What is the formula for calculating number of air changes in an area?

Number of air changes/hour in an area is:- 

                   = Total Room Airflow In CFM x 60

                      Total Volume of room in cubic feet

For calculating Total Room Airflow in CFM, first calculate air flow of individual filter. 

Formula is given below.

Air flow (in cfm) = Avg.air velocity in feet/Minute x Effective area of filter                                                  

Then find Total air flow. The Formula is:-

Total Air flow = Sum of air flow of individual filter.

Air flow Velocity can be measured with the help of Anemometer.