Air Flow Rate Calculator Formula
Understand the math behind the air flow rate calculator. Each variable explained with a worked example.
Formulas Used
Required Airflow
cfm = room_volume * ach / 60Room Volume
volume = room_volumeVariables
| Variable | Description | Default |
|---|---|---|
room_length | Room Length(ft) | 20 |
room_width | Room Width(ft) | 15 |
ceiling_height | Ceiling Height(ft) | 8 |
ach | Air Changes per Hour(ACH) | 6 |
room_volume | Derived value= room_length * room_width * ceiling_height | calculated |
How It Works
Air Flow Rate (CFM)
CFM = Room Volume x ACH / 60
ACH (Air Changes per Hour) is how many times the entire room volume is replaced with fresh or conditioned air in one hour. Divide by 60 to convert from cubic feet per hour to cubic feet per minute.
Worked Example
20 x 15 ft room with 8-ft ceilings at 6 ACH.
room_length = 20room_width = 15ceiling_height = 8ach = 6
- 01Volume = 20 x 15 x 8 = 2,400 cu ft
- 02CFM = 2,400 x 6 / 60 = 240 CFM
Frequently Asked Questions
What ACH values are typical?
Homes: 4-6 ACH for general ventilation. Kitchens: 15-20 ACH. Bathrooms: 8-10 ACH. Workshops: 10-15 ACH. These are guidelines; check local codes.
Is CFM the same for heating and cooling?
Cooling typically requires about 400 CFM per ton of cooling. Heating airflow depends on the furnace type and temperature rise, usually 350-400 CFM per 12,000 BTU/hr.
Ready to run the numbers?
Open Air Flow Rate Calculator