Triangle Area Calculator
Calculate triangle area from base and height, or from three sides with Heron’s formula.
Calculation method
units
units
- 1
Base × height
8 × 5 = 40 - 2
Area = ½ × base × height
40 ÷ 2 = 20Every triangle is exactly half the rectangle sharing its base and height.
Formula
How this is calculated
This calculator finds the area of a triangle in two ways. In base-and-height mode you supply the base (one chosen side) and the perpendicular height — the straight-line distance from that base to the opposite vertex, measured at a right angle, not the slanted edge. The area is simply half their product, ½ × base × height, because any triangle is exactly half of the rectangle that shares its base and height.
In three-sides mode you enter the three side lengths a, b and c, and the tool applies Heron’s formula. It first computes the semi-perimeter s = (a + b + c) / 2, then takes the area as √(s(s−a)(s−b)(s−c)). This needs no angle or height and works for acute, right and obtuse triangles alike. The perimeter is just a + b + c.
All inputs share the same unit (the area is reported in that unit squared), so use one consistent unit throughout. Before computing, the calculator checks the triangle inequality: each side must be strictly shorter than the sum of the other two. If it fails, no real triangle exists and an error is shown instead of a result.
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.
Also known as
TG we-Calculate Editorial Team. (2026). Triangle Area Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/triangle-calculator
TG we-Calculate Editorial Team. "Triangle Area Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/triangle-calculator.
TG we-Calculate Editorial Team, "Triangle Area Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/triangle-calculator
@misc{wecalculate_triangle_calculator, title = {Triangle Area Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/triangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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