How it’s calculated
- CFM = room volume (ft³) × ACH ÷ 60
- ACH = CFM × 60 ÷ room volume
Worked example
A 12 × 10 ft room with 8 ft ceilings needing 6 air changes per hour.
Inputs
- Room length
- 12 ft
- Room width
- 10 ft
- Ceiling height
- 8 ft
- I know the…
- Air changes per hour I need
- Air changes per hour
- 6
- Fan airflow
- 100 cfm
Results
- Airflow
- 96 cfm
- Air changes per hour
- 6 ACH
- Room volume
- 960 cu ft
Assumptions and limits
- Rated fan CFM is reduced by duct length and static pressure in practice.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How many CFM for a bathroom fan?
A common rule is 1 CFM per sq ft for bathrooms up to 100 sq ft, and the IRC sets minimum exhaust rates. Check local code.
What is a good ACH for a room?
It depends on use. Workshops, bathrooms and kitchens need far more than bedrooms. Codes and ASHRAE 62.2 give minimums for dwellings.