About the triangle calculator
This triangle calculator works out the area, perimeter, the three interior angles and the triangle type from the lengths of its three sides. It uses Heron’s formula for the area and the law of cosines for the angles, and checks that the sides can actually form a triangle. Everything is calculated in your browser.
How Triangle works
How to use it
- Enter the three side lengths a, b and c.
- The area, perimeter, angles and triangle type appear instantly.
- The note explains the formulas used.
Heron’s formula
The area is √(s(s−a)(s−b)(s−c)), where s is half the perimeter. The angles come from the law of cosines, and the height to any side is 2 × area ÷ that side.
Triangle inequality
Three lengths form a triangle only if each side is shorter than the sum of the other two — the tool checks this and warns you if they cannot.
Common uses
- Find the area of a triangle from its sides
- Calculate the perimeter
- Work out the interior angles
- Classify a triangle (scalene/isosceles/equilateral)
- Check if three lengths form a triangle
- Solve geometry homework
- Find the height of a triangle
- Verify construction or design measurements
Frequently asked questions
How do I find the area from three sides?
Use Heron’s formula — this tool applies it automatically and also gives the angles.
How are the angles calculated?
By the law of cosines, which finds each angle from the three side lengths.
What is the triangle inequality?
Each side must be shorter than the sum of the other two, or the sides cannot form a triangle.
What makes a triangle right, acute or obtuse?
Its largest angle: exactly 90° is right, less is acute, more is obtuse.
Can I enter decimals?
Yes — any positive side lengths work.
Is my data uploaded?
No — it calculates entirely in your browser.
Does it give the height?
Yes — it shows the height to side a.
Is it free?
Yes — completely free with no sign-up.