ACH and CFM
Air changes per hour (ACH) is how many times the whole volume of air in a room is replaced each hour. Fans are rated in cubic feet per minute (CFM), so divide by 60.
CFM = room volume (ft³) × ACH ÷ 60
ACH = CFM × 60 ÷ room volume
Worked example
An 8 × 10 ft bathroom with an 8 ft ceiling is 640 ft³. At 8 air changes per hour: 640 × 8 ÷ 60 = 85 CFM. That is close to the HVI rule of 1 CFM per square foot of floor (80 CFM).
Typical targets
- Bathrooms: about 8 ACH (HVI). ASHRAE 62.2 minimum: 50 CFM intermittent or 20 CFM continuous.
- Kitchens: about 15 ACH (HVI). ASHRAE 62.2 minimum: 100 CFM intermittent for the range hood.
- Workshops and garages: 6 ACH or more, depending on fumes and dust.
Fan ratings drop with long, flexible or undersized ducts. Compare the fan's airflow at 0.25 in. of static pressure, not the free-air number.
Frequently asked questions
What size bathroom fan do I need?
About 1 CFM per square foot of floor for bathrooms up to 100 ft² with 8 ft ceilings, and at least 50 CFM. Larger bathrooms: use 8 air changes per hour.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. For example, 100 CFM is about 170 m³/h.