Triangle Calculator
Solve area, perimeter, and angles from three side lengths.
Inputs
Results update as you edit.
Area
6
Results update as the calculator inputs change.
Perimeter
12
Angle A
36.87 deg
Angle B
53.13 deg
Angle C
90 deg
How to Calculate Triangle Measurements
How Triangle Side Calculations Work
When all three side lengths are known, a triangle can be checked for validity, then area, perimeter, and angles can be calculated without needing a height.
What Affects Triangle Results?
Common Triangle Formulas
Perimeter
Add all three sides.
Area
Use Heron's formula when three sides are known.
Angles
Use the law of cosines to solve each angle.
Frequently Asked Questions
Can any three lengths make a triangle?
No. The sum of any two sides must be greater than the third side.
How is area calculated from three sides?
Heron's formula uses the semiperimeter and all three side lengths.
Why are angles tied to side lengths?
In a triangle, the largest angle is opposite the longest side.
What units does the calculator use?
Use any consistent length unit. The area result is in that unit squared.
Does this solve right triangles only?
No. It works for any valid triangle when all three sides are known.