Calculate all angles in an isosceles triangle. Input either the vertex angle or base angle to find the remaining angles using the properties of isosceles triangles.
The angle between the two equal sides
Must be between 0° and 180°
Select angle type, enter a known angle, and click "Calculate Angles" to find all angles in the isosceles triangle.
An isosceles triangle has two equal sides and two equal angles. The equal angles are called base angles, and the angle between the equal sides is called the vertex angle.
If vertex angle is known:
Base angle = (180° - vertex angle) ÷ 2
If base angle is known:
Vertex angle = 180° - 2 × base angle
Angle sum theorem:
Vertex angle + 2 × base angle = 180°
Right Isosceles Triangle
Vertex angle = 90°, base angles = 45° each
Equilateral Triangle
All angles = 60° (special case of isosceles)
Obtuse Isosceles Triangle
Vertex angle > 90°, base angles < 45° each
Explore more about isosceles triangles and their properties in our learning section.
Isosceles & Equilateral TrianglesTry other triangle calculators for different properties.