About the triangle calculator
If you know all three sides, you know the whole triangle. This calculator checks the sides can form a triangle, then finds the area, perimeter, angles, and whether it’s acute, right, or obtuse.
How to use this calculator
- Enter sides a, b, and c in the same unit.
- Read the area, perimeter, and angles.
Worked examples
- s = 6
- Area = √(6 × 3 × 2 × 1) = 6
- Largest angle = 90°
Area 6, a right triangle.
Measurement guidance
Checking for square corners
Builders use the 3-4-5 rule: if a triangle’s sides measure 3, 4, and 5 (or 6, 8, 10), the corner is square.
Frequently asked questions
Do the angles always add up to 180°?
Yes, for any flat (Euclidean) triangle.
What units does it use?
Any — just use the same unit for all sides. Area is in square units.