Many home construction projects require a concrete slab foundation or pad. Foundations vary according to their intended use. Lightweight concrete pads, usually 2 to 3 inches thick, are not designed to bear a lot of weight and are generally used for small patio or shed floors. Moderate weight pads, such as standard garage floors, are typically 3 to 4 inches thick. Heavy weight concrete pads, such as home foundations, should be a minimum of 6 inches thick and may include footings or grade beams. No matter what you plan to build, calculating the cubic yards of concrete necessary for the foundation is a fairly easy process.
Calculate the total square footage of the foundation area. Measure the length and width of your foundation area using a tape measure. Multiply these two numbers together to determine the square footage. For example, a 12 foot by 14 foot area equals 168 square feet. For larger jobs, use an available building plan to determine measurements and separate the foundation area into smaller square or rectangular areas for your calculation, then add the square footage calculations for each area together to determine the total square footage.
Determine the thickness of your slab. The thickness of the concrete depends upon foundation usage, but is usually between two and six inches. Convert the thickness measurement into feet by dividing the number of inches by 12. For example, four inches equals 0.333 feet.
Calculate the cubic feet of your foundation area. Multiply the square footage area by the thickness of your slab to determine cubic feet. For example, 168 square feet multiplied by 0.333 feet equals 55.95 cubic feet.
Calculate cubic yards. One cubic yard is equal to 27 cubic feet. Divide the cubic feet of your foundation area by 27 to determine the cubic yards of concrete needed. For example, 55.95 cubic feet divided by 27 equals 2.07 cubic yards.
Calculate for overage. Multiply the cubic yards volume by 1.5 to determine the total cubic yards needed for the slab foundation. This overage will cover extra concrete needed for footings, grade beams and spillage. For example, 2.07 cubic yards multiplied by 1.5 equals 3.1 cubic yards.