How do I know what size tube I need for my bike?

QUESTION: How do I know what size tube I need for my bike? I bought a road bike used, and I need a new tube. A friend told me to look on the sidewall of the tire but there are a lot of numbers on there, so which one do I use? —Pat R.

RBR’S STAN PURDUM REPLIES: The very first thing to determine is whether you actually need a tube. Many bikes produced in recent years have tubeless tires. So ask the seller or Google the model of tire to determine if you need a tube or have to repair the tire itself (which is another whole article). You can also take the wheel and tire with you to your local bike shop and have them tell you, and if you do need a tube, they’ll happily sell you one in the correct size.

But assuming you are on your own and need a tube, your friend is right. The tire into which the tube goes will tell you what size, because the size of tire, which is displayed on the sidewall of the tire, corresponds to the size of the tube you need.

However, I’m using "displayed" loosely. On some tires, the size is clearly marked in a color that contrasts with the color of the tire sidewall, making it easy to read. On most bike tires, though, the size numbers are the same color as the sidewall. Often, the only information provided in a contrasting color is the tire company's name and the tire model name.

The two pieces of information you really need — the tire / tube size for buying the tube and the inflation limits for pumping it up after you have it mounted in the tire — are molded on during the manufacture of the tire, and you have to hunt for them (often with the help of a flashlight) and pick them out from other molded-on info. And sometimes the figures you need are only on one side of the tire — usually the opposite of the one you started looking on!

For example, here is what is displayed on the tires of the three different bikes in my fleet:

Continental Gatorskin (mounted on a road bike)

Side one: 28-622 * 700 x 28c * MAX. LUFTDRUCK [German for "air pressure"] / INFLATION, 8.0 BAR 116 PSI * MADE IN GERMANY * a phrase, I couldn't make out, probably in German

Side 2: MOUNT ONLY ON HOOKED RIMS * 4818, 226527 [probably codes related to the manufacture of the tire] * arrow beside the word ROTATION * Continental website URL

Moab Sweetroll (mounted on a mountain bike)

Side 1: 26 x 1.95 * MADE IN TAIWAN * FITS H/E RIM 48-559 [presumably the style of the rims on which I have them mounted] * KEEP INFLATED 45-65 LBS.

Side 2: all the same as side 1 but with the addition of a stamped code T072 [probably a code related to the manufacture of the tire]

Michelin Protek (mounted on a touring bike)

Side 1: Michelin website URL * 37 x 622, 700 x 35c * arrow beside ROTATING DIRECTION

Side 2: TUBE MICHELIN A-S [presumably a Michelin tube] * INFLATE TO 2.6 BARS (36 PSI) MIN – 6 BARS (87 PSI) MAX * MADE IN TAIWAN

Of the three, the Sweetroll is the easiest to read, mainly because it's a taller tire that makes for a larger sidewall and so the information is given in a bigger font, plus, while the tread area is black, the sidewall is a rust color which makes the information more visible.

The tube size information you want is the one that looks like an equation with an "x" in the middle and with the larger number on the left (700 x 28c for the Gatorskin; 26 x 1.95 for the Sweetroll; and 700 x 35c for the Protek). The 700 (or could be 650 in a slightly different wheel size) refers to the wheel diameter in millimeters (for road, gravel, adventure, hybrid and similar bikes, with the second number giving the width of the tire in millimeters.

In the case of the 26 x 1.95, the measurement is in inches — where the first number is the wheel diameter — and in the case mountain bikes, could be 27.5 or 29, and very old 10-speed road bikes can be 27 or 24). In the case of cruisers and children's bikes, that number could be smaller than 26. The second number in the equation is the width of the tire, in inches.

The "c" at the end of the millimeter sizes is a historic relic from an old French system that indicated the rim sizes using a, b, c and d. The letter has no use today and can be ignored.

When it comes to purchasing the tube, whether it's in the metric or inch system, the second number in the equation may be given as a range, such as 700 x 25-32c or 26 x 1.95-2.125. That's simply because tubes are stretchable enough that they can be inflated within limits to accommodate more than one tire width.

The other two number sets on some tires, which may be notated with either an "x" or a hyphen (28-622 and 37 x 622 in the Gatorskin and Protex examples above) are the equivalent sizes in a different numbering system. But in the United States the system with the larger number first is the one used most often.

One other thing before buying the tube: Get one that has the right valve stem. On most road bikes, the valve stems for inflating the tires are skinny metal devices called Presta valves. Older road bikes and many mountain bikes have valves identical to the ones on your car, which are called Schrader valves. Road bike rims tend to be narrow, and Presta valves are preferred because they require a smaller hole to fit through the rim and thus don't reduce the cross-sectional strength of the rim as much as the larger hole necessary for a Schrader valve. If your existing bike has Schrader valves, however, that's not a problem, as your rims would have been designed to accommodate that size hole.

When you are installing your new tube, if your tire has a rotation direction indicated, mount the wheel so it rolls that way. It shouldn't hurt the tire if you get it backwards, but some tires are designed to roll smoother and have better traction if mounted according to the instructions. Finally, pay attention to the inflation limits so you put enough air into the tube to carry the load without flatting but not so much as to blow the tube. Bars and PSI (pounds per square inch) are competing scales for measuring air pressure. Most pumps sold in the United States use PSI, but some give both.

Stan Purdum has ridden several long-distance bike trips, including an across-America ride recounted in his book Roll Around Heaven All Day, and a trek on U.S. 62, from Niagara Falls, New York, to El Paso, Texas, the subject of his book Playing in Traffic. Stan, a freelance writer and editor, lives in Ohio. See more at www.StanPurdum.com.