Isosceles Triangle Find A Calculator

Enter the base and height of an isosceles triangle to find the equal leg length (a = √((b/2)² + h²)), plus area, perimeter, and apex angle.

Quick Facts

Leg formula
a = √((b/2)² + h²)
Pythagorean theorem applied to the right triangle formed by the altitude.
Area formula
Area = (b × h) / 2
Same base-times-height rule as any triangle.
Apex angle
β = 2 × arctan((b/2) / h)
Angle between the two equal legs.

Your Results

Calculated
Leg Length (a)
-
a = √((b/2)² + h²)
Area
-
(b × h) / 2
Perimeter
-
2a + b
Apex Angle
-
Angle between the two equal legs

Ready

Enter the base and height, then press Calculate.

Formula and Method for Finding the Leg (a) of an Isosceles Triangle

An isosceles triangle has two equal-length sides (the legs, labeled a) and one distinct side (the base, labeled b). If you know the base and the height (the altitude drawn from the apex — the angle between the two legs — straight down to the base), you can find the leg length with the Pythagorean theorem: a = √((b/2)² + h²). This calculator also derives the area, perimeter, and apex angle from the same two inputs.

How the calculation works

Because the two legs are equal, an isosceles triangle is symmetric about the altitude drawn from its apex to its base. That symmetry means the altitude always meets the base at a right angle and at its exact midpoint, splitting the triangle into two congruent right triangles. Each of those right triangles has one leg equal to the height h, the other leg equal to half the base (b/2), and its hypotenuse equal to the isosceles triangle's leg, a. Applying the Pythagorean theorem, a² = (b/2)² + h², so a = √((b/2)² + h²). From there, the area follows the usual triangle rule, Area = (b × h) / 2, the perimeter is P = 2a + b (two legs plus the base), and the apex angle is β = 2 × arctan((b/2) / h), since arctan((b/2) / h) is the angle at the apex in just one of the two right-triangle halves.

Common mistakes

  • Using the full base instead of half: the Pythagorean relationship uses b/2, not b — because the altitude bisects the base, only half of it belongs to each right-triangle half.
  • Mixing units: enter the base and height in the same unit (both in feet, or both in centimeters) before calculating; the result unit follows the units you enter.
  • Confusing leg and base: the leg (a) is one of the two equal sides; the base (b) is the third, distinct side. Swapping them into the wrong formula slot changes every downstream result.

Real-world applications

  • Roof trusses and A-frame structures are often isosceles triangles — knowing the rafter (leg) length from the span (base) and rise (height) is a standard framing calculation.
  • Gable ends, pediments, and decorative arches use the same base-and-height-to-leg relationship to cut angled material to the correct length.
  • Surveying and land-plotting use the leg length and apex angle to lay out symmetric triangular parcels or setbacks.
  • Geometry and trigonometry coursework use isosceles triangles as the standard example for the Pythagorean theorem and angle-sum reasoning.

Frequently Asked Questions

What is the formula for finding the leg (a) of an isosceles triangle from the base and height?
a = √((b/2)² + h²), where b is the base and h is the height (altitude) drawn from the apex to the base. This comes from the Pythagorean theorem: the altitude bisects the base and forms a right triangle with legs h and b/2, and hypotenuse a.
Why does the altitude bisect the base in an isosceles triangle?
Because the two legs are equal length, the triangle is symmetric about the altitude drawn from the apex angle to the base. That symmetry forces the altitude to meet the base at its midpoint and at a right angle, splitting the isosceles triangle into two congruent right triangles.
How do I find the apex angle and base angles?
The apex angle (between the two equal legs) is β = 2 × arctan((b/2) / h). Each base angle is α = (180° − β) / 2, and by the triangle angle sum, 2α + β = 180°.
What if the height is greater than or less than half the base?
Both are valid — a tall, narrow isosceles triangle has h much larger than b/2 (a small apex angle), while a short, wide one has h smaller than b/2 (a large apex angle). The formula a = √((b/2)² + h²) works for any positive base and height.

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