Geometry · free · no sign-up
Triangle Calculator
Enter the three sides of a triangle to find its area, perimeter, angles and type (equilateral, isosceles, scalene, right, acute or obtuse).
About the Triangle Calculator
Enter the three sides of a triangle to find its area, perimeter, angles and type (equilateral, isosceles, scalene, right, acute or obtuse).
How to use the Triangle Calculator
- Enter Side A, Side B and Side C (Base).
- Press Draw & Calculate to see the results and a drawing.
Formula and method
Heron's formula: with s = (a + b + c) ÷ 2, Area = √(s(s − a)(s − b)(s − c)). Angles come from the law of cosines: cos A = (b² + c² − a²) ÷ 2bc.
Worked example
A triangle with sides 3, 4 and 5 has s = 6 and area √(6 × 3 × 2 × 1) = 6. It is a right triangle.
When to use the Triangle Calculator
- Finding a missing side, angle or the area of a triangle.
- Checking whether three lengths can form a triangle.
- Solving geometry homework.
Common mistakes to avoid
- Entering angles in radians when the tool expects degrees.
- Forgetting the triangle inequality: the two shorter sides must add up to more than the longest side.
Tips
- The sum of any two sides must be greater than the third, otherwise no triangle exists.
- Angles in a triangle always add up to 180°.
Frequently asked questions
What is Heron's formula?
It gives the area of a triangle from its three side lengths without needing the height.
What do the angles of a triangle add up to?
180 degrees.
How do I find the area from three sides?
Use Heron's formula: the square root of s(s−a)(s−b)(s−c), where s is half the perimeter.