Triangle Area Calculator
Find a triangle's area from a base and height, or from its three side lengths with Heron's formula.
Area
12
Area = ½ × base × height. A base of 6 and height of 4 give 12.
How it works
When you know the base and the perpendicular height, area is the easy half of base times height. A base of 6 and a height of 4 give an area of 12.
Sometimes you only have the three side lengths, and that's where Heron's formula earns its keep. Add the sides, halve the total to get s, then take √(s(s−a)(s−b)(s−c)). A 3-4-5 triangle works out to an area of exactly 6.
Three lengths only make a triangle if each side is shorter than the other two added together. If that fails — say a side is longer than the sum of the rest — the shape can't close, so the calculator shows a dash instead of a made-up number.
Frequently asked questions
Is the height the same as one of the sides?
Only in a right triangle. In general the height is the straight-line distance from the base to the opposite corner, measured at a right angle to the base — not the slanted side itself.
When should I use Heron's formula?
Reach for it when you have all three sides but no clean height to measure. It skips the height entirely and gets the area straight from the side lengths.
Why did the three-sides mode give me a dash?
The lengths you entered can't form a real triangle. Each side has to be shorter than the sum of the other two; if one is too long, the sides won't meet, so there's no area to report.