What does this tool do?
The Triangle Calculator computes a triangle's area, perimeter, and interior angles from its three side lengths in seconds. It's a handy tool for students, teachers, engineers, and anyone sketching or measuring triangles.
How to use it
- Enter your values into the fields.
- The result is calculated instantly.
- Read or copy the result.
Area from Heron's formula
The calculator finds the area directly from the three side lengths using Heron's formula, so you never need to know the height. It first computes the semi-perimeter s = (a+b+c)/2, then area = √(s·(s−a)·(s−b)·(s−c)). This works for any valid triangle, not just right triangles, and removes the rounding mistakes that creep into manual work. You can copy the result straight into homework, drawings, or reports.
Angles and triangle validity
Interior angles are derived from the sides using the law of cosines and shown in degrees. Because the three angles always sum to 180°, this gives you a quick way to sanity-check your inputs. The tool also enforces the triangle inequality: if the sum of any two sides is not greater than the third, no real triangle exists and you'll see a warning instead of a misleading answer, keeping your geometry consistent and verifiable.
Frequently asked questions
Can I get the area without knowing the height?
Yes. The tool uses Heron's formula, so the three side lengths alone are enough; no height or angle is required.
How are the angles calculated?
Each angle is computed with the law of cosines from its opposite side and the other two sides, then displayed in degrees.
What if my three sides don't form a triangle?
If the triangle inequality fails, the calculator flags an invalid triangle, since the sum of any two sides must exceed the third.
Is Triangle Calculator free?
Yes. Triangle Calculator and all 300+ tools on MagmaNex are completely free with no signup or account required.
Is my data safe?
Yes. Everything runs locally in your browser. The data you enter is never sent to a server, stored, or shared.
Does it work on mobile?
Yes. Triangle Calculator works on phones, tablets and desktops — no app download needed.
🔒 Privacy: everything runs in your browser, your data is never sent to a server.