Triangle Calculator
Enter the three side lengths of a triangle to calculate its area, perimeter, angles and type.
Area
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Perimeter
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Type
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Angle A
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Angle B
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Angle C
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About this triangle calculator
Give this calculator the three side lengths of any triangle and it works out everything else — the area (using Heron’s formula), the perimeter, all three interior angles, and whether the triangle is equilateral, isosceles or scalene, and right, acute or obtuse.
Frequently asked questions
How is the area calculated?
From the three sides we use Heron’s formula, which gives the exact area without needing the height.
How are the angles found?
Using the law of cosines, which calculates each angle from the three side lengths.
Why does it say “not a valid triangle”?
For a triangle to exist, each side must be shorter than the sum of the other two (the triangle inequality). If that fails, no triangle is possible.
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