Air Changes per Hour (ACH)
How many times an hour the air in a room is replaced, against the ventilation target for that space.
Example
You enter
- Room volume (ft³) 10000
- Supply CFM 1000
- Occupancy (comparison band) classroom
You get
- Air changes per hour 6 ACH
- Net / pressurization 6
- Pressurization (CFM) 0
Checked against published worked examples from: ASHRAE.
Details, formula, and sources
ScopeACH = CFM × 60 / volume, net delivered ACH and pressurization airflow from unequal supply/return, and a comparison against ASHRAE 62.1 / 170 occupancy targets (residential to operating room).
Formula or tableACH = supply CFM × 60 / room volume (ft³). Net delivered ACH = min(supply, return) × 60 / volume. Pressurization airflow = supply − return (positive = pressurized).
Source and editionASHRAE 62.1-2022 (ventilation for acceptable indoor air quality) by name; ASHRAE 170-2021 (ventilation of health care facilities) by name.
Free accessASHRAE standard TOCs free at ashrae.org; full standards licensed.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesvolume_ft3, supply_cfm, occupancy, ach, net_ach, pressurization_cfm
- Residential band 0.35-1 ACHthe older ASHRAE 62-1989 0.35 ACH figure (62.2 sets a cfm rate instead)
- Classroom band 4-6 ACHASHRAE 62.1 typical
- Laboratory band 6-12 ACHASHRAE typical
- Operating room 20 ACH minimum, no ceilingASHRAE 170 Table 7-1
- Patient room 4 ACH minimum (6 for a single-bed room with Group D diffusers, note y), no ceilingASHRAE 170 Table 7-1