Use perpendicular height
The height must be measured at a right angle to the base. A sloped side is not the same measurement unless the triangle is right-angled in that position.
Example
A triangle with base 10 and perpendicular height 6 has area 10 x 6 / 2 = 30 square units.
Common mistakes
Do not use a side length as height unless it is perpendicular to the chosen base. Keep base and height in the same unit.
Limitations
This calculator uses the base-height formula only. For three side lengths, coordinates or angles, use a triangle calculator with the matching method.
Last reviewed: 2026-05-17