EHT Tip: How to Calculate Concrete Coverage by Bag Size
To calculate concrete coverage, record your measurements in feet and decimal equivalents of inches to make the math easier to do. For example, 6 inches equals half a foot (or 0.5 foot). Four inches, or a third of a foot, would be 0.333 feet. So, 12 feet, 4 inches would be recorded as 12.33. Those are easy numbers to convert, but it gets trickier. The decimal equivalent of one inch is 0.083 (1 divided by 12), so 5 inches of would be recorded as 0.415 (5 x 0.083).
First, measure the length and width of your slab in feet. Record those two figures, using the decimal equivalents for the inches. Second, determine the depth of your slab. For most applications a depth of 4 inches is standard. To support heavier items such as machinery or vehicles, you may want 6 inches or more. Record the depth of your slab, expressed in its decimal equivalent. Multiply the length by the width. Then multiply the resulting number by the depth you recorded to determine the cubic feet of concrete required.
Next, determine how many bags of ready-mix concrete are required to fill the slab. An 80-pound bag will fill 2/3 of one cubic foot of slab area—about 0.67 cubic feet. Divide the total cubic feet of concrete required by 0.67 to determine the number of 80-pound bags required. (Note: A 60-pound bag of ready-mix concrete will fill 0.45 cubic feet. If using 60-pound bags, divide the cubic feet of the slab by 0.45 to determine the number of 60-pound bags required).
If you decide the required quantity of concrete is too great to be mixing by bag, you can order a concrete truck, but commercial concrete suppliers sell concrete by the cubic yard—not cubic feet. One cubic yard equals 3 x 3 x 3 feet (27 cubic feet). You can convert the cubic feet of your slab into cubic yards by dividing the number by 27 (cubic feet required for slab / 27 = cubic yards required for slab).