# Online Encyclopedia

# Circumference

The **circumference** is the distance around a closed curve. Circumference is a kind of perimeter.

### Circle

The circumference of a circle can be calculated from its diameter using the formula:

*c* = π*d*

Or, substituting the radius for the diameter:

*c* = 2π*r*

Where *r* is the radius and *d* is the diameter of the circle, and π (the Greek letter pi) is the constant 3.141 592 6...

### Ellipse

The circumference of an ellipse is more problematical, the exact solution being an infinite series. A good approximation is Ramanujan's:

where *a* and *b* are the ellipse's semi-major and semi-minor axes, respectively. The two are related by the ellipse's eccentricity as follows:

Which means the circumference can also be written as:

## External link

- Numericana - Circumference of an ellipse http://home.att.net/~numericana/answer/ellipse.htm#elliptic

Last updated: 02-10-2005 01:52:47

Last updated: 02-11-2005 17:47:38