How to Calculate the Area of an Irregular Trapezoid

••• geometry image by Alexey Klementiev from

A trapezoid is a four-sided polygon with one pair of parallel sides. If the two non-parallel sides are the same length, the shape is called an isosceles or "regular" trapezoid. If the non-parallel sides are different lengths, the resulting shape is what's known as an irregular trapezoid. However, despite this additional layer of complexity, the area of an irregular trapezoid can be a calculated with a simple formula.

    Line up the straight edge of a protractor along the shorter of the two parallel sides.

    Use the protractor to draw a line perpendicular (i.e. at a 90-degree angle) from this side of the trapezoid, extending all the way to the opposite parallel side.

    Mark the point where the perpendicular line intersects the shorter side as "A."

    Mark the point where the perpendicular line intersects the longer side as "B."

    Measure the distance between points A and B with a ruler. Call this the "height" of the trapezoid.

    Measure the length of the shorter parallel side.

    Measure the length of the longer parallel side.

    Add the lengths of the sides together. For example, if the length from Step 6 (the shorter side) was 2 in. and the length of Step 7 (the longer side) was 7 in., the sum would be 9 in.

    Divide this sum by 2.

    Multiply the answer from Step 9 by the height of the trapezoid (from Step 5). The result is the area of the irregular trapezoid.


About the Author

A Chicago-based copywriter, Andy Pasquesi has extensive experience writing for automotive (BMW, MINI Cooper, Harley-Davidson), financial services (Ivy Funds, William Blair, T. Rowe Price, CME Group), healthcare (Abbott) and consumer goods (Sony, Motorola, Knoll) clients. He holds a Bachelor of Arts in English from Harvard University but does not care for the Oxford comma.

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