Elipse — Excentricidade

e = √(1 − b²/a²).
Created by
Renato Passos, Eng. de Software
Reviewed by
Renato Passos, Eng. de Software

Last updated: Apr 18, 2026

e
0,8000

Formula

e = √(1 − b²/a²)

About this calculator

The ellipse eccentricity calculator determines how elongated an ellipse is by measuring its deviation from a perfect circle. Eccentricity (e) is a number between 0 and 1, where 0 represents a circle and values close to 1 indicate a very elongated ellipse. To calculate, you need the lengths of the semi-major axis (a) and semi-minor axis (b). The formula used is e = √(1 − b²/a²), which directly relates the difference between the axes.

Practical use of this calculator is common in fields such as astronomy, engineering, and physics. For example, when studying planetary orbits, eccentricity determines the shape of a celestial body's trajectory. The higher the eccentricity, the more eccentric the orbit. It is also useful in designing elliptical mirrors, parabolic antennas, and lenses, where the precise shape of the ellipse is essential for performance.

Be careful when entering the axis values. The semi-major axis (a) must always be greater than or equal to the semi-minor axis (b). If you swap them, the result may be incorrect or even a square root of a negative number, which is invalid. Also, ensure that the units of measurement are consistent; eccentricity is dimensionless, so it doesn't matter if you use meters or centimeters, as long as both are in the same unit.

Another caution: the calculator assumes the ellipse is centered at the origin and that the axes are aligned with the coordinate axes. Otherwise, the eccentricity remains the same, but the formula applies similarly. Remember that eccentricity does not depend on the orientation of the ellipse, only on the relationship between the axes.

Frequently asked questions

What does an eccentricity of 0 mean?

An eccentricity of 0 means the figure is a perfect circle, since the semi-major and semi-minor axes have the same length.

Can I use this calculator for a hyperbola?

No, the formula is specific to ellipses. For hyperbolas, eccentricity is calculated differently (e = √(1 + b²/a²)).

What is the unit of eccentricity?

Eccentricity is dimensionless, meaning it has no unit. It is just a number between 0 and 1.

What if I enter b larger than a?

If b is greater than a, the value inside the square root will be negative, resulting in an imaginary number. So make sure a ≥ b.

Does eccentricity change if I rotate the ellipse?

No, eccentricity depends only on the ratio of the axes, not on the orientation of the ellipse in the plane.

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