Triangle Calculator
Enter the three side lengths to get area, perimeter, all three angles and the triangle's type.
How to use it
- Enter side a The length of the first side.
- Enter sides b and c All three must satisfy the triangle inequality.
- Read the results Area, perimeter, angles and type appear at once.
Examples
| 3, 4, 5 | area 6, right |
|---|---|
| 5, 5, 5 | equilateral |
About this tool
Give this calculator the three sides of a triangle and it works out everything else: the area via Heron’s formula, the perimeter, and all three interior angles using the law of cosines. It also classifies the triangle and flags right angles.
Because it only needs the side lengths, there’s no need to measure heights or angles yourself. If the sides can’t actually form a triangle, the tool says so rather than returning a misleading number. Everything runs in your browser.
Frequently asked questions
How is the area found?
From the three sides using Heron's formula, which needs no height. The angles come from the law of cosines.
What if the sides can't form a triangle?
If any side is longer than the other two combined, no triangle exists and the tool tells you so.
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Updated June 13, 2026