Air Changes per Hour (ACH) Calculator
Calculate how many times per hour the air volume of a room is completely replaced by the HVAC supply system. Used in ventilation design, IAQ assessment, and cleanroom specification.
Air Changes per Hour Calculator
Formula: ACH = (CFM × 60) ÷ Room Volume (ft³)
Reference ACH Targets
Understanding ACH
Air changes per hour (ACH) is a measure of how many times the total air volume of a room is supplied — or exhausted — per hour. It is calculated as:
ACH = (Supply CFM × 60) ÷ Room Volume (ft³)
where the multiplication by 60 converts CFM (cubic feet per minute) to cubic feet per hour.
What Does ACH Tell You?
ACH is a useful metric for comparing ventilation rates across differently-sized rooms. A 400 CFM supply delivering 4 ACH to a small office is very different from 4 ACH in a large warehouse — the same number, but very different air volumes and purposes.
In practice, ACH is used alongside ASHRAE standards to verify that a space is receiving adequate ventilation for its occupancy type. Higher-occupancy and higher-risk spaces (operating rooms, isolation rooms, labs) require higher ACH minimums.
ACH vs. Ventilation Rate
ACH is related to, but distinct from, ventilation rate. ASHRAE 62.1 specifies ventilation rates in CFM per person and CFM per ft² of floor area — not directly in ACH. ACH is most commonly specified in healthcare (ASHRAE Standard 170) and cleanroom (ISO 14644) contexts.
Limitations
ACH tells you total air volume replaced, not how effectively that air mixes with the room air or reaches occupant breathing zones. A room with 10 ACH delivered through a single poorly-placed diffuser may have worse contaminant dilution than a room with 6 ACH from well-designed distribution. Air distribution effectiveness (E_v) is the metric that captures mixing quality.