Trapezoid Area Calculator
Enter the two parallel bases and the perpendicular height. Area uses ((a + b) / 2) × h. Add both non-parallel legs when you also want perimeter.
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How to find trapezoid area
- Enter the lengths of the two parallel bases as a and b. The defaults are 10 and 6.
- Enter the perpendicular height between those bases. The default height is 4.
- Read the area. The default inputs give ((10 + 6) / 2) × 4 = 32.
- Optionally enter both non-parallel legs if you need the perimeter as well.
- Compare with the parallelogram or rectangle pages when both pairs of sides are parallel.
Trapezoid
Quadrilateral with exactly one pair of parallel sides (US school usage)
| Bases | The two parallel sides a and b |
|---|---|
| Height | Perpendicular distance between the bases |
| Area formula | ((a + b) / 2) × h |
| Legs | The two non-parallel sides; optional for perimeter |
| Default example | a = 10, b = 6, h = 4 give area 32 |
| Perimeter | a + b + leg1 + leg2 when both legs are positive |
What a trapezoid is on this page
In US school geometry a trapezoid is a quadrilateral with exactly one pair of parallel sides, called the bases. The height is the perpendicular distance between those bases, not the length of a slanted leg. British texts often call the same shape a trapezium.
How trapezoid area is computed
Area equals the average of the two bases times the height: ((a + b) / 2) × h. Think of it as the area of a rectangle whose width is the mean base length. The default 10, 6, and 4 produce area 32 in whatever length unit you typed.
Optional legs and perimeter
Area needs only the bases and height. Perimeter needs the full boundary, so you must supply both non-parallel legs. If either leg is blank, the page still prints area and notes that perimeter needs both legs. It does not invent missing slant lengths from height alone.
When trapezoid area shows up
Roof sections, retaining-wall faces, and cross-sections of canals often reduce to a trapezoid. Land-survey plats that list two parallel fronts and a height fit this form. Irregular four-sided lots with no parallel sides need a different method.
Limits
Flat Euclidean area only. No 3D prism volume, no unit conversion, and no automatic check that the legs can close the figure in the plane. IEEE floating point applies. CZNull does not receive the numbers.