Calculating the amount of concrete needed to pour a patio requires the use of a mathematical formula. Guessing the amount of concrete for your round patio will result in overordering material or falling short of the material needed during the concrete pour. Falling short will result in a patio weak from a thin slab or low spots in the finished patio. The extra money spent on overordered concrete will cause your project to run over budget.
Things You'll Need
 Calculator
 Notebook
 Pencil

Enter the diameter of your patio in the calculator.

Divide the diameter by two to find the radius.

Press the "times" and then the "equals" button to square the radius. For example, if your patio is 12feet in diameter, dividing by two will result in six. Pressing, "times" then "equals" will yield the number 36 (radius squared.)

Multiply the radius squared amount (36 in the example) by Pi (3.142) and then multiply that number by the depth of your slab.

Write that number in a notebook. Clear the calculator.

Divide the depth of your concrete by 12. For example, a 4inch thick slab is 4 divided by 12, which equals 0.33.

Multiply the saved number by the number you obtained in Step 6. Continuing with the example, the number written, 113.112 multiplied by 0.33 equals 37.33.

Finish the equation by dividing the result from Step 7 by one cubicyard (27). Finishing the example, 37.33 divided by 27 equals 1.38, the cubicyards of concrete needed to pour a patio 12 feet in diameter, 4inches thick.
Tips & Warnings
 Add 10 percent to the final number before you order concrete; it's better to have slightly too much rather than not enough.
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