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Source: www ehow.com/about_6507028_cubic-compared-quarts-potting-soil.html
Cubic Feet to US Liquid Measurement
Materials like potting soil, compost and mulch are usually sold by volume rather than weight. This is because weight can be highly variable based on moisture content. The measurements are generally stated in cubic feet. A cubic foot is 12 inches by 12 inches by 12 inches. When working on smaller gardening projects, you will often find instructions with quarts or gallons used to measure these items. The author isn't trying to confuse you, he's just assuming that you have better access to measuring containers that are calibrated in these measurements. One cubic foot contains 29.9220779 quarts. 30 is precise enough for most applications. This makes the conversion for a gallon 7.48051 or 7.5 when close counts.
Cubic Feet to US Dry Measurements
"Quarts" and "gallons", though, actually have two definitions. Most Americans are accustomed to the US Liquid standard that we use for milk and orange juice every day. There is also a US Dry standard, which converts as 25.71404 quarts per cubic foot and 6.42851 gallons per cubic foot. If precision in a project is vital, confirm whether the liquid or dry standard is in use.
Cubic Feet to Cubic Yards
Going in the other direction, bulk materials like gravel, top soil, concrete and mulch are generally measured in cubic yards rather than cubic feet. A cubic yard is 36 inches by 36 inches by 36 inches. There are 27 cubic feet in a cubic yard.
Calculating Volume to Cover an Area
To calculate how much mulch you need to cover a flower bed, or how much soil you need to fill a new raised bed garden box, measure the length and width of the area in yards (feet divided by three). Then measure the depth and convert it to a fraction of a yard and multiply that fraction times the square area. For example. You have a new box for your garden that is four feet wide by 12 feet long and 18 inches deep. That converts to 1.3333 yards wide by 4 yards long by 0.5 yards deep or 2.67 cubic yards.