Triangle Calculator
Enter three measurements, including at least one side, to calculate missing sides, angles, area and perimeter. Solve SSS, SAS, ASA, AAS and ambiguous SSA cases with worked steps, or start from two perpendicular legs.
Enter your values or load an example to see the answer and worked steps. Calculations run in your browser.
Match sides to opposite angles
Side a lies opposite angle A, b opposite B and c opposite C. Keep this correspondence when copying a diagram. All angles are in degrees; all sides must use the same selected unit. Leave unused measurements empty.
Choose enough independent information
Three sides determine a triangle when the triangle inequality holds. Two sides and their included angle use the cosine rule. Two angles and one side determine the third angle and scale. Three angles alone do not determine lengths or area.
Check the ambiguous SSA case
Two sides and a non-included angle can give zero, one or two valid triangles. The calculator checks both sine-rule angle candidates and only retains triangles with a positive third angle. When two remain, it presents both solutions rather than silently choosing one.
Interpret area, perimeter and rounding
Area uses ½ab sin C and perimeter is a + b + c. Lengths use your chosen unit and area uses its square. A nearly straight triangle is numerically sensitive; extremely small angles are rejected instead of displaying misleading rounded answers.
References
Related free tools
Frequently Asked Questions
Can it solve a right triangle?
Yes. Choose right-triangle mode and enter the two perpendicular legs. The third side is the hypotenuse.
Why are two answers possible?
SSA measurements can place the unknown vertex in two positions. Both triangles satisfy the entered measurements.
Can I enter all six measurements?
Enter exactly three independent measurements. The tool solves a triangle; it does not fit inconsistent measurements.
Does changing units convert my inputs?
No. The selected unit labels every side value you enter. Convert mixed measurements before calculating.
Continue in the app
Use Geometry AI - Math Solver for the full guided result after this quick check.
