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.

## Measurements

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).

## Calculations

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.

## Buying Bags

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.

## Extra Material

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.