How to find what tube size you need.
Tube and tire sizes are measured in 3 ways. Diameter, width, and valve length.
Example: 26" 1.75-2.125, 48mm
The Diameter is the first number - 26"
The Width range is next - 1.75-2.125
Finally is the valve length - 48mm
Kids bikes are usually 12" to 24" in diameter.
Adult bikes are usually 26", 27.5, 29" or 700c.
To find out what size you'll need, simply look at the side of your tire. You should find a number either indented on the side of the tire in black, or painted in white. Simply match the size of the tire to the tube.
Sometimes the width of the tire will not exactly match the tube size. Tubes are meant to stretch like a balloon.
Example, your tire size is 26"x1.95, the correct tube you should get is a 26" 1.75-2.125. This is because the second number, 1.95, is within the range of 1.75-2.125. It falls just in the middle.