Diameter

A diameter is a straight line segment that connects two points on a circle and passes through its center. It is the longest line that can be drawn inside a circle, and it divides the circle into two equal halves.

Diameter is usually denoted by the letters d or D. It is equal to twice the radius (the distance from the center to the edge of the circle): d = 2r. The term is also used for three-dimensional shapes such as spheres and cylinders.

Related calculations

The circumference of a circle is calculated as C = πd, while its area is A = πr² or A = πd²/4. Here π (pi) is a constant approximately equal to 3.14159, representing the ratio between a circle's circumference and its diameter. This ratio is always the same, regardless of the circle's size.

Geometric properties

Any angle inscribed in a circle with the diameter as its base is a right angle (90 degrees); this is known as Thales' theorem. Similarly, every diameter is an axis of symmetry that divides the circle into two halves that are mirror images of each other.

Technical applications

Diameter is a fundamental measurement used to specify tubes, spherical objects, tires, and engineering equipment. When referring to 'a tube with a diameter of 10 centimeters', this usually means either the inner or outer diameter, depending on the industrial standard being followed.