1. Airflow Quantity and Velocity
Q=VA
Quantity of Air “Q” (expressed as CFM)
Velocity of Air “V” (expressed as FPM)
Face Area “A” (expressed as Sq. Ft.)
With 2 known values – 3rd can be calculated

2. Calculating Quantity (CFM)
When face area and velocity are known the
CFM can be determined.
Q=VA
4 x 500 = 2000
4 sq. ft. x 500 FPM = CFM

3. Calculating Velocity (FPM)
When face area and air quantity are known
the velocity can be determined.
V=Q/A
2000 / 4 = 500
2000 CFM / 4 sq. ft. = 500 FPM

4. Calculating Face Area (sq. ft.)
When the velocity and quantity of air
are known, the face area can be
determined.
A=Q/V
2000 / 500 = 4
2000 CFM / 500 FPM = 4 sq. ft.

5. Application Example 40,000 CFM / 400 fpm = 100 sq. ft.
An engineer wants to know how many filters
he needs to use for a system that delivers
40,000 CFM - desired velocity is 400 fpm.
Calculations
40,000 CFM / 400 fpm = 100 sq. ft.
Assume a 24x24 face size with 4 sq. ft.
100 sq. ft / 4 sq. ft. = 25 filters

6. Another Method Knowing he wants 400 fpm you would take
the face area of a 24x24 which is 4 and
multiply x 400 which equals 1600 CFM/filter
40,000 total system CFM divided by 1,600
CFM per filter also equals 25 filters.

7. Square Inches to Square Feet
When dealing with a 24x24 it is easy
to come up with the face area (2x2=4)
Other sizes require conversion
Divide square inches by 144 (sq in/sq ft)
20x20 = 400 / 144 = round to 2.78
20x25 = 500 / 144 = round to 3.47

8. Application Example An engineer doesn’t have CFM data for his
system and wants to know what it is. He has
measured the velocity at 375 fpm and the
unit contains x20 filters.
Calculations
Convert sq. in. to sq. ft. 20x20 = 400 / 144
which equals round to 2.78 sq. ft.
24 x 2.78 = total sq. ft.
66.72 sq. ft. x 375 fpm = 25,020 CFM

9. Application Example A maintenance lead is having air delivery
problems and wants to lower the pressure
drop across his filter bank. He has room to
expand the filter bank. The bank contains
10 filters that were originally MERV 7 pleats
but have been upgraded to MERV 13. System
CFM is 24,000 and he wants to reduce his
velocity to 300 fpm to cause a big reduction in
pressure drop. How many filters does he need?

10. Calculation Examples Alternate Method
He is asking for 300 fpm velocity
4 x 300 = 1200 CFM per 24x24 filter
Total system CFM 24,000/1,200 = 20 filters
Alternate Method
24,000 / 300 = 80 sq. ft. face area needed
80 / 4 = 20 filters needed.

11. Media Velocity Media velocity is calculated by starting with
the CFM per filter and dividing by the total
media area. It is an important consideration
because lower media velocities normally
improve the efficiency of a filter. Excessive media
velocities can adversely affect media performance!
Pleat – 2,000 CFM / 17.6 sq. ft. = FPM
113.6 is the media velocity for this pleat
Bag or pocket filter – 2,000 CFM / 90 sq. ft.
equals FPM media velocity

12. Additional Examples Box – 2,000 CFM / 58 sq. ft. = 34.48 FPM
Cell – 2,000 CFM / 100 sq. ft. = 20 FPM
4V – 2,000 CFM / 194 sq. ft. = FPM
Hepa – 1,000 CFM / 160 sq. ft. = 6.25 FPM