Triangle Calculator
Calculate the area (Heron’s formula) and perimeter of a triangle from its three sides.
How it works
- 1Enter the three side lengths a, b, and c.
- 2Read the perimeter and the area from Heron’s formula.
- 3If the sides cannot form a triangle, the area shows “—”.
Use cases
- Find the area of a triangular plot or sail.
- Check the perimeter for trim or edging.
- Verify homework using Heron’s formula.
Frequently asked questions
What is Heron’s formula?
With sides a, b, c and s = (a + b + c) / 2, the area is √(s(s − a)(s − b)(s − c)). It finds the area from the three sides alone.
Why does the area show “—”?
The three sides do not satisfy the triangle inequality (each side must be shorter than the sum of the other two), so no triangle exists.
What is the triangle inequality?
For any valid triangle, the sum of any two sides must be greater than the third. For example, 1, 2, and 10 cannot form a triangle.
Related tools
See all →Calculate the area and perimeter of a rectangle from its width and height, with a labeled diagram.
Calculate the area and circumference of a circle from its radius, with a labeled diagram.
Calculate the area and perimeter of a trapezoid from its two parallel sides, height, and legs.
Calculate the area and perimeter of a parallelogram from its base, height, and side.