Find air changes per hour (ACH) — how many times a ventilation system fully replaces the air in a room each hour.
How it works
ACH is airflow scaled to an hourly rate, divided by room volume: ACH = (CFM × 60) ÷ room volume. A system moving 200 CFM through a 2,000 cubic foot room achieves 6 air changes per hour.
What this does not include
This calculates a theoretical ACH from stated airflow and volume — actual air mixing in a real room is never perfectly uniform, so effective ventilation can differ somewhat from this idealized calculation.
How to use this calculator
- Enter the airflow rate in CFM.
- Enter the room’s volume.
A worked example
A ventilation system delivering 200 CFM into a 2,000 cubic-foot room: ACH = (200×60) ÷ 2,000 = 6 air changes per hour.
500 CFM into a 6,000 cubic-foot room: ACH = 5.
What the variables mean
| Variable | Meaning |
|---|---|
| CFM | Cubic feet per minute of airflow delivered |
| Room volume | Total room volume, in cubic feet |
Edge cases worth knowing
A larger room needs proportionally more CFM to hit the same ACH target — the second example moves more than double the air of the first, but into a room three times as large, landing on a slightly lower ACH.
Zero CFM makes ACH zero, a degenerate but valid case — the calculator declines to show a result only when CFM is genuinely missing.
What ACH is considered good for indoor air quality?
Recommended ACH varies significantly by space type — a bathroom or kitchen typically needs more air changes than a bedroom, and specific building codes and health guidelines set different targets for different room types.
Why does ACH matter for airborne particle or odor removal?
A higher ACH clears contaminants, odors, and airborne particles from a space more quickly, which is why hospitals and labs often specify very high minimum ACH values for sensitive areas.
How is CFM measured for a real HVAC system?
A balometer or anemometer at the supply vents, or the equipment’s rated airflow specification, are common ways to determine actual or expected CFM for this calculation.
