# Perimeter of an Ellipse

On the Ellipse page we looked at the definition and some of the simple properties of the ellipse, but here we look at how to more accurately calculate its perimeter.

## Perimeter

Rather strangely, the perimeter of an ellipse is **very difficult to calculate**!

There are many formulas, here are some interesting ones.

Note: **a** and **b** are measured from the edge to the center, so they are like "radius" measures.

### Approximation 1

This approximation will be within about 5% of the true value, so long as **a** is not more than 3 times longer than **b** (in other words, the ellipse is not too "squashed"):

### Approximation 2

The famous Indian mathematician **Ramanujan** came up with this better approximation:

### Infinite Series 1

This is an **exact formula**, but it requires an "infinite series" of calculations to be exact, so in practice you still only get an approximation.

Firstly you must calculate e (the "eccentricity", **not** Euler's number "e"):

Then use this "infinite sum" formula:

Which may look complicated, but expands like this:

The terms continue on infinitely, and unfortunately you must calculate a lot of terms to get a reasonably close answer.

### Infinite Series 2

But my favorite **exact formula** (because it gives a very close answer after only a few terms) is as follows:

Firstly you must calculate "h":

Then use this "infinite sum" formula:

(Note: the is the Binomial Coefficient with half-integer factorials ... wow!)

It may look a bit scary, but it expands to this series of calculations:

The more terms you calculate, the more accurate it becomes (the next term is 25**h**^{4}/16384, which is getting quite small, and the next is 49**h**^{5}/65536, then 441**h**^{6}/1048576)

## Comparing

Just for fun, I used the two approximation formulas, and the two exact formulas (but only the first four terms, so it is still just an approximation) to calculate the perimeter for the following values of **a** and **b**:

a |
b |
Approx 1 | Approx 2 | Series 1 | Series 2 | Exact* | |
---|---|---|---|---|---|---|---|

Circle |
10 | 10 | 62.832 | 62.832 | 62.832 | 62.832 | 20π |

10 | 5 | 49.673 | 48.442 | 48.876 | 48.442 | ||

10 | 3 | 46.385 | 43.857 | 45.174 | 43.859 | ||

10 | 1 | 44.65 | 40.606 | 43.204 | 40.623 | ||

Lines |
10 | 0 | 44.429 | 39.834 | 42.951 | 39.884 | 40 |

*** Exact:**

- When
**a=b**, the ellipse is a circle, and the perimeter is**2πa**(62.832... in our example). - When
**b=0**(the shape is really two lines back and forth) the perimeter is**4a**(40 in our example).

They all get the perimeter of the circle correct, but only **Approx 2** and **Series 2** get close to the value of 40 for the extreme case of b=0.

My guess is that Infinite Series 2 is the best of them.