How this calculator works
Multiply floor area by ceiling height to obtain room volume. Divide hourly air delivery by that volume. This is a nominal mixing calculation: supply air, outdoor air and filtered clean air are different quantities. Use the airflow relevant to your purpose and do not count the same air twice.
Formula
ACH = 3600 x Q / (A x H), where Q is m3/s, A is m2 and H is m. Rearrange the same equation for airflow, area or height.
Worked example
A 20 m2 room with a 3 m ceiling contains 60 m3. An airflow of 360 m3/h provides 6 air changes per hour; 3 changes require 180 m3/h.
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Frequently asked questions
Does one air change replace every molecule?
No. Mixing leaves some of the original air in the room after one nominal air change.
Can I use recirculated airflow?
Use it only for the question you are assessing. Unfiltered recirculation is not equivalent to fresh outdoor air or filtered clean air.
What rate should I target?
The appropriate rate depends on occupancy, activity and applicable ventilation guidance. The calculator converts a chosen rate; it does not set a universal target.