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One air change means a volume of air equal to the room is supplied (or exhausted) once. CFM is the airflow in cubic feet per minute; multiplying by 60 gives cubic feet per hour, and dividing by the room volume gives changes per hour. The right target depends on occupancy, heat load and contaminants.
- Room 40 × 25 × 10 = 10,000 ft³.
- Target 6 ACH.
- CFM = 6 × 10,000 ÷ 60 = 1,000 CFM.
Reference tables
Typical air-change targets by space
| Space | ACH |
|---|---|
| Office | 4–8 |
| Classroom | 6–12 |
| Restaurant kitchen | 15–30 |
| Warehouse | 4–6 |
| Restroom | 8–15 |
| Laboratory | 8–12 |
A number the tables do not give: a 10,000 ft³ office at 6 ACH needs 1,000 CFM — about a 20-inch round duct at 700 fpm.
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FAQ
Is ACH the same as CFM?
No. CFM is raw airflow; ACH normalizes it to the room volume, so it tells you how quickly the whole room is refreshed.
What ACH should I target?
It depends on the space and code. Offices sit around 4–8, kitchens and labs are much higher. Use the table as a starting point and check the local mechanical code.
Does ceiling height matter?
Yes — volume is length × width × height, so a taller room needs proportionally more CFM for the same ACH.
Supply or exhaust?
Either can define ACH; balanced systems match them. For contaminant control, exhaust ACH is usually the governing number.
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