DIY Cost Tools
Triangle Calculator
MATH CALCULATOR
Calculate a triangle’s perimeter, area, type, and three angles from the side lengths.
Perimeter12
Area6
Side typeScalene
Angles A, B, C36.87°, 53.13°, 90°
How it works
Enter all three side lengths in the same unit. Heron’s formula gives area, and the law of cosines gives each angle.
Formula
With semiperimeter s = (a + b + c) ÷ 2, area = √[s(s − a)(s − b)(s − c)].
Worked example
Sides 3, 4, and 5 form a scalene right triangle with perimeter 12, area 6, and angles about 36.87°, 53.13°, and 90°.
Important limits
- All sides must use the same unit.
- Nearly degenerate triangles can be sensitive to rounding.
- The calculator classifies side equality; angle type can be inferred from the reported angles.
Frequently asked questions
What is the triangle inequality?
The sum of any two side lengths must be greater than the third side.
Does the calculator know the unit?
No. The output area uses the square of whatever length unit you enter.
Sources and review notes
The calculation uses Heron’s formula and the law of cosines, standard geometry relationships. Last reviewed September 29, 2026.