- 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). 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). 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). 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.
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