Diameter of a Circle

Diameter of a circle

The diameter of a circle is the straight-line distance across the circle passing through its center — it's the longest distance you can measure inside the circle and is exactly twice the radius.

Formula

d = 2r

(or equivalently r = d / 2)

Where:

d = diameter

r = radius

How to Calculate the Diameter?

You usually calculate the diameter given the radius, circumference, or area.

1. If you know the radius (r)

d = 2 × r

(Simplest and most direct method.)

2. If you know the circumference (C)

d = C / π

π ≈ 3.14159 (or use 3.14 for quick estimates).

3. If you know the area (A)

First, find the radius:
r = √(A / π)

Then: d = 2 × r

Or directly: d = 2 × √(A / π)

Circle Circumference Calculation Examples
Example 1 – From radius

Radius = 10 cm
d = 10 × 6

d = 20 cm

Example 2 – From circumference

Circumference = 31.42 inches (using π ≈ 3.14159)
d = 31.42 / 3.14159

d ≈ 10 inches

Example 3 – From area

Area = 78.54 m² (using π ≈ 3.14159)

First, r = √(78.54 / 3.14159) = √25 = 5 m

Then, d = 2 × 5

d = 10 m