Trapezoid Area Calculator
Find the area of a trapezoid from its two parallel sides and height.
What the Trapezoid Area Calculator does
A trapezoid has one pair of parallel sides. Its area equals the average of those two sides multiplied by the perpendicular height — effectively treating it as a rectangle of the average width.
Formula
Area = ((a + b) ÷ 2) × h
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Parallel side a | number | Yes | Accepts more than 0. |
| Parallel side b | number | Yes | Accepts more than 0. |
| Height | number | Yes | Accepts more than 0. |
How to use it
- Enter Parallel side a, Parallel side b and Height.
- Select Calculate.
Worked example
A garden bed with parallel edges of 8 m and 5 m, 4 m deep.
- Side a
- 8
- Side b
- 5
- Height
- 4
((8 + 5) ÷ 2) × 4 = 26 m².
Reading the result
- Averaging the two parallel sides turns the trapezoid into a rectangle of the same area, which is why the formula works.
- When the parallel sides are equal the shape is a parallelogram, and the formula reduces to base × height.
Common mistakes
- Using a slanted side as the height. The height is the perpendicular distance between the two parallel sides.
- Averaging the non-parallel sides. Only the parallel pair enters the formula.
Frequently asked questions
Is the height the slanted side?
No — it is the perpendicular distance between the two parallel sides.