In geometry, the diameter of a circle is a line from one side directly to the opposite side through the center. It can also be defined as the longest chord of a circle. The same explanations can be also used to describe the diameter in a sphere.

The letter d or the symbol ⌀ are often used as a symbol for the diameter.^[1] The diameter's length is often called the diameter itself.

The diameter's length is twice the radius,^[2] and therefore the relationship between the diameter's length and radius can be shown with the equations:^[3]

${\displaystyle d=2r}$ and ${\displaystyle r={\frac {d}{2))}$

• Circumference
• Pi

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