Triangle Area Calculator
Calculate the area of any triangle two ways: from its base and height, or from all three side lengths using Heron's formula.
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- 1Enter method in the form on the left.
- 2Fill in the remaining fields — the result updates automatically as you type.
- 3Review the highlighted result and the supporting breakdown on the right.
- 4Use Copy, Share or Print to save or send your result.
Two ways to find a triangle's area
If you know the base and the perpendicular height, area = ½ × base × height. When you only know the three sides, Heron's formula works: compute the semi-perimeter s = (a+b+c)/2, then area = √(s(s−a)(s−b)(s−c)).
The triangle inequality must hold — each side must be shorter than the other two combined — otherwise the three lengths cannot close into a triangle, and the calculator will tell you so.
Frequently Asked Questions
▸What is Heron's formula?
Area = √(s(s−a)(s−b)(s−c)) where s is half the perimeter. It finds a triangle's area from its three sides alone, no height needed.
▸Does the height have to be perpendicular?
Yes — the height in ½ × base × height is the perpendicular distance from the base to the opposite vertex, not the slanted side length.