Square footage of a trapezoid: in short
Add the two parallel sides, divide by 2 and multiply by the height between them. Sides of 14 ft and 8 ft that are 10 ft apart give (14 + 8) ÷ 2 × 10 = 110 sq ft.
The formula
Square feet = (a + b) ÷ 2 × h. The two parallel sides are a and b; h is the distance between them, measured at a right angle. In effect you take the average width and multiply it by the depth.
Examples
| Side a | Side b | Height | Area |
|---|---|---|---|
| 10 ft | 6 ft | 8 ft | 64 sq ft |
| 12 ft | 8 ft | 10 ft | 100 sq ft |
| 14 ft | 8 ft | 10 ft | 110 sq ft |
| 16 ft | 10 ft | 12 ft | 156 sq ft |
| 20 ft | 14 ft | 15 ft | 255 sq ft |
| 30 ft | 20 ft | 25 ft | 625 sq ft |
Where you find trapezoids
A room with one slanted wall, a bay window seen from above, a patio that narrows toward the garden, or a lot that is wider at the street than at the back. Each is a trapezoid when two of its sides run parallel.
Measure the height, not the slanted side
The slanted sides are always longer than the height. Using one of them gives too large an area: in the example above a slanted side is 10.4 ft, and using it would add 4.8 sq ft that are not there.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
About this page · Written by the Squaremetry team. Last checked on October 1, 2026. Found a mistake? Send us a message.