To convert square feet to cubic feet, you're going to need to do some measuring. The formula for a square or rectangular object's cubic feet is its length times width times height, or L × W × H. You might use this formula to figure out how many cubic feet of soil to buy for your garden, for example, or to determine how many ounces of pesticide to use when releasing a bug bomb inside a room of your house.
TL;DR (Too Long; Didn't Read)
To convert a measurement in square feet to cubic feet, multiply by the measurement of the missing dimension (either length, width or height).
Measure the object's height in feet. (You have already measured the object's length and width to get its measurement in square feet.)
Divide that number by 12 to get its measurement in feet, if you measured the object in inches. If the object is 6 inches tall, it is 0.5 feet.
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Multiply the object's height in feet by its square foot measurement to convert from square feet to cubic feet. So if you are measuring a garden bed that is 3 feet wide by 6 feet long and 0.5 feet high, its dimensions in cubic feet are 3 ft × 6 ft × 0.5 ft = 9 ft3. Note that the size of the garden bed could also be expressed as 18 ft2 by 0.5 ft high, and thus calculated as 18 ft2 × 0.5 ft, but the result is the same: 9 ft3.
This formula works only for square or rectangular objects.