Booster fans are rated at 250 cfm when boosting an existing current of air. They are typically used in HVAC when the register is far from the source <heater,A/C> and it needs a boost of air. They typically use one size under the current size ducting also. So those are FREE air CFM ratings, get anything behind it, 90's,HEPAfilters, long runs of duct and it will diminish at an astounding rate.