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Trapezoid Area Calculator

How do I find the area of a trapezoid?

How it is calculated

The area is the average of the two parallel sides multiplied by the height: A = (a + b) ÷ 2 × h. Here a and b are the lengths of the two parallel sides (bases) and h is the perpendicular distance between them, all in the same length unit. The result is in the square of that unit and is rounded to two decimals; the tool prints only the number, so you add the unit. All three values must be greater than zero, otherwise nothing is shown. The order of a and b does not matter. The slanted legs are not used, which is why the sloping side length is not an input.

Worked example

A trapezoid has parallel sides of 8 and 5 and a height of 4. Add the bases: 8 + 5 = 13. Halve it to get the average base: 13 ÷ 2 = 6.5. Multiply by the height: 6.5 × 4 = 26. The tool shows 26.00, which is 26 square units, for instance 26 m² if the sides were in meters.

Try this example →

Common mistakes

  • Entering the slanted side as the height. The height is measured at a right angle between the two bases, not along a leg.
  • Using two sides that are not parallel. Only the parallel pair counts as a and b; the other two sides are the legs.
  • Mixing units, such as bases in meters and height in centimeters. Convert everything to one unit first.
  • Forgetting to divide the sum by 2. Multiplying (a + b) by h gives double the true area.
  • Reading the result as a perimeter or in linear units. It is an area, so the unit is squared.

About this calculator

This calculator finds a trapezoid's area from its two parallel sides (bases) and its height — the perpendicular distance between those two bases — using the standard formula of averaging the two base lengths and multiplying by the height.

It is used by students working through geometry problems, and by tradespeople and DIYers measuring real trapezoid-shaped spaces like a sloped garden bed, a countertop with an angled edge, or a piece of land with two parallel boundary lines, where the shape is not a simple rectangle or triangle.

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Comments (1)

  • Editorial team

    A quick sanity check for any result: the area must fall between the shorter base times the height and the longer base times the height. With bases of 8 and 5 and a height of 4, anything outside 20 to 32 means a typo. Since the answer is a flat area, multiply it by a depth in the same unit if you are ordering soil or mulch for a sloped bed. What odd-shaped plot or countertop are you measuring?

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