How to Calculate a Round Concrete Patio

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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 over-ordering 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 over-ordered 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 12-feet 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 4-inch 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 cubic-yard (27). Finishing the example, 37.33 divided by 27 equals 1.38, the cubic-yards of concrete needed to pour a patio 12 feet in diameter, 4-inches 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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References

  • Photo Credit concrete factory image by dinostock from Fotolia.com
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