Triangle Area Calculator
Calculate triangle area from base and height, or from three sides with Heron’s formula.
Calculation method
units
units
Formula
Examples
| Input | Result |
|---|---|
| base = 8, height = 5 | Area = 20 |
| sides a = 3, b = 4, c = 5 | Area = 6, Perimeter = 12 |
About this calculator
The most common way to find a triangle’s area is to multiply half the base by the perpendicular height. When you only know the three side lengths, Heron’s formula lets you compute the area without measuring any height: first find the semi-perimeter s, then take the square root of s(s−a)(s−b)(s−c).
For three-side mode this tool also checks the triangle inequality — every side must be shorter than the sum of the other two — and reports the perimeter. Heron’s formula works for any valid triangle, whether acute, right or obtuse.
Frequently asked questions
Heron’s formula finds a triangle’s area from its three sides: compute the semi-perimeter s = (a+b+c)/2, then Area = √(s(s−a)(s−b)(s−c)).
Only when the triangle inequality holds: each side must be strictly shorter than the sum of the other two sides. Otherwise the sides cannot close into a triangle.
Use the perpendicular height measured from the chosen base to the opposite vertex, not the slanted side length.
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