Choose what you know: base and height, or the three side lengths.
Formula
Area = base × height / 2 Heron: Area = √(s(s−a)(s−b)(s−c)), s = (a+b+c)/2
Worked example
Base 6 and height 4 give an area of 12. Sides 3, 4 and 5 give an area of 6.
Frequently asked questions
What is the height of a triangle?
The perpendicular distance from the base to the opposite corner.
When do I use Heron’s formula?
When you know the three sides but not the height.
Why can some sides not form a triangle?
Each side must be shorter than the sum of the other two.
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Results are estimates for information only. Check important figures with a professional.