CFM or Cubic Feet per Minute is a measure of the volume of air being exhausted through your range hood. This measurement is always based on when the hood is running at full speed and at zero pressure, i.e. without the resistance of any ducting. Depending on its distance to the outside, the type, number of bends, and the resistance of the blow-back damper and wall plate, this output may be considerably reduced. The CFM requirement for your kitchen therefore will depend on the type and output of your range top, the size of the hood, the positioning of the hood, plus the length and configuration of your ductwork. Every situation is different but we can say with confidence that it is better to have too high a CFM rating than too low.