Triangle Area by Heron's Method Calculator

How it is calculated

The tool applies Heron's formula to the three side lengths a, b and c, all in the same length unit. First the semi-perimeter s = (a + b + c) / 2, then Area = √( s(s − a)(s − b)(s − c) ). It also reports the perimeter a + b + c. Both values are rounded to two decimals.

Every side must be positive and the sides must form a real triangle: the sum of any two sides has to be strictly greater than the third. If not (for example 1, 2, 3, which collapses into a line), the tool returns no result. The area comes out in the square of the unit you typed, while the perimeter stays in the unit itself.

Common mistakes

• Entering sides that cannot close into a triangle, such as 2, 3, 10; the tool gives no result because the triangle inequality fails.
• Using the full perimeter in place of the semi-perimeter when doing the check by hand; s is half of a + b + c.
• Mixing units, such as one side in metres and another in centimetres, which gives a meaningless area.
• Reading the area as a length: if the sides are in metres, the area is in square metres, not metres.
• Rounding the sides heavily before entering them; for a very flat triangle a small measuring error changes the area a lot.