TYRE SIZE CHART
Tyre size is defined by ETRTO to ensure perfect fit and safety. It is recommended to replace your bicycle’s tyres by the same size as original equipment. Tyre width and wheel diameter are specified in millimeters by ETRTO but measurement in inches may also be used as alternative.
Alternative system considers the diameter of the complete system (wheel + tyre), while the norm considers wheel and tyre separately as you can see below: