Triangle Calculator
Solve any triangle by SSS or SAS — find all sides, angles, area, and type.
How to Use
- SSS Mode: Enter all three side lengths to calculate all angles, area, and perimeter.
- SAS Mode: Enter two sides and the included angle to solve the triangle.
- Results appear instantly — sides, angles (in degrees), area, perimeter, and triangle type.
About Triangle Calculator
This triangle calculator can solve any triangle given sufficient information. SSS mode (all three sides known) uses the Law of Cosines to calculate each angle: cos(A) = (b² + c² – a²) / (2bc), and Heron's formula for area. SAS mode (two sides and the included angle) uses the Law of Cosines to find the third side, then resolves the remaining angles.
The calculator classifies the triangle automatically: Equilateral (all sides equal), Isosceles (exactly two sides equal), Right (one angle is 90°), or Scalene (all sides different). All angles are reported in degrees to 4 decimal places.
The triangle inequality theorem is enforced: any two sides must sum to more than the third side. If this is violated, an error message is displayed instead of an invalid result.
You may also find our Area Converter and Scientific Calculator helpful.
Frequently Asked Questions
What is SSS mode?
SSS (Side-Side-Side) solves a triangle when all three side lengths are known using the Law of Cosines.
What is SAS mode?
SAS (Side-Angle-Side) solves a triangle when two sides and the angle between them are known.
What is Heron's formula?
Area = √(s(s-a)(s-b)(s-c)) where s = (a+b+c)/2 is the semi-perimeter. Used in SSS mode.
Can any three lengths form a triangle?
No. The triangle inequality requires that any two sides must sum to more than the third side.
How is triangle type determined?
Equilateral (all sides equal), Isosceles (two sides equal), Right (90° angle), or Scalene (all sides different).
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