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