cubic foot per minute
Volumetric flow rateexact by definition
The cubic foot per minute is the customary unit of air flow in North American ventilation, compressed air and dust collection work. It equals exactly 4.719474432e-4 cubic metres per second, since the international foot is exactly 0.3048 metres. The conversion is exact by definition, though the physical meaning of a cfm figure depends on the air's density.
Watch out: Cfm is a volume rate, so the mass of air it represents changes with pressure, temperature and altitude. A compressor rated in scfm has been corrected to a standard reference condition, while acfm is the actual volume at the inlet, and the two differ substantially at altitude or on a hot day. There is also more than one standard condition in circulation: 68 degrees Fahrenheit at 14.696 psia is common in the compressed air trade and 70 degrees Fahrenheit in some HVAC standards. Mass flow in pounds per hour does not have this ambiguity.
| 1 cfm | 28.316847 L/min |
The convention of rating fans, blowers, air compressors and duct systems in cubic feet per minute is entrenched throughout the North American HVAC and compressed air trades.
About the cubic foot per minute
Air flow in cfm is the basic currency of ventilation design. A bathroom exhaust fan moves 50 to 110 cfm, a residential furnace blower 800 to 2000 cfm, and a commercial air handler tens of thousands. Ventilation codes state requirements per person and per unit floor area, and the sensible heat carried by an air stream follows \(q = 1.08 \ imes \ ext{cfm} \ imes \Delta T\) in BTU per hour, where 1.08 bundles together air density, specific heat and the minutes-to-hours conversion at standard conditions.
That constant is exactly where the density caveat bites. At 5000 feet of elevation air is about 17 percent less dense, so the same cfm through the same coil carries about 17 percent less heat, and the 1.08 should be roughly 0.90. Fans are constant-volume devices to a good approximation, meaning they move nearly the same cubic feet per minute regardless of density while delivering less mass and drawing less power. This is why altitude corrections appear on every serious equipment selection, and why specifying mass flow in pounds per hour sidesteps the whole problem when the process actually cares about how much air rather than how much space it occupies.