Manually calculate concrete amounts in cubic yards for square or rectangular areas by multiplying the length by the width by the depth and dividing by 27. Convert all measurements to the units of feet before calculating. Multiply the total cubic yards by 1.05 to allow 5 percent for errors.

To convert inches to feet, divide the number of inches by 12. Using this concept, a 4-inch slab is 1/3 feet. Many concrete suppliers, such as WestrocInc.com, offer a free volume calculator on their website that automatically makes the unit conversions. When using these calculators, make sure the numbers you enter are in the correct units.

When the concrete surface is not rectangular, calculations become more complex. Calculating the volume of concrete to order sometimes requires breaking down the surface area into geometric shapes. Use the appropriate formula to calculate the surface area in square feet and multiply the results by the thickness in feet before dividing by the conversion factor and multiplying to account for errors.

Some small jobs do not require the use of ready-mix cement. For these jobs, the user needs to determine the number of bags of product to buy. Concrete product manufacturers, such as QUIKRETE.com, offer online calculators that provide an easy way to determine the amount of product the job requires.