There are three main standards for measuring bicycle tire dimensions, which you'll find on one (or both) sidewalls of the tire:

ETRTO

European Tire and Rim Technical Organization

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

English standard

Inches

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 outer diameter of 26 inches and a width of 2.10 inches.

French standard

Millimeters

This standard uses two numbers followed by a letter. The first number indicates the tire's outer diameter in millimeters, the second number its width in millimeters, and the letter corresponds to the inner 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 inner diameter of 622 mm.