Air Changes per Hour Calculator

Calculate how many times per hour the air in a room is fully replaced by the HVAC system.

Calculator

Airflow to Room (CFM)

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Calculated Air Changes per Hour

0.0 ACH

Enter values above and click calculate.

How It Works

This calculator determines how many times per hour the entire volume of air in a room is replaced by the HVAC system. ACH is used to evaluate ventilation effectiveness, indoor air quality, and whether a space meets code requirements for fresh air delivery.

ACH = (CFM x 60) ÷ Room Volume

Where:
CFM = airflow delivered to the room in cubic feet per minute
× 60 = converts per-minute airflow to per-hour
Room Volume (ft³) = length × width × ceiling height in feet
ACH = number of times the room air is fully replaced per hour

Typical ACH targets by application:
Residential living spaces: 4–6 ACH
Commercial offices: 6–10 ACH
Kitchens and bathrooms: 10–15 ACH
Medical and cleanroom: 15–20+ ACH

Common example: 150 CFM delivered to a 12 × 14 × 9 ft room produces approximately 5.95 ACH - within the recommended residential range.