Construction suppliers sell concrete by the cubic yard. If you pour concrete for a new driveway or patio, you need to calculate the volume of concrete in cubic yards. This will help you create a realistic remodelling budget and help you avoid running out of concrete when you pave. The most common shapes of concrete driveways and patios are rectangles and circles, so there are two different formulas for calculating concrete. Use the method that applies to your paving project.
Measure the width and length of the rectangle in feet, and call these measures W and L, respectively. For example, suppose the width is 12 feet and 6 inches and the length is 15 feet and 3 inches. Then W = 12.5 and L = 15.25. (Note that 6 inches = 0.5 feet and 3 inches = 0.25 feet.)
Measure the depth of the driveway or patio in feet, and call this measure D. For example, if the depth is 6 inches, then D = 0.5.
Multiply W, L and D to obtain the volume in cubic feet. Since 12.5 * 15.25 * 0.5 = 95.31, this project has a volume of 95.31 cubic feet.
Divide the cubic feet by 27 to obtain the number of cubic yards. Since 95.31 / 27 = 3.53, this project requires 3.53 cubic yards of concrete.
Measure the radius of the patio in feet, and call this measure R. The radius is half of the diameter. For example, if the diameter is 18 feet, then R = 9.
Measure the depth of the project in feet, and call this measure D. For example, if the depth is 1 foot, then D = 1.
Compute 3.1416 (Pi) * D * R^2 (radius squared) to obtain the cubic feet of concrete. Since 3.1416 * 1 * 9^2 = 254.5, this project requires 254.5 cubic feet of concrete.
Divide the number of cubic feet by 27 to obtain the number of cubic yards. Since 254.5 / 27 = 9.4, you need 9.4 cubic yards of concrete.