There are three main standards for measuring bicycle tire dimensions, that you will find on one of the sidewalls of the tire :

Inches

English standard

This standard is commonly used for mountain bikes, BMX bikes and children's bikes. It expresses the outer diameter of the tire in inches. For example, a 26x2.10 tire has an outside diameter of 26 inches and a width of 2.10 inches.

ETRTO

European Tire and Rim Technical Organization

This system uses millimeters to indicate both the inside diameter of the tire and its width. For example, an ETRTO 40-559 tire has an inner diameter of 559 mm and a width of 40 mm.

French standard

Dimension expressed in millimeter

This standard uses two numbers followed by a letter. The first number indicates the outer diameter of the tire in millimeters, the second number indicates its width in millimeters, and the letter corresponds to the internal diameter of the rim. For example, a 700x23C tire has an outer diameter of 700 mm, a width of 23 mm and is designed for a rim with an internal diameter of 622 mm.