Triangle Area Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/3/2026
The area of a triangle is calculated with (base × height) ÷ 2. For a base of 8 cm and a height of 5 cm, the area is 20 cm².
Explanation
This formula works for any triangle, whatever its shape (equilateral, isosceles, right, or scalene), as long as you use a base and the height that exactly matches it: the height is the perpendicular segment connecting the base to the opposite vertex, not just any side of the triangle. The underlying geometric idea is this: a triangle occupies exactly half the area of the rectangle (or parallelogram) that would share the same base and height, hence the division by two. If you only know the lengths of the three sides, with no height, Heron's formula is better suited, but it isn't covered by this calculator. For a right triangle specifically, where you know the two legs forming the right angle, our hypotenuse calculator (Pythagorean theorem) finds the third side. For other geometric shapes, see also our circle area and circumference calculator.
Example: a triangle with base 8 cm and height 5 cm
Inputs
Base: 8 cm. Height: 5 cm.
Calculation
Area = (8 × 5) ÷ 2 = 40 ÷ 2 = 20.
Result
The area of this triangle is 20 cm².
Frequently asked questions
Which side should I choose as the base?
Any of the three sides can serve as the base, as long as the height you use is the one measured perpendicularly from that side to the opposite vertex. The resulting area is the same no matter which side you choose as the base.
How do I find the height if I don't know it?
In a right triangle, one of the two legs forming the right angle can often serve directly as the height relative to the other. In the general case, without a direct height measurement, you'll need another formula such as Heron's, using only the three side lengths.
Does this formula work for a right triangle?
Yes: in a right triangle, the two sides that form the right angle can directly serve as base and height for each other, which makes the calculation especially simple.
Why do we divide by 2 in the formula?
Because a triangle is exactly half of a rectangle (or parallelogram) with the same base and height. It's this geometric relationship that justifies dividing by two.