A circle is a geometric object characterized as a line of points in a plane that are equidistant from a single point. There are essentially three different measurement values used to describe the size of a circle — radius, diameter and circumference. The diameter, in particular, is described as the length of a line between two points on a circle that intersects the center point; it is equal to twice the value of the radius. The units used to describe the diameter ultimately depend on the context in which it is being measured and reported.

The most widely accepted units for scientific measurements are those identified by the metric system. The units of a linear measurement, such as diameter, are reported in meters. The value may also be reported in different derivations of the meter depending on the object being measured, including millimeters, centimeters and kilometers. For example, kilometers would be the preferred measurement units used to report the diameter of the Earth, whereas millimeters or centimeters would be the ideal units in reporting the diameter of a coin.

In the United States, the metric system is not normally used for common measurements. Instead, customary units are used such as pounds for weight measurement and inches for linear measurement. The diameter, in non-scientific situations, may therefore be reported in inches, feet or miles based on the respective size of the circular object being measured.

The circumference of a circle describes the measure of the distance around the edge of the circle. It is calculated as the measured diameter of the respective circle multiplied by the mathematical constant pi. The reported unit of circumference is dependent on the unit used for the diameter. A circumference calculated with a diameter in inches would therefore also be reported in inches.

The area of a circle is calculated as the square of the diameter multiplied by one-fourth of the constant pi. The units of area are, therefore, reported as square units of the diameter measurement. For example, the area of a circle calculated with a diameter in centimeters would be reported in square centimeters.