How to choose an inner tube
Check all the information indicated on the sidewall of the tyre (diameter and width). For example, 20 x 1.60 means your tyre is 20 inches in diameter and 1.60 inches wide.
On the tyre sidewall you will also find the dimensions in mm in ETRTO international units. For example, 40-406 means a tyre that is 40 mm wide and has an inner diameter of 406 mm (or 20 x 1.60).