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I need to replace a tube in my bike. I know there are many tire sizes. I bought 26', assuming that my bike was that size, and realized when I got home that I didn't even know where to measure from. What is the correct location to measure in order to get the correct size of tubes?

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Unless your bicycle is very special-purpose or old, by far the most common is 20 (bmx), 26 (mountan-bikes for example) and 700 (road bikes). Also, not only the wheel size, but also the WIDTH is necessary to choose the proper tube. Skinny tires need skinny tubes, and so on. By the way, did you have any problem with the tube you bought? Did it fit? – heltonbiker Nov 11 '11 at 18:41

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If you still have the old tire, then the easiest method is to check the markings on the tire. But if you don't, then you have to measure the rim's bead seat diameter (see illustration). Here are the most common sizes:

  • 622 mm : 700c, or 29 inch. Common on road bikes and some mountain bikes.
  • 630 mm : 27 inch. Used to be common on road bikes, now obsolete.
  • 559 mm : 26 inch. Most mountain bikes, some hybrids.

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Source: Sheldon Brown. See his article for more obscure sizes, and an explanation of what it all means.

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Note that tubes are interchangeable between 700c and 27". Tires don't stretch, so you need the exact diameter; tubes stretch quite a bit, so you have a margin of error of at least a few millimeters. – Mike Baranczak Nov 11 '11 at 16:35
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As a side note tires sizes on older bikes were not standardized.Not all 26inch tires from different bike makers will interchange. A 26x13/8" will not interchange with a 26x1.90 tire, the tubes however will.Always reference the size on the side wall of the tire rather than trying to measure the wheel diameter.

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Look at the tire. It will have the size embossed on the sidewall -- either inches diameter & width (like 26x1.5) or metric sizes (like 700C-32). Use a tube that says it's for that size.

Tubes generally are made to fit a range of sizes. Eg, a single tube may fit both 27" and 700C tires, and 1.25-1.5 inches or 32-38 mm wide.

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