How do you calculate how many yards of concrete to pour in a footer?

First, you need to calculate the volume to be filled with concrete.

Say the footer is 12" wide and 8" in height and 21 feet long. ( I picked these numbers simply to make the following arithmetic easier for me)

First, change the width and height to feet. 12" = 1 ft 8" = 0.67 ft.

Volume, the, = 1 ft x 0.67 ft x 21 feet = 14 cubic feet.

But you wanted the volume in "yards" which is really cubic yards. To convert our cubi feet of 14 to cubic yards, simply divide it by 27. ("27" = 9ft x 9ft x 9ft, which is a cube that contains 1 cubic yard.)

So, to finish up, 14 cubic feet/27 cubic feet = a little more than 1 1/2 cubic yards, so you had better order 1 5/8 cubic yards of concrete. Most places only sell by the full cubic yard, so you will have to order 2 cubic yards. Some places only sell a complete truck full, so you will have to order 8 or 10 cubic yards . . . $ that is expensive! $

You may be able to share a load with a different customer.

Or, of course, you may need several trucks full . . .