Trapezoid Area Calculator
From the two parallel sides and the height between them, get the enclosed area — the one figure that describes how much surface a trapezoid covers.
Three inputs, one answer
Enter the two parallel sides and the perpendicular height, and the calculator returns the area as ((a + b) / 2) × h.
Height is perpendicular
The height must be the straight-across distance between the parallel sides, not the length of a slanted leg.
What is a trapezoid area calculator?
Two parallel sides plus a height
A trapezoid area calculator turns three measurements — the two parallel sides and the perpendicular height between them — into the area enclosed by the shape. A trapezoid (called a trapezium in the UK) is a four-sided figure with exactly one pair of parallel sides. Because only those parallel sides and their separation matter, three numbers are enough to size a land plot, a roof cross-section, a drainage channel, or a deck panel.
Enter the two parallel sides and the perpendicular height in any unit to get the area instantly.
The formula averages the two parallel sides, then multiplies by the height. Averaging the parallel sides is the key idea: a trapezoid is "between" two rectangles, so its area equals that of a rectangle whose width is the mean of the two parallel sides.
area = ((a + b) ÷ 2) × hHere a and b are the two parallel sides and h is the perpendicular distance between them. When a equals b, the average is just that side length and the formula collapses to width × height — a rectangle. The wider the gap between a and b, the more the shape leans, but the averaging keeps the area correct.
Suppose a trapezoid has parallel sides of 8 and 4 and a height of 5.
Add the parallel sides
8 + 4 = 12 — the combined length of the two parallel sides.
Halve the total
12 / 2 = 6 — the average width of the trapezoid.
Multiply by the height
6 × 5 = 30 square units — the area enclosed by the shape.
The area answers a practical question: how much surface does the shape cover? For our example, 30 square units is the space inside the trapezoid — the turf on a tapered garden plot, the cross-section of a channel that carries water, or the material in a sloped roof panel. The single most important thing to check is that your height is the perpendicular distance between the two parallel sides, measured at a right angle, and not the length of a slanting side; a slanted leg is always longer, so using it inflates the area. A useful sanity check is the special case where the two parallel sides are equal: the trapezoid becomes a plain rectangle, the average equals that side, and the area is simply width times height. If your two sides are close in length, expect a result near that rectangle; if they differ a lot, the averaging still gives the exact area because it treats the shape as the equivalent middle-width rectangle. Real uses span land plots, roof cross-sections, drainage channels, and deck panels — anywhere one pair of edges runs parallel.
The formula is exact, but a couple of practical points are worth keeping in mind.
Right shape, consistent units
This formula applies to a true trapezoid — a flat figure with exactly one pair of parallel sides — and uses the perpendicular height, never a slanted leg. The inputs are also unit-agnostic, so the answer is only meaningful if you keep one unit throughout: sides in metres with a height in metres give an area in square metres, never a mix of metres and feet.