Interactive Triangle Calculator
Choose the set of values you know, enter numbers, and click Calculate to solve the full triangle (sides, angles, area, perimeter, and more).
Angles are in degrees. Side lengths can be in any consistent unit (cm, m, in, etc.).
What a Triangle Calculator Does
A triangle calculator helps you solve unknown sides, angles, perimeter, and area when you know a valid set of starting values. Instead of manually applying trigonometric identities each time, you can enter known dimensions and instantly compute the complete geometry of a triangle.
This is useful for students, engineers, surveyors, architects, DIY builders, and anyone working with geometric layouts.
Supported Input Modes
1) SSS (Side-Side-Side)
When all three sides are known, the calculator uses the Law of Cosines to determine each angle. This is one of the most common triangle-solving workflows.
2) SAS (Side-Angle-Side)
When two sides and their included angle are known, the calculator first computes the third side using the Law of Cosines, then solves the remaining angles.
3) ASA/AAS (Angle-Angle-Side)
When two angles and one side are known, the third angle comes from the triangle angle sum rule, and unknown sides are solved via the Law of Sines.
Core Triangle Formulas Used
- Angle sum: A + B + C = 180°
- Law of Cosines: a² = b² + c² − 2bc cos(A)
- Law of Sines: a/sin(A) = b/sin(B) = c/sin(C)
- Area (general): ½bc sin(A)
- Perimeter: a + b + c
- Inradius: r = 2 × Area / Perimeter
- Circumradius: R = abc / (4 × Area)
How to Use This Calculator Correctly
- Select the mode that matches your known values.
- Enter positive side lengths and realistic angles.
- For angle-based modes, ensure angles are greater than 0° and less than 180°.
- Click Calculate and review all computed measurements.
- Use the Clear button to start a new problem quickly.
Common Input Mistakes to Avoid
- Invalid SSS values: side lengths must satisfy triangle inequality (sum of any two sides must exceed the third).
- Impossible angle pairs: in ASA/AAS, A + B must be less than 180°.
- Mixing units: if one side is in meters, all sides should be in meters.
- Degree/radian confusion: this calculator expects degrees for angles.
Practical Applications
Triangle calculations appear in roof framing, land measurement, navigation, machine design, game development, and physics problems involving vectors and forces. Being able to quickly solve triangles saves time and reduces arithmetic mistakes in both educational and professional settings.
Quick FAQ
Can I use decimal values?
Yes. The calculator accepts decimals for both sides and angles.
Does it work for right triangles?
Absolutely. Right triangles are a special case and are solved naturally by these formulas.
What does triangle type mean?
The result classifies your triangle by side equality (equilateral, isosceles, scalene) and by angle category (acute, right, obtuse).