Dump truck bodies are measured by the cubic yard.
One yard is one cubic yard, a measure 3' x 3' x 3', or 27 cubic feet.

To estimate your project, measure the length x width x depth, using the same units of measure for each.
Use foot units for easy calculations.

Example: Say you have an area that is 12' x 18 '. You want to cover it with 6" of material.
Multiply, 12' x 18' x .5' to get 108 cubic feet. You see, 6" is .5 feet or 1/2 a foot.
Now, divide 108 cubic feet by 27 cubic feet(1 yard) to get your total yardage at 4 yards.

If you want a 3" cover you use ¹/₄' or .25'. A four inch cover is 1/3 foot or .33', etc.

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