How it’s calculated
Air changes per hour is the volume of air supplied (or exhausted) each hour divided by the room volume. CDC’s infection-control guideline writes it as ACH = Q / V with Q in ft³ per hour; with airflow in CFM multiply by 60.
Example: a 20 × 15 ft room with an 8 ft ceiling is 2,400 ft³. For 6 ACH: 6 × 2,400 ÷ 60 = 240 CFM. At 6 ACH a perfectly mixed room needs about 46 minutes to remove 99% of an airborne contaminant, matching CDC Table B.1.
Required ventilation rates come from codes and standards (e.g. ASHRAE 62.1/62.2, health-care guidelines), not from this calculator. Only outdoor or filtered air counts toward contaminant removal.
Frequently asked questions
How many air changes per hour do I need?
It depends on the space and the code that applies: homes, offices, labs and hospital rooms all differ. CDC’s health-care guideline, for example, lists a minimum of 12 total ACH (including 2 ACH of outdoor air) for airborne infection isolation rooms.
How do I convert CFM to m³/h?
1 CFM = 0.0283168 m³/min = 1.699 m³/h (= 0.4719 L/s).
Why does it take longer than 1/ACH to clear the air?
Incoming air mixes with room air, so contaminant concentration decays exponentially: one air change removes about 63%, and 99% removal takes about 4.6 air changes.
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Sources
- Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: Air — U.S. Centers for Disease Control and Prevention
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement — U.S. National Institute of Standards and Technology
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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