Home/Math/Triangle

Triangle Calculator

Side A

Side B

Side C

👤

Reviewed by Dr. Alex M., Ph.D.

Last updated: November 2025

< > Embed This Calculator

<iframe src="https://calcs.top/math/triangle-calculator/" width="100%" height="600" frameborder="0"></iframe>

Add this free triangle calculator to your website or blog.

💡 Expert Tips

Check if it's Possible

Not any three numbers make a triangle. The two shorter sides MUST add up to more than the longest side (e.g., 2, 3, 6 is impossible because 2+3 < 6).

Heron's Formula

This calculator uses Heron's Formula, which is perfect when you only know the three side lengths (SSS) and not the height.

Pythagorean Triples

Memorize common right triangles like 3-4-5, 5-12-13, and 8-15-17. They appear frequently in math problems and standardized tests.

⚠️ Common Mistakes

Confusing Area and Perimeter

Perimeter is the distance AROUND the triangle (add sides). Area is the space INSIDE (squared units). Don't mix them up!

Assuming Right Angles

Just because a triangle looks like it has a 90-degree angle doesn't mean it does. Always verify with a² + b² = c².

Unit Mismatch

Ensure all sides are in the same unit (e.g., all cm or all inches). If one side is in feet and another in inches, convert them first.

❓ Frequently Asked Questions

How do you calculate the area of a triangle with 3 sides? +

We use Heron's Formula. First, find the semi-perimeter (s = (a+b+c)/2). Then, Area = √(s(s-a)(s-b)(s-c)).

What is the perimeter of a triangle? +

The perimeter is simply the sum of the lengths of all three sides: P = a + b + c.

What are the different types of triangles? +

Based on sides: Equilateral (3 equal sides), Isosceles (2 equal sides), Scalene (no equal sides). Based on angles: Acute (all <90°), Right (one=90°), Obtuse (one>90°).

What is the Triangle Inequality Theorem? +

It states that the sum of any two sides of a triangle must be greater than the third side. If a+b ≤ c, a triangle cannot be formed.

How do I know if it's a right triangle? +

Use the Pythagorean Theorem: a² + b² = c². If the square of the two smaller sides equals the square of the longest side, it's a right triangle.