Air Changes per Hour Calculator
This tool uses room size and airflow to help you find air changes per hour, or ACH. It also shows the CFM needed to reach your target ACH, so you do not have to guess at fan size.
1Measure the room and airflow
The right target depends on the goal, number of people, pollutants, and the standard that applies.
Estimated air-change rate
5.00 ACH
In theory, the airflow replaces one room volume every 12.0 minutes.
Room volume
2,400 ft³
CFM for target
200.0 CFM
For 5.0 ACH in this room.
Entered airflow
200.0 CFM
ACH assumes that air mixes evenly. Air taking a short path from supply to return, closed doors, dirty filters, duct leaks, the amount of outdoor air, and fan pressure can lower the real ventilation or cleaning rate.
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Quick planning answer
To find ACH, multiply airflow in CFM by 60 and divide by room volume in cubic feet. A room that is 20 feet long, 15 feet wide, and 8 feet high holds 2,400 cubic feet. An airflow of 200 CFM gives 5 ACH if the air mixes evenly.
Choose an ACH target that fits the goal
There is no single ACH target for every job. Ventilation, air cleaning, drying, odor control, and pollutant removal have different needs. Bedroom ventilation may depend on people and floor area. An air cleaner uses clean-air delivery. Some work areas need a hood or local exhaust instead of more air mixing in the whole room.
Start by naming the goal. Then use the right standard or professional guide to choose the target. More outdoor air can also add heating, cooling, and humidity load, especially on a hot, humid day.
Installed airflow may be lower than rated CFM
A fan's free-air rating does not promise the same CFM after installation. Filters, grilles, ducts, elbows, backdraft dampers, and coils add resistance. Use the fan curve at the system pressure or measure the airflow. For a portable air cleaner, smoke CADR is a cautious choice for general particle planning when no rating exists for the exact pollutant.
- Include all connected rooms when the doors normally stay open.
- Use average ceiling height for a sloped ceiling, or split the room into simple shapes.
- Keep supply and exhaust openings far enough apart so air does not take a short path between them.
- Check combustion safety before adding a large exhaust fan to a home with naturally vented fuel appliances.
ACH50 does not mean normal ventilation
A blower-door result called ACH50 measures air leakage while the building is held at 50 pascals of pressure. It helps compare how tightly buildings are sealed and supports energy models. It is not the normal outdoor airflow during everyday weather.
This tool can find ACH50 when you enter CFM50 and use the full tested building volume. Always keep the 50-pascal label with the result so it is not mistaken for normal ventilation.
Questions people ask before buying
How do you calculate air changes per hour?
Multiply airflow in CFM by 60. Then divide by room volume in cubic feet. Room volume is length times width times average height.
How many CFM do I need for 6 ACH?
Multiply room volume by 6, then divide by 60. A 3,000-cubic-foot room needs 300 CFM if the air mixes evenly.
Is higher ACH always better?
No. The right target depends on whether the air is filtered, exhausted, or brought in from outside. It also depends on the pollutant, number of people, noise, energy use, humidity, and the standard that applies.
Is ACH the same as ACH50?
No. ACH50 measures building leaks during a blower-door test at 50 pascals. Normal ventilation ACH uses airflow while the system runs under normal conditions. Always label where the airflow number came from.
Method, sources and limits
Room volume equals length × width × height. ACH equals CFM × 60 ÷ room volume. Minutes per air change equals 60 ÷ ACH. Target CFM equals target ACH × room volume ÷ 60.
These equations assume steady airflow and even mixing. The tool does not choose a health, code, work-process, or moisture target for you. It also does not lower a fan's rated CFM to account for system pressure.
- Ventilation in buildings (Centers for Disease Control and Prevention)
- Residential ventilation standard 62.2 (ASHRAE)
- Enclosure leakage testing (U.S. Department of Energy Building America)