Trapezoid Side Calculator

Created by Luciano Mino
Reviewed by Anna Szczepanek, PhD
Last updated: Sep 28, 2022

With our trapezoid side calculator, you can easily find the missing side of a trapezoid, given the perimeter and the length of its other sides.

If you're wondering:

  • How do I find the missing side of a trapezoid?
  • Does a trapezoid have one pair of parallel sides?
  • Can any side of a trapezoid be called a base?

We will answer all those questions in a short text. Keep reading to learn how to use the trapezoid side length calculator!

What is a trapezoid?

A trapezoid is a 2D geometric figure with four sides where at least two are parallel to each other. Given that there is no further restriction to what constitutes a trapezoid, different shapes can fit the description, such as a parallelogram (two pairs of parallel sides), isosceles trapezoid (two congruent sides), or a right trapezoid (one side perpendicular to the bases).

Trapezoid with abcd sides.
Trapezoid with a,b,c,d sides.

🙋 You can get a more detailed description of trapezoids in Omni's trapezoid calculator paired with different examples in solving trapezoid problems.

Using the trapezoid side calculator

The trapezoid side calculator takes four inputs and calculates the missing side based on the following equation:

a=Pbcda = P - b - c - d

Here aa, bb, cc and dd are the sides of the trapezoid and PP is its perimeter. Notice how one of the lengths is on the other side of the equation? That's because we chose aa to be the missing side in this case.

To use this equation, we would need to leave the missing length on the left side of the expression and then subtract the sum of all the other lengths from the perimeter on the other side.

With the trapezoid side calculator, you just need to leave the missing side empty and input all other parameters. It doesn't matter which side you call 'a', 'b', 'c', or 'd' in your trapezoid. As long as you leave one empty and input the others, the calculator will automatically fill in the blank field.

How do I find the missing side of a trapezoid?

To find the missing side of a trapezoid:

  1. Identify which side is missing.
  2. Add together all the other sides and write down the result.
  3. Subtract that number from the perimeter. The result is the missing side's length.

As you can see, obtaining the missing side given the length of the other sides and the trapezoid's perimeter is really easy.


Can any side of a trapezoid be called a base?

No, only any of the parallel sides can be called a base. The non-parallel sides of a trapezoid are called the 'legs'.

Does a trapezoid have one pair of parallel sides?

No, but a trapezoid must have at least one pair of parallel sides. It could have up to two pairs of parallel sides. In that case, it would be called a parallelogram.

Luciano Mino
A trapezoid with sides, and height marked
Perimeter (P)
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