Trapezoid Calculator

A trapezoid (called a trapezium in British English) is a four-sided polygon with exactly one pair of parallel sides. The parallel sides are called bases; the non-parallel sides are called legs. The height is the perpendicular distance between the two bases.

Take the average of the two parallel bases and multiply by the height: A = ((base 1 + base 2) ÷ 2) × height. For bases 8 and 12 with height 5, the area is ((8 + 12) ÷ 2) × 5 = 10 × 5 = 50 square units.

Add all four sides: P = base 1 + base 2 + side 1 + side 2. The two non-parallel sides (legs) are not equal to the height; the height is perpendicular to the bases while the legs slant. Enter both side lengths to get the perimeter.

An isosceles trapezoid has two equal non-parallel sides. The formulas are the same: enter the same value for both sides and the calculator returns the symmetric result. The same area formula works because it depends only on the bases and height.

A trapezoid is like a rectangle with one base that varies linearly from base 1 to base 2. Averaging the two bases gives the effective width of an equivalent rectangle with the same area, and multiplying by the height completes the area.

Technically a parallelogram has two pairs of parallel sides, so if you enter base 1 = base 2, the area reduces to base × height, which is the parallelogram formula. For a dedicated parallelogram calculator with its own input style, use the parallelogram calculator.