How to calculate cement slabs

Written by l.p. klages
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How to calculate cement slabs
A patio can add more functional living space to a home. (patio image by fotogisèle from

Cement patios can add character, charm and additional living space to the outside of a home. Many homeowners choose to save a bundle of money by pouring the patio themselves using ready-mix concrete that they add water to. Before you begin the project you'll need to determine how many bags of ready-mix concrete you'll need for your size patio.

Skill level:

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  1. 1

    Measure the length and width of the area you want to cover with cement. For example, you might want to build a patio in your backyard measuring 10 by 18 feet.

  2. 2

    Multiply the length and width you found in Step 1 by each other. In the example, 10 feet times 18 feet is 180 square feet.

  3. 3

    Multiply the answer from Step 2 by one third of a foot to find the volume of the concrete slab. Patios are generally 4 inches deep, which is one third of a foot. One hundred eighty square feet times one third of a foot is 60 cubic feet.

  4. 4

    Divide the answer from Step 3 by two thirds. An 80-pound bag of ready-mix concrete fills two thirds of a cubic foot. Sixty cubic feet divided by two thirds is 90 bags of concrete.

Tips and warnings

  • Some speciality types of concrete such as structural concrete used on elevated decks have different volumes, so check the front of your concrete bags and adjust the divisor in Step 4 if necessary.

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