“The calculation for the CFM of a fume hood is obtained by multiplying the total opening (comprised of the face opening and the bypass opening) and the face velocity.
The face opening is defined as the width of the sash opening multiplied by the height of the sash opening. The bypass opening is defined as the area of additional opening above and below the sash. The bypass performs an important role by allowing air to enter the hood while the sash is closed and preventing the buildup of chemical fumes.
The face velocity is the speed of the air entering the hood, expressed in units of feet per minute (FPM). The typical range of face velocity falls between 80 FPM to 120 FPM, although new high performance fume hoods like the Kewaunee Venturi fume hood can be operated as low as 50 FPM. “