Diameter

Diameter

In geometry, a diameter of a circle is a straight line segment that passes through the center of the circle and ends on the circle. It is the longest chord of the circle. We can also talk about the diameter of a sphere.

When we talk about the diameter, we often mean the length of this line. For any circle or sphere, all diameters have the same length. This length is always twice the length of the radius, which is the distance from the center of the circle to any point on its edge.

The word "diameter" comes from Ancient Greek. It is sometimes shortened to DIA, dia, d, or ∅.

Constructions

You can make a diameter of a circle using a straightedge and compass. First, pick any line across the circle, called a chord. Then, draw a line that cuts this chord exactly in half and goes straight through the middle of the circle. If you draw two of these lines, where they cross is the center of the circle.

To make a circle with a specific line as its diameter, find the middle point of that line. Then, draw the circle so it centers on that middle point and goes through one end of the line.

Symbol

"⌀" redirects here. For other uses, see ⌀ (disambiguation).

The symbol ⌀ is used to show the diameter of something. For example, "⌀ 55 mm" means the diameter is 55 millimeters. People also use this symbol for sizes of camera filters.

The symbol has a special code in Unicode called U+2300 ⌀ DIAMETER SIGN. It is part of the Miscellaneous Technical set. It looks similar to some other symbols, like Ø or ∅, but these have different meanings.

Generalizations

The idea of a diameter can be used for many shapes, not just circles and spheres. For any group of points, the diameter is the farthest distance between any two points in that group.

For shapes like ellipses, a diameter is any line that passes through the center of the shape. The longest diameter of an ellipse is called the major axis, and the shortest is called the minor axis.

We can also find an equivalent diameter for objects that are not perfect circles or spheres. This is the diameter of a circle or sphere that would work the same as the object. For example, the hydraulic diameter measures the effective size of a pipe.

The diameter of a circle is always twice its radius, but this rule only works for circles in flat geometry. Other shapes have different relationships between their size and their diameters.