Welcome to the isosceles triangle angles calculator, where we'll show you how to calculate the angles of an isosceles triangle. Along the way, we hope to teach you more about:

  • What kinds of angles an isosceles triangle has;
  • What the vertex angle of an isosceles triangle is; and
  • What angles an isosceles right triangle has.

How do I use the isosceles triangle angles calculator?

The isosceles triangle angles calculator gets to the point without cutting any triangles' corners. Here's how to use it:

  1. Enter the length of your triangle's legs and the base length (aa and bb, respectively).
  2. See how the calculator works out the vertex angle and base angles (β\beta and α\alpha).
  3. The calculator can work backward, too! Try inputting your mystery triangle's angles together with one side's length (the isosceles triangle angles calculator needs some sense of proportion) and see how it works out the remaining side's length.

If you still want to learn what an isosceles triangle is and how to calculate its angles, read on!

What is an isosceles triangle? What kind of angles does an isosceles triangle have?

An isosceles triangle is a triangle with two sides of equal length. We usually call these two sides the "legs" (aa below) and the remaining side the "base" (bb).

An isosceles triangle with its sides, angles and heights marked.
An isosceles triangle. See how the legs marked aa are the same length, and the corners marked α\alpha are the same angle?

Because the legs of an isosceles triangle are the same length, the two angles they form with the base are also equal. We call the angles adjacent to the base side the base angles (α\alpha) and the remaining angle the vertex angle (β\beta).

How do I find the angles of an isosceles triangle?

Finding the angles α\alpha and β\beta of an isosceles triangle is easy — all you need is some geometry tricks!

  1. Find aa and bb, the length of the isosceles triangle's base and legs.
  2. Split the isosceles triangle down its axis of symmetry (i.e. from its vertex angle straight down the middle of the base) to obtain two mirrored right triangles.
  3. Use trigonometry to work out the angles, which will be 9090^\circ, α\alpha, and β/2\beta/2.

What is an isosceles right triangle?

An isosceles right triangle is the child of a right triangle and an isosceles triangle, and so it has all of its parents' attributes. It has two equal sides and one angle (in this case, the vertex angle) that is 9090^\circ. The two base angles are then 4545^\circ each.


What is the vertex angle of an isosceles triangle?

The vertex angle of an isosceles triangle is the angle formed by the triangle's two legs (the two sides that are of equal length). It is unique in the triangle unless all three sides are equal and the triangle is equilateral.

Can an isosceles triangle have a 90 degree angle?

Yes — an isosceles triangle can have a 90-degree angle. This angle would be its vertex angle. We can call an isosceles triangle with a 90-degree angle a right isosceles triangle. The base angles of an isosceles triangle can't be 90 degrees — a shape with two or more 90° angles can't even be a triangle at all!

What are the angles of an isosceles triangle with a vertex angle of 90°?

If an isosceles triangle has a vertex angle β = 90°, we only need to calculate one more angle — the base angle, α, which features twice.

  1. The sum of a triangle's angles is 180°, i.e.:
    2α + β = 180°.
  2. Make α the subject of the equation:
    α = (180° − β) / 2
  3. Substitute β = 90°:
    α = (180° − 90°) / 2
  4. Work out the equation to obtain α = 45°.
Rijk de Wet
isosceles triangle with sides, angles and heights marked.
Leg (a)
Base (b)
Vertex angle (β)
Base angle (α)
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