How to Choose a Range Hood - Build.com You compensate for this by adding to the CFM power of the range hood. For each foot of pipe between the range hood and the external vent, add one CFM; for each 90-degree or elbow turn that the pipe takes, add 25 CFM; and for the external vent cap, add 40 CFM. 1 foot of duct pipe = 1 CFM; 1 pipe elbow = 25 CFM; 1 external vent cap = 40 CFM