How do you find the area of an isosceles triangle?

Use one half of the base times the perpendicular height. If height is not given, recover it from the equal side and half the base or from the given angle.

Can an equilateral triangle be isosceles?

Yes. An equilateral triangle is a special case with all three sides equal, but this calculator is for the more common two-equal-sides workflow.

Why does the calculator reject my base and side?

Twice the equal side must be greater than the base. Otherwise the sides cannot meet above the base to form a triangle.

Isosceles Triangle Area Calculator

Calculate isosceles triangle area from base and equal side, base and height, equal side and height, apex angle, or base angle, with perimeter, height, angles, inradius, and circumradius.

Known values

Unit

Base (m)

Triangle inequality: twice the equal side must be greater than the base.

Equal side (m)

Isosceles triangle area from base, side, or angle

English search results for this intent focus on a practical isosceles triangle solver: users may know the base and equal side, base and height, an equal side with height, or an angle. The calculator recovers the missing height first, then reports area and related triangle properties.

A = \frac{1}{2} bh

A is area; b is the base; h is the height to the base.

h = a^{2} - (\frac{b}{2})^{2}

h is height; a is an equal side; b is the base.

P = 2 a + b

P is perimeter; a is an equal side; b is the base.

Supported known values

Known values	What is calculated
Base and equal side	Height, area, perimeter, base angles, apex angle, inradius, and circumradius.
Base and height	Equal side first, then area and angles.
Equal side and height	Base first, then area and angles.
Base or equal side with apex angle	Missing side or base, height, area, and perimeter.
Base and base angle	Height, equal side, area, and apex angle.

A single equal side does not define one isosceles triangle. A second measurement such as base, height, or angle is required.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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