Triangle Calculator — Sides, Area & Angles
Enter three side lengths to test the triangle inequality, then calculate area and angles for a valid triangle.
Triangle valid?Yes
Perimeter12
Area6
Angles A / B / C36.87° / 53.13° / 90°
How this tool works
With semiperimeter s=(a+b+c)/2, Heron's formula gives A=√[s(s−a)(s−b)(s−c)]. Angles use the law of cosines.
Example
Sides 3, 4 and 5 form a valid right triangle with area 6 and angles about 36.87°, 53.13° and 90°.
Common questions
What makes three lengths a valid triangle?
Each pair of side lengths must sum to more than the remaining side.
Does this assume a right triangle?
No. It handles any valid triangle defined by three side lengths.