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

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