Concrete Foundation Calculation Guide

Definitions

• Though cement and concrete are often used interchangeably, they are two different substances. Cement is the basic ingredient of concrete, with 85 percent of its mass made up of lime and silica. Concrete combines paste and aggregates to form a hard substance. It is made up of 11 percent cement, 41 percent gravel of crushed stone, 26 percent sand, 6 percent air and 16 percent water. The International Residential Code, on which U.S. building codes are based, states that concrete slabs on the ground must be a minimum of 3.5 inches thick.

Mixes

• The most convenient way to pour a concrete foundation is to use premixed concrete, which is available in several types including regular for most foundation work, fiber reinforced to reduce cracking and eliminate the need for wire mesh, and lightweight, which provides more cubic feet than the same amount of regular concrete. The amount of mix needed depends on the slab thickness, the square footage, and the size of the mix bags.

Four-Inch Slabs

• Assuming a slab that is 4 inches thick, the number of 60-pound bags required for 9 square feet is 6, for 15 square feet is 10, for 20 square feet is 13.33, for 35 square feet is 23.33, for 45 square feet is 30, for 50 square feet is 33.33 and for 100 square feet is 66.66. The number of 80-pound bags required for 9 square feet is 4.5, for 15 square feet is 7.5, for 20 square feet is 10, for 35 square feet is 17.5, for 45 square feet is 22.5, for 50 square feet is 25 and for 100 square feet is 50.

6-Inch Slabs

• For 6-inch thick slabs, the number of 60-pound bags required for 9 square feet is 9, for 15 square feet is 15, for 20 square feet is 20, for 35 square feet is 35, for 45 square feet is 45, for 50 square feet is 50 and for 100 square feet is 100. The number of 80-pound bags required for 9 square feet is 6.75, for 15 square feet is 11.25, for 20 square feet is 15, for 35 square feet is 26.25, for 45 square feet is 33.75, for 50 square feet is 37.5 and for 100 square feet is 75.

Costs

• To calculate the cost of concrete, the consumer information site ConcreteNetwork.com recommends using $2.50 to $4.50 per square foot for plain, undecorated concrete. These prices were accurate at the time of publication. This means a 100-square-foot slab would cost anywhere from $2,500 to $4,500 total. Breakdowns for the price of a concrete foundation include $75 per cubic yard for the cost of concrete, $50 per hour for the cost of grading, $12 per cubic yard for a grave or sand subbase, $1 to $1.50 per square foot for concrete forms and finishing, and $0.10 per square foot for reinforcement in the form of wire mesh, rebar, fiber or plastic mesh.