2 Answers

courtesy of ONLINE CONVERSION

The formula for Square feet is:
length × width

The formula for Cubic feet is:
length × width × height

Since we know the Square feet of the garden, we now have the conversion to Cubic feet:
(cubic feet) = (square feet) × (height in feet)

So.... If you wanted the topsoil to be 3 inches thick, you would need:
12 × 0.25 = 3
You would need 3 cubic feet of topsoil.

Notice that I convert 3 inches to 0.25 feet before using the formula. All the measurements must be in the same units for it to work. And it will work for anything. Meters, feet, inches, just as long as all are the same.

Once again, here is the formula:
(cubic feet) = (square feet) × (height in feet)

Or for meters:
(cubic meters) = (square meters) × (height in meters)

Calculating cubic feet from square feet is impossible since the two are not the same measurement: cubic feet is of volume, whereas square feet is of area.