size group is called 700C. These are found on most road bikes and hybrid bikes. They come in widths ranging from 20-23 millimeters for fast road bikes, to 38-42mm wide for hybrids and touring bikes.
A question we are often asked is "what does 700C mean?" The name comes from years back when there were 700A, 700B, 700C, and 700D tires available; the letters referred to different widths. All of these tires were 700mm in diameter, measured from one outer edge to the other. However, they all had different inner diameters; this measurement is also referred to as the "bead seat diameter," or BSD for short, because it's the diameter of the tire's "bead," i.e. the part that hooks onto the wheel's rim.
For various reasons, 700A, 700B, and 700D eventually fell out of common use, and 700C became the de facto standard. The standard BSD of 700C tires was 622mm, and over time up through the present, the term 700C came to be used for any tire with a BSD of 622mm, no matter what the width. This can be a source of confusion, because some tires will have the size stamped on the tire itself using the BSD measurement. Thus, a tire labeled 25-622 is 25mm wide with a 622mm BSD, or what would more commonly be called a "700x25."
The upside of this is that any modern tire labeled as 700C will work on a modern 700C rim, as long as the widths are close enough. The downside of this is that there is a huge variation of the "true" outer diameter among 700C tires; for example, a 700x20 road bike tire may be as small as about 660mm in diameter. Another downside of the variations in tire diameter (as well as width) is that there is no foolproof way to know whether any given tire will fit in your frame, other than trial and error.
The next common size is 27-inch tires. This size was popular on the "10-speed" bikes that were common from the 1960's through the 1980's, but not many (it would probably be safe to say "none") bikes are being made today based on this wheel size. However, replacement 27-inch wheels are still available for these bikes, and the tires will probably continue to be produced for years to come. 27-inch tires use the same inner tubes as 700C tires, but the tires themselves are NOT the same. So, you should NOT use the terms "27-inch" and "700C" interchangeably.
The prevalent tires on mountain bikes and comfort bikes is 26-inch. This size first became common on beach cruiser bikes beginning in the 1930's. In the 1970's, when the early mountain biking pioneers first cobbled together their bikes, 26-inch tires became the de facto standard, because the 26-inch balloon tires on the beach cruisers were all that were available in a tire wide enough to handle off-road terrain. Today, 26-inch tires are commonly available in widths from 1.25 inches up to 2.5 inches. The standard bead seat diameter on modern 26-inch tires is 559mm. As with 700C tires, the true outer diameter of a 26-inch tire will vary from one brand and model of tire to the next.
Finally, some triathlon/time trial bikes and smaller-framed women's-specific bikes use 650C tires. They are typically one inch in width. Contrary to common belief, 650C tires are NOT the same as 26-inch tires. 650C uses a BSD of 571mm. However, similar to 700C and 27-inch, 650C and 26-inch inner tubes are interchangeable, given a close enough width.
What about the new 29er mountain bike wheels, you may ask? The name "29er" comes from the fact that the fatter tires have an outer diameter that is near 29 inches. However, the rims use the same BSD of 622mm that is standard for 700C tires, so the growing popularity of the 29er market has not complicated things as much as it could have.
One new complication is a resurgence of the 650B tire size (with a 584mm BSD) among some touring and mountain bike enthusiasts. Many claim that this size provides the "best of both worlds" when comparing the pros and cons of 26-inch versus 29er/700C wheels. We'll leave that discussion for another day...
I have to credit the late, great Sheldon Brown for providing reference for much of this article. If you want even more details and history of the tire size issue, read his article on the subject here.