The following formula may be used to calculate the approximate amount of compost needed for a specified area. To use this calculation, substitute your own values for area and depth. See the example for details.
Volume Required (cubic yards) = area (sq. ft.) x depth (inches) x 0.0031
Example:
A vegetable farmer would like to add an amendment of compost to his field of lettuce to improve water retention. The field is 350 feet long and 200 feet wide. The farmer has determined that compost needs to be applied at a depth of 2 inches throughout the field.
Here is how the farmer performs the calculation.
First, the area of the field needs to be determined. The area of a rectangle is equal to length x width. In this case, area equals 350 feet x 200 feet = 70,000 square feet.
Area was the only preliminary calculation needed in this case, so the farmer can plug the numbers into the formula. It looks like this...
Volume Required (cubic yards) = 70,000 (sq. ft.) x 2 (inches) x 0.0031
Using a simple calculator, the vegetable farmer determines that a total of 434 cubic yards of compost will be needed for the field.

2003 Cornell Waste Management Institute
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