Math
Triangle Calculator
Use this triangle calculator when you know all three side lengths and need the area, perimeter, and interior angles.
Area
—
- Perimeter
- —
- Angle A
- —
- Angle B
- —
Student quick launch
Grade planning, algebra checks, and formulas students reach for most.
Study path
Use this calculator with
Follow these when you want the formula behind the answer, a short lesson, or nearby tools in the same topic.
What the triangle calculator solves
Use this triangle calculator when you know all three side lengths and need the area, perimeter, and interior angles.
Formula
How to use it
- Enter all three side lengths.
- Confirm they satisfy the triangle inequality.
- Use the area, perimeter, and angle outputs together.
How to read the answer
The area is the space inside the triangle, while the three angles show its shape.
Common mistakes and edge cases
- The longest side must be shorter than the sum of the other two.
- Use the same unit for all side lengths.
- This version solves SSS triangles, not angle-side-angle cases.
Worked examples
Scalene triangle
Area
14.6969
Equilateral triangle
Area
43.3013
Frequently asked questions
What if I know angles instead of sides?+
This version focuses on three known sides. Angle-based triangle solving can be added as a future advanced mode.
Why does the triangle inequality matter?+
If one side is too long, the other two sides cannot meet to form a triangle.
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Last updated: May 8, 2026