a (leg)
b (base)
h (from apex)
h (from other two vertices)
Vertex angle
Base angle

Isosceles triangle calculator is the best choice if you are looking for a quick solution to your geometry problems. Find out the isosceles triangle area, its perimeter, inradius, circumradius, heights and angles - all in one place. If you want to build a kennel, find out the area of Greek temple isosceles pediment or simply do your maths homework, this tool is here for you. Experiment with the calculator or keep reading to find out more about the isosceles triangle formulas.

What is an isosceles triangle?

An isosceles triangle is a triangle with two sides of equal length, which are called legs. The third side of the triangle is called base. Vertex angle is the angle between the legs and the angles with the base as one of their sides are called the base angles.

Properties of the isosceles triangle:

  • it has an axis of symmetry along its vertex height
  • two angles opposite to the legs are equal in length
  • the isosceles triangle can be acute, right or obtuse, but it depends only on the vertex angle (base angles are always acute)

The equilateral triangle is a special case of a isosceles triangle.

Isosceles triangle formulas for area and perimeter

To calculate the isosceles triangle area, you can use many different formulas. The most popular ones are the equations:

  1. Given arm a and base b:

    area = (1/4) * b * √( 4 * a² - b² )

  2. Given h height from apex and base b or h2 height from other two vertices and arm a:

    area = 0.5 * h * b = 0.5 * h2 * a

  3. Given any angle and arm or base

area = (1/2) * a * b * sin(base_angle) = (1/2) * a² * sin(vertex_angle)

Also, you can check our triangle area calculator to find out other equations, which work for every type of the triangle, not only for the isosceles one.

To calculate the isosceles triangle perimeter, simply add all the triangle sides: perimeter = a + a + b = 2 * a + b

Isosceles triangle theorem

Isosceles triangle theorem, also known as the base angles theorem, claims that if two sides of a triangle are congruent, then the angles opposite to these sides are congruent.

Also, the converse theorem exists, stating that if two angles of a triangle are congruent, then the sides opposite those angles are congruent.

Golden triangle calculator

A golden triangle, which is also called sublime triangle is an isosceles triangle in which the leg is in the golden ratio to the base:

a / b = φ ~ 1.618

The golden triangle has some unusual properties:

  • It's the only triangle with three angles in 2:2:1 proportions
  • It's the shape of the triangles found in the points of pentagrams
  • It's used to form a logarithmic spiral

How to find the area with this isosceles triangle calculator?

Let's find out how to use this tool on a simple example. Have a look at this step-by-step solution:

  1. Determine what is your first given value. Assume we want to check the properties of the golden triangle. Type 1.681 inches into leg box.
  2. Enter second known parameter. For example, take a base equal to 1 in.
  3. All the other parameters are calculated in the blink of an eye! We checked for instance that isosceles triangle perimeter is 4.236 in and that the angles in the golden triangle are equal to 72° and 36° - the ratio is equal to 2:2:1, indeed.

You can use this calculator to determine different parameters than in the example, but remember that there are in general two distinct isosceles triangles with given area and other parameter, e.g. leg length. Our calculator will show one possible solution.

Hanna Pamuła, PhD student

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