The diameter of a circle is the line segment that passes through the center of the circle and has its two endpoints on the circle. It is the longest distance from one end of the circle to the other and is twice the radius.

Shown below is the diameter of a circle.

Symbol

Diameter is also sometimes represented by the symbol ‘∅’. It is used in technical drawings or specifications as a prefix or suffix for a number. For example, ‘∅ 66 cm’ is read as ‘diameter 66 cm’.

Formulas

Calculating the diameter of the circle is easy if we know any of the other dimensions such as the circumference, radius, or area. It is also possible to figure out the diameter if we do not have any of the dimensions but the circle is drawn. We will discuss each of the situations in detail.

How to Find the Diameter of a Circle Using the Circumference

When the circumference of the circle is known, the equation to calculate the diameter is derived below.

As we know,

Circumference (C) = πd, here π = 3.141 = 22/7, d = diameter

=> d = C/π

Thus,

Diameter (d) = C/π

Let us solve an example.

Find the measure of the diameter of the circle with circumference 20 feet.

Solution:

As we know, Diameter (d) = C/π, here C = 20 feet, π = 3.141 = 20/3.141 = 6.36 feet

How to Find the Diameter of a Circle from the Area

When the area of the circle is known, the equation to get the diameter is derived below.

As we know,

Area (A) = πd^{2}/4, here d = diameter, and π = 3.141 = 22/7

=> d^{2} = 4A/π

=> d = √4A/π

What is the diameter of the circle with an area of 22 m^{2}.

Solution:

As we know, d = √4A/π, here A = 22 m^{2}, π = 3.141 = 22/7 = √4 × 22/3.141 = √28.01 = 5.29 m

How to Find the Diameter of a Circle Using the Radius

When the diameter of the circle is known, the equation to calculate the radius is derived below.

As we know,

Diameter (d) = 2 × r, here r = radius

Determine the diameter of the circle having a radius of 16.5 inches?

Solution:

As we know, Diameter (d) = 2 × r, here r = 16.5 = (2 × 16.5) inches = 33 inches

How to Find the Diameter of a Circle from the Drawn Circle