Air Changes Per Hour Calculator
Determine room air turnover rates, required exhaust fan CFM, and compliance with ASHRAE standards.
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How Air Changes Per Hour (ACH) Are Calculated
Air changes per hour (ACH) measures how many times the total air volume of a room is fully replaced or filtered within 60 minutes.
Room Volume (Cubic Feet) = Length × Width × Height
ACH = (Fan CFM × 60) / Room Volume
Required CFM = (Room Volume × Target ACH) / 60
For COVID-19 and airborne pathogen risk reduction, the CDC and ASHRAE recommend a minimum of 5.0 to 6.0 clean air changes per hour in classrooms, offices, and occupied rooms.
Frequently Asked Questions
What ACH is recommended for home bathrooms?
The Home Ventilating Institute (HVI) recommends 8 ACH for bathrooms to rapidly exhaust shower moisture and prevent ceiling mold.
How many ACH does a residential kitchen require?
Kitchen exhaust range hoods should achieve 6 to 10 ACH (or at least 100 CFM for standard 30" electric ranges and 250+ CFM for gas ranges).
What is the ASHRAE 62.2 residential baseline?
Standard 62.2 mandates approximately 0.35 total air changes per hour (or ~7.5 CFM per person + 3 CFM per 100 sq ft) for continuous mechanical ventilation in energy-efficient airtight homes.