DocomintTriangle Calculator
Calculate a triangle's area, perimeter, and all three angles from its three side lengths.
Given just three side lengths, this computes the area using Heron's formula (no need to know the height) and all three interior angles using the law of cosines — and checks the triangle inequality first, so three lengths that couldn't actually form a triangle are flagged rather than silently producing a nonsense answer.
Example: A triangle with sides 7, 9, and 12 has a perimeter of 28, an area of about 31.3 (via Heron's formula), and angles of roughly 35.4°, 48.2°, and 96.4° — which correctly sum to 180°.
How it works
- Enter the three side lengths, a, b, and c
- Press Calculate
- Area, perimeter, and all three angles appear instantly, computed on your device
Frequently asked questions
How is triangle area calculated from just three side lengths?
Using Heron's formula: compute the semi-perimeter s = (a+b+c)/2, then area = √(s(s−a)(s−b)(s−c)). It works without knowing the triangle's height, which is what makes it useful when you only have the three side lengths to start with.
Why might three side lengths not actually form a triangle?
The triangle inequality requires that any two sides added together must be longer than the third side — otherwise the two shorter sides can't physically reach each other to close the shape. Side lengths like 1, 1, and 10 fail this test.
Is my numbers uploaded anywhere?
No — Triangle Calculator runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.