Airflow, room size, and how often the air turns over
Air changes per hour is how many times a room's entire volume of air is replaced in 60 minutes. The 60 converts the per-minute airflow into a per-hour count. Reverse it to size a fan: CFM = ACH × volume ÷ 60. Room volume is length × width × ceiling height — height matters, so always measure it.
Recommended air changes by room type
Typical ASHRAE-based targets. Higher numbers clear moisture, odors and airborne particles faster, at the cost of more fan energy.
| Room | Minimum | Recommended | One air change every |
|---|---|---|---|
| Bedroom | 3 | 5 | 12 min |
| Living room | 4 | 6 | 10 min |
| Kitchen | 5 | 8 | 7.5 min |
| Bathroom | 6 | 8 | 7.5 min |
| Office | 6 | 8 | 7.5 min |
| Restaurant kitchen | 10 | 15 | 4 min |
| Operating room | 15 | 20 | 3 min |
The last column is 60 ÷ ACH: at 6 air changes an hour the whole room of air is swapped every 10 minutes.
What to watch when you measure a real room
- Use volume, not floor area. A 400 sq ft room with an 8 ft ceiling (3,200 ft³) needs less airflow than the same floor with a 10 ft ceiling (4,000 ft³) for the same ACH.
- Design ACH and actual ACH differ. Many homes run below 3 air changes an hour even where the plan called for 6. Measure the airflow at the vent with an anemometer to find the real number.
- Metric inputs convert cleanly. 1 CFM is 1.699 m³/h and 1 cubic foot is 0.0283 m³, so the same formula works once both are in matching units.
Common questions
How do you calculate air changes per hour?
Multiply the airflow in CFM by 60, then divide by the room volume in cubic feet. A 300 CFM supply into a 3,000 cubic-foot room gives 6 air changes per hour.
What is a good ACH for a room?
Around 5 to 6 for bedrooms and living spaces, 8 for kitchens and offices, and 15 to 20 for operating rooms and busy labs. Below 1 means the air is barely being replaced.
What is the difference between ACH and CFM?
CFM is the raw airflow, in cubic feet per minute. ACH counts how many times that airflow replaces the whole room of air each hour. Converting between them needs the room volume, since a big room with the same CFM has a lower ACH.
