Perimeter (circumference) of an Ellipse
The total distance around the line that forms the ellipse
When the circumference of a circle is so easy to find, it comes as a surprise that there is no easy way to find the circumference of an ellipse. There is simply no easy way to do it. Calculating it precisely requires the use of calculus, and is beyond the scope of this volume. If the ellipse has equal major and minor axes, it is a circle and the circumference is obvious.
There are however several formulae that give an approximation to the perimeter, some of which are surprisingly accurate, such as the one by Ramanujan. Some are described in the links below.

Other ellipse topics