Find the Numbers
Tube sizes are marked on the box of a new tube. Match this tube size to the size of your tire. Find the size of your tire by looking on your tire's sidewall. This will be marked as two numbers separated by the letter x.
The first number on the tube box and the tire sidewall refers to the diameter of the tire. Typical sizes are 26 (inches) and 700 (millimeters). Make sure the tube you buy has the same diameter as your tire.
The second number on the tube box and sidewall is the tire's width. You'll notice only one number on the sidewall, but a range of numbers on the tube box. For example, your tire may read 700x23, but the tube box will say 700x18-23. Don't get confused by the width range on the tube box. This simply means that the tube will fit a tire with a width measuring from 18mm to 23mm.
Choose a Range
If your tire's width falls within the range of two different tubes, the tube with the smallest number range will be lighter. For example, your tire's width is 23mm, but there are two tube choices that will fit: 18-23 and 23-25. In this case, the 18-23 tube will be lighter and add less weight to your bike.
Match the Valve
Tubes come with two different valve styles. Presta valves are more narrow and screw open. Schrader valves are what you see on car tires. If you choose the wrong valve, it will either not fit through the rim of your tire, or may make you more prone to flat tires.
Article Written By Kathrine Cole
Kathrine Cole is a professional outdoor educator. She teaches rock climbing, backpacking, cycling, and bike maintenance classes. She is a graduate of the National Outdoor Leadership School, a Wilderness First Responder, and a Leave No Trace Trainer.