Math
Triangle Calculator
Enter three side lengths to confirm the triangle is valid, then instantly view its area, angles, and classification.
Enter all three sides; get type, area, and precise internal angles.
Heron’s formula & law of cosines
Area = √[s(s − a)(s − b)(s − c)]
The calculator first uses Heron’s formula to get area from the semi-perimeter s = (a + b + c)/2. Internal angles come from the law of cosines: cos(A) = (b² + c² − a²) ÷ (2bc), and so on.
How to use
- Enter the three side lengths; units can be any consistent measure.
- Confirm the triangle is valid—if not, adjust the sides until the triangle inequality is satisfied.
- Read the perimeter, area, angle sizes, and detected triangle type.
Example
Input: Sides = 7, 8, 9
Output: Area ≈ 26.83 square units, Angles ≈ 48.19°, 58.41°, 73.40°, Type = Scalene
FAQ & notes
Why do I get an invalid triangle warning?
A triangle must satisfy the inequality that any two sides summed together exceed the third. If that rule is broken, adjust the inputs until it holds.
Can I use this for right triangles?
Yes. If the inputs form a right triangle the tool labels it accordingly and still reports angles and area.