Concrete fills the volume of a specific area. The area is measured in cubic feet for doityourself projects, or in cubic yards for large projects. Most doityourself projects such as small patios can be completed by using 80pound bags of concrete. The number of bags you need for your project are calculated using mathematic equations that are usually forgotten after high school  but are really very simple to do and remember after you use them a few times.
Dimensions

Dimensions are calculated by measuring the length and width of the area where you want to put a concrete pad. Assume your concrete pad is 10 feet long and 12 feet wide. The dimension  when multiplied together  equals 120 square feet. The square footage dimension is then used with the depth of the pad to determine cubic feet in another calculation.
Depth

Depth represents how deep your concrete pad is. A common depth for a concrete pad is 4 inches. To obtain cubic feet, multiply the square feet of the surface by the depth and then divide by twelve. You must divide by 12 as the area is measured in feet, while the depth is measured in inches. Dividing by 12 will convert the dimensions and depth to cubic feet. For example,10 feet x 12 feet x 4 inches divided by 12 inches equals 40 cubic feet. Forty cubic feet equals the volume of concrete required to pour your concrete pad.
Cubic Feet in 80Pound Bags of Concrete

Each 80pound bag of concrete contains .60 cubic feet when mixed with water as directed. A 40 cubic foot concrete pad will take 67, 80pound bags of concrete. This amount is calculated by dividing 40 by .60. You can't purchase a partial bag of concrete so you must round up your result to the next highest whole number.
Round Holes

Round holes are not calculated the same as a square concrete pad. To calculate the amount of concrete for a large round hole, you will need to calculate the area of a circle (pi multiplied by the radius squared), then multiply the area by the depth of the hole, then divided by the total cubic inches in one cubic foot. Cubic inches in one cubic foot are 1,728 (12 x 12 x 12). For example, the volume of a round hole that is 12 inches wide by 24 inches deep is calculated as 3.14 (pi) x 6 x 6 (radius squared) x 24 (depth), divided by 1,728 (cubic inches in a cubic foot). The result is 1.57 cubic feet. Now, divide 1.57 cubic feet by .60, which is the cubic feet in one 80pound bag of concrete. The result is 2.62 cubic feet  it would take almost three bags of concrete to fill the hole.
Tips

A very large area of concrete is measured in cubic yards. It is more efficient to have the concrete brought to you in a cement truck.
When using bagged concrete, you have approximately 1 hour to use it before it starts to set up, depending on weather conditions. Concrete sets up faster when temperatures are warm and the worksite is in direct sun.
To calculate cubic yards, follow the steps to determine cubic feet and divide cubic feet by 27 (there are 27 cubic feet in a cubic yard).
References
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