The cubic foot is a U.S. unit of measurement for volume. It is the space created by a cube one foot wide by one foot long by one foot deep, or the equivalent of 1,728 cubic inches. The gallon is a commonly used measure of liquid volume. One gallon of liquid occupies a 231-cubic-inch space. Thus, one cubic foot equals about 7.48 gallons of liquid. To convert cubic feet to gallons, simply multiply the volume in cubic feet by 7.48 for the liquid volume in gallons.
Measure the width, length and depth of the fluid in feet. Note: These are the three parameters for determining volume in cubic feet.
Multiply the measurements of length, width and depth of the fluid to produce the volume in cubic feet (length x width x depth = volume). Note as an example, a container 2 feet wide by 2 feet long by 2 feet deep would measure 8 cubic feet (2 feet x 2 feet x 2 feet = 8 cubic feet).
Multiply cubic feet by the conversion factor of 7.48 for the volume in gallons. Note again the example above; an 8 cubic-foot container will hold 59.84 gallons of fluid (8 cubic feet x 7.48 gallons/cubic foot = 59.84 gallons).