Trapezoid Area Calculator
Enter the two parallel sides and the height to get the area of a trapezoid from A = ½(a + b) × h — with the substitution shown step by step.
Example: with Base a (one parallel side) 8 · Base b (other parallel side) 5 · Height h (perpendicular distance) 4 → Area: 26.00.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outHow to find the area of a trapezoid
A trapezoid has exactly one pair of parallel sides — the bases a and b. The trapezoid area formula averages them and multiplies by the height: A = ½(a + b) × h. With bases of 8 and 5 and a height of 4, the average base is 6.5, and 6.5 × 4 = 26 square units. That average, (a + b) ÷ 2, is the trapezoid’s median (midsegment) — the formula is really just median × height, the same idea as a rectangle’s length × width.
The height must be the perpendicular distance between the bases, not the length of a slanted leg. In a right trapezoid one leg already meets the bases at 90°, so that leg is the height — bases 10 and 6 with a vertical leg of 5 give ½(16)(5) = 40. Units multiply too: measurements in feet produce square feet, so keep every input in the same unit.
How it’s calculated
Area = ½ × (a + b) × h, where a and b are the parallel sides and h is the perpendicular distance between them. The median row reports (a + b) ÷ 2, the midsegment whose product with h equals the same area. The formula holds for every trapezoid — right, isosceles, or scalene — and results carry the square of whatever unit you enter.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Using a slanted leg as the height — h is the perpendicular distance between the parallel sides, always the shortest path.
- Forgetting the ½ — adding the bases and multiplying by height double-counts; average the bases first.
- Mixing units, like bases in feet and height in inches — convert everything to one unit before multiplying.
Frequently asked questions
What is the trapezoid area formula?
A = ½(a + b) × h: add the two parallel sides, halve the sum, and multiply by the perpendicular height. Bases 8 and 5 with height 4 give ½(13)(4) = 26.
How do I find the area of a right trapezoid?
The same formula applies — and the leg that meets both bases at 90° is the height. Bases 10 and 6 with a perpendicular leg of 5 give ½(10 + 6)(5) = 40.
Which sides are the bases of a trapezoid?
The two parallel sides, whatever their orientation. The other two sides are the legs and never enter the area formula directly.
Why does averaging the bases work?
A trapezoid has the same area as a rectangle whose width is the median, (a + b) ÷ 2. Multiplying that average width by the height gives the exact area, not an approximation.