Free Triangle Calculator

🟦 How to Use the Triangle Calculator

The Triangle Calculator lets you solve any type of triangle — whether you know all three sides, a mix of sides and angles, or just one angle in a right triangle.
It uses the Law of Sines, Law of Cosines, and Heron’s formula to calculate missing values with high precision.

1️⃣ Choose a Calculation Mode

Use the dropdown menu at the top to select which type of information you already know:

• SSS (Side-Side-Side) – You know all three sides (a, b, and c). The calculator finds all angles, area, perimeter, and more.

• SAS (Side-Angle-Side) – You know two sides and the angle between them (the included angle).

• ASA / AAS (Angle-Side-Angle or Angle-Angle-Side) – You know two angles and one side. The third angle and remaining sides are calculated automatically.

• Right Triangle – For triangles with one 90° angle. You can calculate missing sides and angles using combinations such as “two legs”, “one leg and one angle”, or “hypotenuse and angle”.

Each mode automatically shows the correct input fields for your case.

2️⃣ Enter Your Known Values

Enter the sides or angles you know in the corresponding boxes.

• Use any unit you prefer — cm, m, mm, or inches.

• Angles are always entered in degrees (°).

• You can leave one angle blank in ASA mode; the calculator will complete it using the rule A + B + C = 180°.

If you make a mistake (for example, angles that don’t sum to 180° or sides that can’t form a triangle), the tool will show an error message explaining the issue.

3️⃣ Select Rounding and Units (Optional)

Use the Rounding dropdown to control how many decimals you want in your results — from 0 to 6.
The Unit field lets you add a label (like “cm” or “m”) that will appear automatically beside the results.

4️⃣ Click “Calculate”

After you click Calculate, the tool instantly computes:

• All sides (a, b, c)

• All angles (A, B, C)

• Perimeter (P)

• Area (K) using Heron’s formula

• Heights (hₐ, hᵦ, h꜀) drawn from each side

• Inradius (r) – the radius of the inscribed circle

• Circumradius (R) – the radius of the circumscribed circle

Each result is rounded to your chosen precision, and all values are displayed clearly in a results box.

5️⃣ Reset or Try Another Mode

Use the Reset button to clear all inputs and start a new calculation.
You can switch between SSS, SAS, ASA, and Right Triangle at any time — the tool will automatically adjust the input layout.

💡 Tips & Notes

• The calculator works for acute, right, and obtuse triangles.

• If you only know two sides, you must also know the included angle (SAS) to get a valid triangle.

• If your inputs give an impossible triangle (for example, one side longer than the sum of the other two), an error will appear so you can correct it.

• The formulas used are accurate for both small and large triangles, so you can use it for geometry homework, 3D modeling, or construction planning.