Cubic Feet per Minute (CFM)
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 and 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 so I can only make average suggestions for the cfm required; but I can say with confidence that it is better to have too much than not enough. 

Suggested CFM (Cubic Feet per Minute) requirements for Cooktops:
Electric Cooktops: 
Generally speaking an electric cook top is low powered and should only need a range hood output between 150cfm - 300cfm. If you have an electric deep fat fryer you will need to treat it as if it was a gas stove top. 

Gas Cooktops:
  Gas cook tops have outputs of up to around 40,000btu [British Thermal Units] and, since they are almost always used in an "against the wall" situation, you can use this simple formula to help make your choice. Calculate your cook top's btu rating by adding the power of each burner and then divide the total by 100, e.g. a 30,000btu top will require a 300cfm fan."