How to choose your inner tube
Check all the information indicated on the sidewall of the tyre (diameter and width). For example, 20 x 2.00 means your tyre is 20 inches in diameter and 2.00 inches in width.
On the sidewall, you will also find the dimensions in mm in international ETRTO measurements.
For example 50-406 means the tyre is 50 mm wide with an inner diameter of 406 mm (or 20 x 2.00).