Air Changes & CFM Calculator
Airflow from room volume and air changes per hour, or ACH from a measured airflow, in m³/h, L/s and CFM, with minutes per change and flow per occupant.
The fan or ventilation flow a room needs for the air-change rate you specify — or the rate an existing flow achieves — in the units the fan curve and the specification use.
Example: A 6 × 4 × 2.7 m room (64.8 m³) at 6 air changes per hour needs 388.8 m³/h = 108 L/s = 229 CFM, one full change every 10 minutes; for 4 occupants that is 27 L/s each.
One definition.
The target rate is yours.
What an air change is, why this page recommends no rate, and how the per-person figure relates.
Air changes per hour
ACH is the volume of air supplied (or extracted) in an hour divided by the volume of the room: at 6 ACH the hourly airflow equals six room volumes, and a full volume passes every 10 minutes. The relation is exact and works both ways — enter a target rate to size the flow, or a measured or rated flow to see the rate it gives. The room volume is length × width × height of the ventilated space; deduct large fixed equipment if you want the free volume.
Why no rate is recommended
Required ventilation rates depend on the use of the space, the occupancy, the contaminants, the jurisdiction and the standard in force (ASHRAE 62.1, EN 16798, workplace and cleanroom rules, process requirements), and many of them are expressed per person or per floor area rather than as ACH. A single table of “typical ACH by room type” would be wrong for most readers, so this page carries none: take the rate from your standard, specification or process engineer and enter it.
Flow per person and units
With an occupant count the same airflow is also shown per person (L/s per person or CFM per person), the form most ventilation standards use, so a rate given that way can be compared directly. Airflow is listed in m³/h, m³/s, L/s, CFM and ft³/h; 1 CFM = 1.699 m³/h from the exact cubic foot. Note that “air changes” says nothing about mixing: a well-mixed room and a short-circuited one at the same ACH ventilate very differently.
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SOURCES
- ASHRAE Handbook — Fundamentals, chapter "Ventilation and Infiltration": air change rate defined as airflow ÷ space volume (definition only; required rates are in the applicable standard)
- NIST Special Publication 811 (2008), Guide for the Use of the SI — cubic foot 0.028 316 846 592 m³
Last reviewed 19 September 2026. How results are checked: How we verify.