Trapezoid Area Calculator

Trapezoid area formula explained

The area of a trapezoid is A = ½ × (a + b) × h. The values a and b are the two parallel sides, often called the bases. The height is the perpendicular distance between those bases.

You can also think of the formula as average base length times height. That makes trapezoid area practical for sloped lots, retaining walls, roof faces, fabric patterns, and geometry diagrams.

Worked example: bases 8 m and 14 m, height 5 m

Why average the bases?

A trapezoid can be transformed into a rectangle-like shape whose effective width is the average of the two parallel sides. Multiplying that average by the height gives the total area.

Common mistakes

• Using the slanted leg as the height.
• Adding all four sides instead of only the two parallel bases.
• Forgetting to multiply by one-half.

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Step-by-step method

1. Identify the two parallel sides.
2. Add the parallel side lengths together.
3. Divide that sum by two to get the average base.
4. Multiply the average base by the perpendicular height.

Second example: bases 5 ft and 11 ft, height 4 ft

Practical interpretation

Trapezoids appear often in site plans, sloped panels, retaining walls, roof sections, and pattern cutting. The formula works when the top and bottom edges are parallel. If the sides are not parallel, the shape may need to be divided into triangles or measured with another method.

If you are estimating material, draw a simple diagram and label only the two parallel bases and the perpendicular height. This reduces the chance of using a sloped side by mistake.