To calculate the number of air changes per hour (N) you will need:

  • area of the space (length, width, height) in cubic feet (ft3)
  • fan or ventilation (in cubic feet per minute, or CFM)

The number of air changes per hour is calculated by:

N= (60 min/hour) (CFM of ventilation) / Total Cubic feet

How useful is this? Well, it’s a mixed bag.

On the downside:

  • It assumes that the air mixes perfectly (which it never does) and that the fan is running properly and accurately. 

On the plus side:

  • It does give you an idea of how fast the air is moving in proportion to the total area. That information can be very useful when comparing spaces, comparing the types of ventilation available, or attempting to control a hazardous atmosphere by dilution ventilation.

But, be warned: dilution ventilation is NOT a recommended control for a “dangerous” hazard. As they say, “dilution is not the solution to pollution”.