Triangle Calculator
Free triangle calculator: enter the three side lengths to get the perimeter and exact area with Heron\u2019s formula. Invalid triangles are flagged automatically.
About this tool
Know the three sides of a triangle but not its height? Heron\u2019s formula finds the area from the sides alone - no height measurement needed.
Enter sides A, B and C, and this triangle calculator returns both the perimeter and the exact area, while checking the sides genuinely form a triangle.
How to use
Type the three side lengths in the same unit and press Calculate Triangle. The tool first validates the triangle inequality, then shows perimeter (a + b + c) and area.
Useful for land plots, roof trusses and any triangular frame where measuring the height is awkward.
The math behind it
Perimeter = a + b + c. Area = sqrt(s(s-a)(s-b)(s-c)), where s is the semi-perimeter (a+b+c)/2 - this is Heron\u2019s formula.
A valid triangle needs each side shorter than the other two combined; the calculator flags impossible sets like 2, 3, 10.
Frequently asked questions
What is Heron\u2019s formula?
When should I use this instead of the area calculator?
Why does it reject sides like 2, 3 and 10?
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