What is the Irregular Trapezoid Area Calculator?
A trapezoid (trapezium) is a quadrilateral with one pair of parallel sides. Even when the trapezoid is irregular — meaning its legs are unequal and slanted — its area depends only on the two parallel sides and the perpendicular distance between them. This calculator finds that area instantly from sides a, b and height h.
How to use it
Enter the lengths of the two parallel sides (the top and bottom edges) and the height, which is the perpendicular distance between those parallel sides — not the slanted leg length. Use the same unit for all three inputs (cm, m, in, etc.) and the result will be in those units squared. Press calculate to see the area and the midline.
The formula explained
The area of any trapezoid is given by:
$$A = \frac{a + b}{2} \times h$$
The term \(\frac{a + b}{2}\) is the average of the two parallel sides, also called the midline or median. Multiplying that average width by the height gives the area, exactly as you would for a rectangle of equivalent average width.
Worked example
Suppose the parallel sides are \(a = 8\) and \(b = 5\), with height \(h = 4\). Then the midline is $$\frac{8 + 5}{2} = 6.5,$$ and the area is $$6.5 \times 4 = 26 \text{ square units.}$$
FAQ
Does this work for irregular trapezoids? Yes. The formula only needs the two parallel sides and the perpendicular height, so unequal slanted legs do not change the result.
What if I only know the slant side length? You must convert it to the perpendicular height first; the slant length alone is not enough.
What units does the answer use? Whatever unit you used for the inputs, squared — e.g. centimeters give square centimeters.
