Geometry Tutorials: How to calculate the surface area of a trapezoid

To calculate the surface area of a trapezoid we need to think of a rectangle. This might sound a bit strange at first, but if you bear with me you will see why the formula for calculating the surface area of a rectangle, and that for calculating the area of a trapezoid is not that different.

First of all, the main difference between a rectangle and a trapezoid is that the top and bottom borders on a trapezoid have different lengths. Once you add these two lengths up however, and divide them by two, you are left with an average border length that gives the trapezoid the same format of a rectangle. Without changing the amount of surface area of course.

This in turn allows us to apply the same old rectangle formula for calculating surface areas. Height times width. Or in this case:

A = ((b1 + b2)/2)h

So, the surface area of a trapezoid equals border1, plus border2, divided by 2, and then multiplied by height. Piece of cake! Note: make sure that you solve the brackets first. Do not for instance divide b2 by 2, and then add it to b1. That is a recipe for disaster!