How to Figure Concrete Flatwork

Determine the measurements of your concrete flatwork project by measuring the proposed slab. Accurate measurements at this point are absolutely crucial, as a mistake in measurement could result in too much concrete, or even worse, not enough concrete.

Convert the thickness of your slab to a decimal in order to use it in the concrete formula, and round the decimal to the nearest .05 or .10. For reference, 3 inches equals .25 foot, while four inches equals .35 foot. A 5-inch thickness would be .41 foot, while a 6-inch thickness would be .50 foot. Six inches is likely the thickest depth at which you'll pour your concrete.

Translate your measurements into the volume of concrete you need for the project using the following formula: ((length x width x depth)/27 cubic feet). For example, a four inch (.35 foot) thick slab measuring 28 feet long by 15 feet wide would look like this in the formula: (28x15x.35)/27=5.44 cubic yards of concrete.

Always add 5 to 10 percent extra to adjust for any mistakes or discrepancies. To determine the amount of extra concrete, multiply the total cubic yards by .05 for 5 percent or by .10 for 10 percent, and add that amount to the total cubic yard amount. For example, 5.44 cubic yards multiplied by .05 equals .27. Add .27 to 5.44 for a total amount needed of 5.71 cubic yards. Ten percent extra would be .54, for a total of 5.98 cubic yards.

Call your local concrete company and tell them how much concrete you need according to your calculations. Often, they'll ask for the measurements and thickness of your project to ensure that you have calculated correctly.