In these formulas, height means the shortest perpendicular distance from a base to the opposite vertex or parallel side. It is a relationship between lines, not simply whichever side looks vertical on a drawing.
A triangle can use different bases, each with its own height
Any side may be chosen as the base, but the entered height must meet that same base at 90°. Using another side length as height is valid only for a right triangle whose sides are perpendicular.
For parallelograms and trapezoids, height spans the parallel lines
A parallelogram's slanted side is generally longer than its perpendicular height. In a trapezoid, the two values called bases must be parallel, and height is the perpendicular gap between them.
The calculator cannot verify the angle of your measurement
AreaCalc applies the formula to numbers you supply. It does not inspect a plan, image, coordinate set, or physical object, so verify perpendicularity with the appropriate drawing or measuring method before relying on the result.