Simple formula for the focus position f I think the article would benefit from an easy to find and simple formula for the position of the focus as a function of the radii especially for drawing ellipses with office programs where the width 2a and the height 2b are usually given If I read correctly then the focus is at and the eccentricity is Taking the square of both equations results in a very simple formula or after taking the square root the square I found this equation also in the eccentricity subsection So I ll split this off and make a new subsection named Focus