Triangle Calculator
Free online triangle calculator. 100% client-side, no upload.
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What is Triangle Calculator?
A Triangle Calculator solves a triangle from the measurements you know. Give it three sides, two sides and the included angle, or a base and height, and it instantly computes the area, perimeter, missing sides, and all three angles. It uses the Heron formula and the law of cosines, and it classifies the triangle by sides (equilateral, isosceles, scalene) and by angles (acute, right, obtuse). Everything runs in your browser, so your data never leaves your device.
Key Benefits
- Area, perimeter, and all angles in a single click
- Solves SSS and SAS triangles with validated inputs
- Automatic triangle type classification
- Base & Height mode for quick area calculations
- 100% client-side - nothing is uploaded
Common Use Cases
- •Checking homework or exam geometry problems
- •Finding the area of a plot, roof, or triangular space
- •Calculating a missing side for construction or design
- •Learning Heron formula and the law of cosines
How to Use the Triangle Calculator
- Choose an input mode: Pick between 3 Sides (SSS), Side-Angle-Side (SAS), or Base & Height depending on the measurements you already know.
- Enter your values: Type the known side lengths and/or the included angle. For SSS enter all three sides, for SAS enter two sides and the angle between them, and for Base & Height enter the base and the height.
- Calculate: Click Calculate. The tool validates the inputs (positive values and the triangle inequality) and shows the area, perimeter, and angles.
- Read the triangle type: For SSS inputs the result also classifies the triangle by sides (equilateral, isosceles, scalene) and by angles (acute, right, obtuse). Use Copy to grab the full summary.
Key Features
- Three input modes: SSS (3 sides), SAS (side-angle-side), and Base & Height
- Area via Heron formula and the law of cosines
- Computes perimeter, missing sides, and all three angles
- Classifies triangles by sides and by angles automatically
- Validates triangle inequality and invalid inputs
- 100% client-side - nothing is uploaded
Frequently Asked Questions
How do you calculate the area of a triangle?
The simplest formula is area = ½ × base × height. When you know three sides (SSS), the tool uses Heron formula: s = (a+b+c)/2, then area = √(s(s−a)(s−b)(s−c)). When you know two sides and the included angle (SAS), it uses area = ½ × a × b × sin(C).
What is the triangle inequality?
The triangle inequality states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side. In other words, a + b > c, a + c > b, and b + c > a must all hold. If any of these fails, the three lengths cannot form a valid triangle, and the tool flags the input as invalid.
How do I find a missing side of a triangle?
Use the law of cosines. Given two sides a and b and the included angle C, the third side is c = √(a² + b² − 2ab·cos(C)). This is exactly what the Side-Angle-Side (SAS) mode computes for you.
What types of triangles can this tool identify?
By sides it identifies equilateral (all three equal), isosceles (two equal), and scalene (no equal sides). By angles it identifies acute (all angles under 90°), right (exactly one 90° angle), and obtuse (one angle over 90°). Equilateral triangles are always acute.