Formulas, without the guesswork.
Choose a relationship from the measurements you know. The variable names below belong to each calculator; a letter can have a different role in another method. π is evaluated using the browser’s full numerical constant, rather than replacing it with 3.14.
Two semi-axes
Inputs: Semi-major axis a (length units); Semi-minor axis b (length units).
Full axis lengths
Inputs: Major axis length (length units); Minor axis length (length units).
Area and semi-major axis
Inputs: Area A (square units); Semi-major axis a (length units).
Semi-major axis and eccentricity
Inputs: Semi-major axis a (length units); Eccentricity e (unitless).
Reading the notation
√ means a square root, ∛ means a cube root, and a superscript ² or ³ means a square or cube. Sine, cosine and tangent use angles converted from entered degrees. Where an inverse trigonometric function is used, its displayed angle is converted back to degrees. The ellipse perimeter is approximate; other formula labels describe the ideal mathematical model.
Semi-axes are half the full axis lengths. The approximation label applies to perimeter, not to the area formula.
Reference reading: Ellipse at Wolfram MathWorld.