Circumference of a Circle

The circumference is the distance all the way around a circle. Given the radius, you only have to multiply by $2\pi$.

• $L$ — the circumference
• $r$ — the radius
• $\pi$ — pi, the constant $3.14159\ldots$

Since the diameter is $d = 2r$, the same formula can be written $L = \pi d$.

Example

With the default radius $r = 5$:

The circumference is about 31.4159.

What pi actually is

Divide the circumference of any circle by its diameter and you always land on the same number, no matter how big or small the circle. That number is $\pi$. It is the ratio of circumference to diameter, $3.14159\ldots$, a decimal that never ends and never settles into a repeating pattern, and it cannot be written as a fraction.

Watch out

This calculator takes the radius, not the diameter: a circle 10 across has radius 5. Keep the circumference distinct from the area $\pi r^2$. The circumference is proportional to the radius, so doubling the radius doubles the circumference, while the area becomes four times larger.