Air Changes Per Hour (ACH) Calculator
Air Changes Per Hour (ACH) measures how many times the entire volume of air in a room is replaced with fresh air each hour. ACH = (airflow rate in CFM × 60 minutes/hour) ÷ room...
Formula
Source: Engineering Toolbox, ASHRAE 62.1 | Last reviewed: June 8, 2026
Examples
0 CFM
= 3 CFM
- cfm = 400
- room_vol_ft3 = 8000
400 CFM into 8,000 ft³ room = 3 ACH
0 CFM
= 12 CFM
- cfm = 2000
- room_vol_ft3 = 10000
2,000 CFM into 10,000 ft³ = 12 ACH (hospital isolation room)
0 CFM
= 6 CFM
- cfm = 500
- room_vol_ft3 = 5000
500 CFM into 5,000 ft³ = 6 ACH (office)
Where is this used?
Cleanroom classification: ISO 14644 cleanroom classes are achieved through specific ACH rates (e.g., ISO Class 7 = 60+ ACH).
Laboratory ventilation: chemical fume hood makeup air and general lab exhaust ensuring safe contaminant dilution.
Infection control: hospital isolation rooms require minimum ACH (typically 12 ACH).
Frequently Asked Questions
What are typical ACH requirements for different spaces?
Offices: 4-6 ACH. Classrooms: 6-8 ACH. Laboratories: 6-12 ACH. Hospital patient rooms: 6 ACH. Hospital isolation rooms: 12 ACH. Cleanrooms: 20-600+ ACH depending on ISO class. Restrooms: 10-15 ACH. Parking garages: 4-6 ACH.
How do I calculate room volume for ACH?
Room volume (ft³) = length × width × ceiling height. Use interior dimensions. For rooms with sloped ceilings, use the average height. For open-plan spaces, the volume of the entire interconnected space should be used.
What ACH do I need for COVID-19 ventilation?
ASHRAE recommends 6-12 ACH for general occupied spaces during pandemics, prioritizing outdoor air and high-efficiency filtration. CDC/ASHRAE guidance emphasizes equivalent air changes (eACH) which accounts for filtration and air cleaning in addition to ventilation.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.