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This is the Ultimate Ellipse Page, everything you ever wanted to know/need to know about ellipses and of course, some calculators for those who are in Algebra-2/Geometry.  First there are some definitions, just to refresh your memory:

An ellipse is the locus of points the sum of whose distances from two fixed points, called foci, is a constant.
Line AB is the Major Axis (also called Long Axis or Line of Apsides).
Lines AO and OB are the Semi-Major axes.
Line CD is the Minor Axis and is the perpendicular bisector of the Major Axis.
Points f₁ and f₂ are the foci of the ellipse.
Points A and B are called apses.

The eccentricity of an ellipse equals (f₁ f₂ / AB)

As the eccentricity value goes from 0 to 1, the ellipse goes from circular to highly elongated.

The ratio of the Minor Axis to the Major axis, which we will call Y/X, is equal to the square root of (1-e²).

The perimeter of an ellipse approximately equals
The area of an ellipse equals PI * Line AO * Line CO which equals
PI * (Major Axis * Minor Axis)/4 .

Johannes Kepler's First Law states that the planets move in elliptical orbits with the Sun located at one of the foci.
If we were dealing with planetary orbits and we were to say the Sun were at f₁ then Line A f1 would be the perihelion distance, Line f1 B would be the aphelion distance and the planet's average (or mean) distance would be one half of the major axis.

Now the calculators:

Eccentricity and X/Y ratio Converter

This calculator converts from one to another, just click what you know and then put it in.  After that click Convert and you've got it.

The Ultimate Ellipse Calculator

This one gives a lot of information taken from very little.  First choose what you know and then click Let's Do It and it'll do it.