Diameter

A diameter of a circle is a line segment with both endpoints on the boundary which passes through the center. The word is also used to mean the length of such a segment. Diameters are the longest chords that a circle has. A diameter is twice as long as a radius (this formula is sometimes written d = 2r).

The diameter d is related to the circumference C by the formula

C = π d,

where π is pi, an irrational number approximately equal to 3.14.