Enter all three side lengths.
Geometry Calculator
Triangle Calculator
Use three side lengths to validate a triangle and calculate perimeter, semiperimeter, area, angles, and triangle type.
Instant answer
Formula breakdown
Browser-based
Mobile friendly
Interactive calculator
Enter your values
Use the example values or replace them with your own. Required validation happens before the calculation.
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Calculated answer
Result
Simple workflow
How to use the Triangle Calculator
Follow these steps to get a reliable result and understand how it was produced.
Check the triangle inequality.
Apply Heron’s formula and the law of cosines.
Understanding the calculation
The triangle inequality validates the sides. Heron’s formula gives area, and the law of cosines gives each angle.
Heron: A=√[s(s−a)(s−b)(s−c)]The result panel substitutes your numbers into this relationship and shows the important intermediate values.
Common uses
- Triangle construction checks
- Geometry and surveying
- Classifying triangles
Accuracy tips
- The sum of any two sides must exceed the third.
- Angles may differ slightly from exact values because of decimal rounding.
- Use one consistent unit for all sides.
Helpful answers
Triangle Calculator FAQs
Important details about formulas, inputs, limitations, and result interpretation.
How is the triangle type determined?
Sides classify it as equilateral, isosceles, or scalene; angles classify it as acute, right, or obtuse.
What is semiperimeter?
It is half the perimeter and is used in Heron’s formula.
Why can three positive lengths still be invalid?
They may fail the triangle inequality and therefore cannot close into a triangle.
Does the area have square units?
Yes. If sides are metres, area is square metres.