Air Changes Per Hour Calculator
Air changes per hour (ACH) tells you how many times the entire volume of air in a room gets replaced in one hour. This calculator works two ways: enter your airflow in CFM to find the resulting ACH, or enter a target ACH to find how much CFM you need. Essential for HVAC design, exhaust fan sizing, and indoor air quality.
The formula is ACH = (CFM x 60) / Room Volume. CFM is cubic feet per minute, the 60 converts minutes to hours, and room volume is length x width x ceiling height in cubic feet. Flip it around and you get Required CFM = (ACH x Volume) / 60.
Different rooms need different ACH rates. Bathrooms typically need 8 ACH (that is why exhaust fans are sized for the room). Kitchens need 15-20 ACH for cooking fumes. Offices need 4-6 ACH. Hospitals require 6-12 ACH depending on the area. Warehouses may only need 1-2 ACH since they are large, open spaces.
For residential ventilation, ASHRAE 62.2 recommends a minimum continuous rate of 0.35 ACH or 15 CFM per person, whichever is greater. This is much lower than the rates needed for kitchens and bathrooms because it is a whole-house average, not a spot ventilation rate.