Home Repair and Paint Guide

Here’s a question that keeps popping up during home construction projects - “How do I calculate the amount of concrete needed to fill an area?” The answer is simple once you know a few basic terms. For this article we will use an example - pouring a slab for a detached garage.

Concrete is ordered by the cubic yard, and is abbreviated to just “yard”. A cubic yard contains 27 cubic feet, and the measurement is 3 feet wide, 3 feet high, and 3 feet long, or 3′ x 3′ x 3′ = 27 cubic feet. This forms the basic unit of measurement for ordering concrete.

Getting back to our example of pouring a slab for a detached garage, let’s use measurements of 20 feet wide, 24 feet long, and 4 inches thick. Start by calculating the square feet of surface area. The formula is:

20′ x 24′ = 480 square feet of surface area.

That part is easy enough. It’s the standard square foot measurement that everyone is used to. But now we have to get those inches in there somewhere, and that’s the part that seems to confuse a lot of people. How to do it? Convert the inches to a fraction of a foot. Since 1 foot = 12 inches, divide the thickness (in this case 4 inches) by 12. Here’s the answer:

4″ / 12″ = 0.33333333, or simply .333.

Now we have everything needed to complete the calculations.

480 x .333 = 159.84 cubic feet. This is the total area expressed in cubic feet. To convert this to yards, divide by 27:

159.84 / 27 (cubic feet per cubic yard) = 5.92. Round this up to 6 cubic yards.

Add about 10% for waste and you should be OK. In this case, that would be 6 x 1.10 = 6.6 yards. Most concrete companies deliver whole yards only, so for this project I would order 7 yards. Since most areas to be filled with concrete have a few low spots, you will quite likely use the entire amount of material.