Height means perpendicular height
For triangles, parallelograms and trapezoids, the height is the perpendicular distance between the base and the opposite point or side — not the length of a slanted edge. The diagram marks it with a dashed line and a right-angle square precisely because this is the single most common error in area problems.
A parallelogram leaning further over does not gain area. Its slanted sides get longer while the perpendicular height stays the same, and so does the area.
Why some shapes have no perimeter here
Rectangles and circles have perimeters determined by the same measurements as their area, so both are shown. A triangle given only base and height does not — many different triangles share them, with different side lengths. The same is true of a trapezoid without its slanted sides.
Ellipses are a special case: their circumference has no exact elementary formula at all, only approximations, so it is omitted rather than quietly rounded.
Units
The calculator is unit-agnostic — put in centimetres and the area is square centimetres. What matters is that every measurement uses the same unit. Mixing feet and inches is the other classic source of wrong answers, and no calculator can detect it for you.
Frequently asked questions
How do I find the area of an irregular shape?
Split it into shapes on this list, work out each one, and add them up. An L-shaped room is two rectangles; a house-shaped outline is a rectangle plus a triangle.
Is anything uploaded?
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