Quick answer: for any triangle with three known sides, use Heron's formula for area. For right triangles, use the Pythagorean theorem to find a missing side. Try our free Triangle Calculator.
Heron's Formula
Given sides a, b, c: first find the semi-perimeter s = (a+b+c)/2, then Area = √(s(s-a)(s-b)(s-c)). This works for any triangle, not just right triangles.
Pythagorean Theorem
For right triangles: a² + b² = c², where c is the hypotenuse. Knowing any two sides lets you solve for the third.
Frequently Asked Questions
Does Heron's formula work for right triangles too?
Yes, it works for any triangle type, including right triangles.
What if I only know two sides and an angle?
The calculator can also solve using the Law of Cosines when an angle is known instead of the third side.
Can this calculate perimeter too?
Yes, perimeter is simply the sum of all three sides.
✨ Try it now — free & instant
Open the Tool →