Triangle Calculator
Last updated: November 5, 2026
Calculate a triangle’s area, perimeter and classification from three side lengths.
Triangle area
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- Perimeter: —
- Semiperimeter: —
- Triangle type by sides: —
- Largest angle: —
On this page
Quick summary: Calculate a triangle’s area, perimeter and classification from three side lengths.
Read the complete guide
Updated reference sections in this guide are in English.
What this calculator estimates
Calculate a triangle’s area, perimeter and classification from three side lengths.
How to use it
Enter Side a; Side b; Side c. Select Calculate to update the results.
How the calculation works
Area is calculated with Heron’s formula after validating the triangle inequality. The largest angle uses the law of cosines.
Worked example: starting scenario
| Side a | 3 |
|---|---|
| Side b | 4 |
| Side c | 5 |
| Triangle area | 6 square units |
|---|---|
| Perimeter | 12 |
| Semiperimeter | 6 |
| Triangle type by sides | Scalene |
| Largest angle | 90° |
Important limitations
All side lengths must be positive and form a valid triangle.
Sides determine one triangle up to reflection
All three side lengths must be positive, and the sum of the two shorter sides must be greater than the longest side. Lengths 3, 4 and 5 give area 6, perimeter 12 and a largest angle of 90°. Scaling every side by 10 multiplies perimeter by 10 and area by 100. The largest-angle output belongs opposite the longest side; the shape classification uses relative floating-point tolerance.