How to calculate the square footage of a trapezoid
A trapezoid is a four-sided shape with exactly one pair of parallel sides. Those two parallel sides are the bases, usually labelled a and b, and the perpendicular gap between them is the height, h. To find the square footage you average the two bases and multiply by the height: Area = ½ × (a + b) × h. Averaging matters because the shape is wider at one base than the other, and the average width behaves exactly like the width of a rectangle of the same height.
The most common mistake is measuring the wrong distance for the height. The height is the straight-line, perpendicular distance between the two parallel sides — not the length of either slanted edge. If you measure along a sloped wall, you will overstate the area. Pull the tape square across the shape instead. Once you have both bases and a true perpendicular height, the calculator above returns the square footage instantly.
Two easy ways to use the calculator above
The embedded tool has rectangle and triangle tabs, and either one handles a trapezoid. The fastest method is the rectangle-average trick: on the rectangle tab, enter the height as the length and enter the average of the two parallel sides — (a + b) ÷ 2 — as the width. With 12 ft and 8 ft bases the average is 10 ft, so a 10 ft × 10 ft rectangle gives the same 100 sq ft.
The second method splits the trapezoid into two familiar pieces: one rectangle plus one right triangle. Use the shorter base as the rectangle width and the height as its length, then treat the leftover wedge as a triangle whose base is the difference between the two parallel sides. Add the two results together for the total. Both methods give an identical answer — pick whichever matches how you took your measurements.
When a shape is more than one trapezoid
Real rooms and lots are often a combination of shapes rather than a single clean trapezoid. When that happens, divide the floor plan into non-overlapping rectangles, triangles, and trapezoids, calculate each piece separately, and add the areas. For heavily broken outlines with several angled walls, the irregular shapes approach — slicing the plan into simple pieces and summing them — is the reliable way to reach an accurate total.