Isosceles Triangles - Calculator

With this calculator you can calculate the angles, altitudes, area, perimeter, inradius and circumradius of an isosceles triangle. You can select to how many decimal places the calculated values should be rounded and further calculations are always performed with the rounded values.

On this page, formulas are used and lengths are named as they appear on German websites about isosceles triangles. For example, the length of the base of the triangle is called c instead of b. (German Wikipedia article about isosceles triangles)

Formulas

Altitude	ha=hb= c 2∙a ∙4∙a²−c² hc= 4∙a²−c² 2
Angle	γ=180°−2∙α α=β=arcsin( hc a )
Area	A= a∙ha 2 A= c∙hc 2 A= a²∙sin(γ) 2
Perimeter	P=2∙a+c
Inradius	r= c∙hc 2∙a+c
Circumradius	R= a 2∙sin(α) R= c 2∙sin(γ) R= a² 4∙a²−c²

What is an isosceles triangle?

An isosceles triangle is a triangle in which at least 2 sides have the same length. On this site, the lengths of the two sides of equal length are called a and b, where a = b. The length of the third side is called c on this site. The angle opposite from the side with length c is called γ and the other two angles α and β.

Altitudes

The two altitudes h_a and h_b have the same length in an isosceles triangle. If the side length c differs from a and b, then the altitude h_c also differs from the altitudes h_a and h_b.

The altitude h_c divides the isosceles triangle into 2 right triangles. The side lengths and the angles of both triangles are equal.

Calculation of the altitudes:

The altitude h_a can be calculated with the following formula:

The altitude h_b corresponds to h_a.

The following formula can be used to calculate h_c:

Example:

An isosceles triangle has the side lengths a = 5 cm, b = 5 cm and c = 4 cm. The altitudes are to be calculated.

ha	=	c 2∙a ∙4∙a²−c²
	=	4 cm 2∙5 cm ∙4∙(5 cm)²−(4 cm)²
	=	4 cm 10 cm ∙4∙25 cm²−16 cm²
	=	0.4∙100 cm²−16 cm²
	=	0.4∙84 cm²
	≈	3.66606 cm

h_b=h_a≈3.66606 cm

hc	=	4∙a²−c² 2
	=	4∙(5 cm)²−(4 cm)² 2
	=	84 cm² 2
	≈	4.58258 cm

Angles

The angle γ (gamma) is between the two sides of equal length (resp. opposite from the side with length c). If the isosceles triangle is not also an equilateral triangle, then γ is different from the two angles α and β. However, the two angles α and β are equal. The sum of the 3 interior angles is 180°. Since the two angles α and β are equal in size:

If the angle γ is known and you want to calculate the angle α, then you can solve the equation for α.

Example:

It is known that the angle γ of an isosceles triangle is 40°. We are looking for α and β.

α	=	180°−γ 2
	=	180°−40° 2
	=	140° 2
	=	70°

Since α and β are equal, β = 70° is also valid.

The angles α and β are each smaller than 90°. The angle γ can also be larger than 90°, but is smaller than 180°.

Calculate c with the help of γ:

If you draw the altitude h_c in the above isosceles triangle, then the isosceles triangle is divided into 2 right triangles, which have the same side lengths and the same angles. Since a = b and α = β, b and β have been replaced by a and α in the following figure.

In a right triangle, the following applies for the acute angles:

If you solve the formula for c you get:

This formula can be used if the length of the two sides of equal length and the angle γ are known and the side length c is to be calculated.

Calculate γ with the help of the side lengths:

If the side lengths are known and the angle γ is to be calculated, then you can solve the above formula for γ and obtain:

Calculate α with the help of a and h_c:

The opposite side of α is h_c and the hypotenuse is a. Thus:

If you solve this for α you get:

Area of an isosceles triangle

The area of any triangle can be calculated by choosing one side as the base side, multiplying it by the corresponding height, and dividing the result by 2. The following therefore applies:

Example:

A triangle has the side lengths a = 2 cm, b = 2 cm and c = 3 cm. The area is to be calculated.

First, one of the altitudes is calculated. In this example, the altitude h_c is chosen.

hc	=	4∙a²−c² 2
	=	4∙(2 cm)²−(3 cm)² 2
	=	7 cm² 2
	≈	1.3229 cm

Then h_c is used in the formula for the calculation of the area:

A	=	c∙hc 2
	≈	3 cm∙1.3229 cm 2
	=	3.9687 cm² 2
	=	1.98435 cm²

Calculate the area with the help of γ:

If only the angle γ and the side length a are known, the area can still be calculated. The following formula can be used for this purpose:

Perimeter of an isosceles triangle

The perimeter of a triangle is calculated by adding the lengths of the 3 sides. a and b have the same length. Therefore applies:

Inradius

An incircle of a triangle is a circle that lies inside the triangle and touches each side of the triangle at exactly one point. For an isosceles triangle with side lengths a, b and c, where a = b and a ≠ c, the incircle touches the side with length c exactly in the middle. The sides with lengths a and b, on the other hand, are touched at a point other than the center.

The inradius can be calculated with the following formula:

Circumradius

The circumcircle of a triangle is a circle whose circular line passes through each of the 3 corners. If one of the angles is known, then the radius of the circumcircle can be calculated using one of the following formulas:

However, the following formula can also be used to calculate the radius of the circumcircle: