How to find the area of a trapezoid
The area of a trapezoid averages the two parallel bases and multiplies by the height: A = ((a + b) ÷ 2) × h. With bases 6 and 10 and height 4, A = ((6 + 10) ÷ 2) × 4 = 8 × 4 = 32 square units. For a slanted four-sided shape with equal bases, use the parallelogram area calculator.
Averaging the bases works because the trapezoid covers the same area as a rectangle whose width is the midpoint between the two parallel sides — here 8 units wide and 4 tall.
How to use this calculator
Enter both parallel sides as base a and base b, plus the perpendicular height between them — all in the same unit. The height is the straight distance between the bases, not the slanted side. Compare with the rectangle area calculator.
Using a slanted leg as the height is the standard mistake and always overstates the area. If the two bases are equal the shape is a parallelogram, and the formula still returns the correct answer.
Worked example
A garden bed has parallel edges of 6 m and 10 m, 4 m apart. Area = ((6 + 10) ÷ 2) × 4 = 8 × 4 = 32 square metres.
Sanity-check any result against two rectangles: the area must exceed 6 × 4 = 24 and fall below 10 × 4 = 40, and 32 sits between them. To work out the edging or fencing around the shape instead, use the perimeter calculator.
Frequently asked questions
- What is the formula for the area of a trapezoid?
- A = ((a + b) ÷ 2) × h, where a and b are the parallel bases and h is the perpendicular height.
- What is the area of a trapezoid with bases 6 and 10 and height 4?
- 32 square units (((6 + 10) ÷ 2) × 4).
- Which sides are the bases?
- The two parallel sides. The height is the perpendicular distance between them, not the slanted leg.
- What units is the area in?
- Square units of whatever you measured the bases and height in.
- Does it matter which base I enter as a and which as b?
- No. The two bases are added together before being halved, and addition works in either order, so swapping them gives the same area. Only the height needs care, since it must be measured perpendicular to both bases.