Understand what cubic feet per minute is. A cubic measurement, no mater what it is, is a volume. Therefore, cubic feet is feet to the third power. An easier way to explain this is, volume equals the product of the length, width, and height of a solid object (V=l x W x H). These three measurements are measured in feet, and when you multiply "foot" against itself three times, it becomes foot to the third power or foot cubed or cubic feet (as it is commonly known). With that known, cubic feet per minute looks like this: ft^3/min, which translates into a speed or a velocity. A specific volume of the substance (liquid, gas or matter) that is being measured travels out of whatever it is every minute.

Find the area of the pipe. The flow rate is used is to calculate the rate at which a substance leaves a pipe such as water or a gas. So you will need to know the area of the pipe. The equation to find cubic feet per minute is Q = V x A where Q is the velocity in cubic feet per minute, V is the velocity in ft/sec, and A is measured in square feet or feet squared. The area of any pipe is A = pi x r (squared). Pi is a constant which equals 3.14. R is the radius of the pipe, which is the distance half way across the opening of the pipe and then multiplied by itself. Say that the radius of the pipe is 2 feet, then the area of the pipe would be: A = pi x r^2 or (3.14) (2x2) = 12.56ft^2

Find the velocity of the liquid or gas coming out of the pipe. The easiest way of doing this is drop something in the liquid and time how many feet it went in a minute or most systems will tell you what speed gas it coming out of a pipe. This is the velocity which we will give as 1.5 ft/min^2.

Find the flow rate. Plug the items into the equation of Q=V x A. See that 1.5 x 12.56 = 18.84 cubic feet per minute. The more you work with this, the easier it becomes.