Concrete Ideas, pt. 2

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Mar 1, 2001

Trying to figure out just how much concrete you will need can cause more headaches than any other part of concrete work. But with a few simple math tricks, it is quite easy. First of all, concrete is purchased in "yards". This refers to the volume of concrete in cubic yards. To arrive at the number of yards you will need is to figure out the volume in cubic feet and divide by 27, which is the number of cubic feet per cubic yard.

If your project is a walk, patio or other slab type project, you first need to figure out the thickness of the slab in fractions of a foot. A 4 inch thick slab is the average thickness and is .33 feet thick (or 1/3.) Next, measure the length and the width of the slab for the total surface area. If you are pouring a patio that is 10 feet by 15 feet, your surface area is 10 x 15, or 150 square feet. Then multiply the square footage by the thickness of your slab to find the cubic footage (150 x .33, or 49.5 cubic feet.) Divide that by 27 to arrive at the cubic yardage (49.5/27, or 1.8 cubic yards.) Add another 5 % for waste and you come to 2 cubic yards of concrete for a slab that is 10 feet by 15 feet and 4 inches thick.

If your patio is round, its a bit trickier, but just as easy to figure once you have the formula. To find the surface area of a circle, find the radius, which is the length from the center of the circle to the edge (or half of the diameter.) Multiply the radius by itself and then multiply the answer by 3.14 (or pi.) Multiply this by the thickness of your slab and you have the cubic footage. For example, you have a circle that is 10 feet across. The radius is 5 feet. 5 x 5 is 25 and 25 x 3.14 gives you 78.5 square feet of surface area. Then multiply 78.5 by .33 and you get 25.9 cubic feet. To find the number of cubic yards, divide by 27 (25.9/27, or .9 cubic yards) with waste figured in, your patio should use about 1 yard of concrete.

If your project is an odd shape, just divide it into smaller shapes and figure the area of each shape separately. Then add everything up and multiply by the thickness to arrive at the total cubic feet and find the cubic yards as before. Remember that all your measurements must be in feet or fractions of a foot, not inches.

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