measures how much air is being moved in cubic feet. A Kirby Sentria II moves 80-85 cubic feet/minute of air. Direct-air vacuums move much more air than clean-air vacuums. I'd guess that a Dyson moves 5-10 CFM at best, because the air has to enter the nozzle, swirl around in the bin, split up into each of the cyclones, and then finally reach the motor. You would think that that kills the airflow, right? A good direct-air cleaner should move enough air that you hear a ROOOAR when you first turn the machine on. Agitation, airflow, and suction, in that order, are the 3 most important components of a good vacuum. You have to remove the surface and embedded dirt from the carpet, and be able to carry that dirt into the bag or bin. 

Jaker15: I don't know where you get your Kirby specifications, but everywhere I've ever seen Kirby Specs, it has always been 115 CFM for all the G series machines and 127 CFM for the Sentria I and II.