(Jump to Area of a Trapezoid or Perimeter of a Trapezoid)

A trapezoid is a 4-sided flat shape with straight sides that has a pair of opposite sides parallel.

Trapezoid (or Trapezium)
Isosceles Trapezoid
A pair of parallel sides

Called an Isosceles trapezoid when the sides that aren't parallel are equal in length and both angles coming from a parallel side are equal.

Called a "Trapezium" in the UK

Play with a trapezoid:


The parallel sides are the "bases"

The other two sides are the "legs"

The distance (at right angles) from one base to the other is called the "altitude"

Area of a Trapezoid


The Area is the average of the two base lengths times the altitude:

Area = a+b2 × h

Example: A trapezoid's two bases are 6 m and 4m, and it is 3m high. What is its Area?

Area  =  6 m + 4 m2 × 3 m  =  5 m × 3 m  =  15 m2

The Area of Polygon by Drawing tool is helpful when you can draw your Trapezoid.

Perimeter of a Trapezoid

The Perimeter is the distance around the edges.


The Perimeter is the sum of all side lengths:

Perimeter = a+b+c+d

Example: A trapezoid has side lengths of 5 cm, 12 cm, 4 cm and 15 cm, what is its Perimeter?

Perimeter = 5 cm + 12 cm + 4 cm + 15 cm = 36 cm

Median of a Trapezoid


The median (also called a midline or midsegment) is a line segment half-way between the two bases.

The median's length is the average of the two base lengths:

m = a+b2

You can calculate the area when you know the median, it is just the median times the height:

Area = mh


A trapezium (UK: trapezoid) is a quadrilateral with NO parallel sides.

The US and UK have their definitions swapped over, like this:

  Trapezoid Trapezium
US: a pair of parallel sides NO parallel sides
UK: NO parallel sides a pair of parallel sides