Concrete is ordered by the bag -- builders use the dry mix by adding water to create this durable substance. Before you begin a project, you may need to estimate how many bags of concrete you will need. Here's how.
To estimate the amount of concrete needed for a slab, you will need to know the depth (thickness) of the slab in addition to the length and width. For example, if you want to create a slab measuring 20 feet by 20 feet, you also need to determine average depth of the slab, typically around 4 inches (.33 feet).
Determine the volume of the slab by multiplying length X width X depth. In our example, that would be 20 X 20 X .33, for a volume of 136 cubic feet. Because concrete is ordered by the cubic yard, you need to divide the cubic feet by 27 (the total cubic feet in a cubic yard). Continuing with our example, 136 cubic feet divided by 27 equals 4.9 cubic yards of concrete.
Once you know how many cubic yards your project requires, you will need to translate that information into bags of concrete. As a general rule, for every cubic yard of concrete you need, you will require 90 40-pound bags. Many manufacturers make 80-pound bags, of which you will require 45 per cubic yard.
Consider ordering extra concrete for your project, around 5 percent, to account for spillage. Multiply the total cubic yards by 1.05 to add 5 percent to your calculations.