Step 1 Inspect the sidewall of the bicycle tire that needs a new tube. The sidewall is the area of the tire next to the rim on either side that does not touch the ground when the bike is in motion. The tire size will be printed there in raised numbers along with the recommended tire pressure and possibly other information. Alternatively, check the tube that needs replacing or your bike’s owner's manual for the information. The tire size notation will consist of the wheel diameter and the tube width separated by the letter "x." For example, 26 x 1.95 means that the wheel diameter is 26 inches and the tire width is 1.95 inches. Although mountain bike tires are 26 inches in diameter, you must still find the width. Step 2 Choose a tube with the same diameter number and a range of widths into which your tire falls. In the previous example, a tube labeled 26 x 1.9-2.125 has the correct diameter of 26 inches. The width of your tire, 1.95 inches, falls within the range of the widths displayed on the tube package, or 1.9-2.125. This is the correct tube size for your bike. Step 3 Learn what tubes will fit your tires if you can’t find the exact right size. Because the stated tire width is usually slightly larger than the actual tire and because the tube stretches, tubes close in width serve just as well. For example, choose a tube for a mountain bike if the width is within ¾ inch of the tire’s stated width. Use a tube within 3/8 inch of the stated width for road bikes. A tube with a close diameter measurement will also work. For example, a 700c tube can be substituted for a 27-inch tube and vice versa.