Triangle inequality matters
The calculator checks whether the side lengths can actually form a triangle. If the two shortest sides do not add up to more than the longest side, the shape collapses into a line and no triangle area exists.
Example
For sides 5, 6 and 7, the semiperimeter is (5 + 6 + 7) / 2 = 9. Heron's formula gives sqrt(9 x 4 x 3 x 2), or about 14.70 square units.
Common mistakes
All three side lengths must use the same unit. If you know base and height instead of all three sides, use a basic area formula rather than Heron's formula.
Limitations
Heron's formula works for any valid triangle when all three sides are known, without requiring height or angles. It does not determine orientation or draw the triangle to scale.
Last reviewed: 2026-05-17