CFM Calculator
Air changes per hour from a fan’s airflow and the room’s size, or the CFM a fan must move for the air changes you are aiming for.
Result
5.21
Assumes the air mixes evenly; real rooms have corners the airflow misses.
- Airflow
- 100 cfm (169.9 m³/h, 47.2 L/s)
- Room volume
- 1,152 ft³ (32.6 m³)
- Time for one air change
- 11:31
https://calcgallery.com/home-systems/cfm-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the CFM Calculator
- Enter the room's length, width and ceiling height.
- To find air changes per hour, enter the fan's airflow in CFM, cubic metres an hour or litres a second.
- To find the airflow you need, switch the mode and enter the number of air changes an hour you are aiming for.
The formula
room volume = length × width × ceiling heightair changes per hour = airflow per minute × 60 ÷ room volumeairflow needed = air changes per hour × room volume ÷ 60The CDC’s guidelines for environmental infection control write it as Q ÷ V, with Q the airflow in cubic feet per hour. Fans are rated per minute, hence the 60. In metric the same arithmetic runs in cubic metres an hour, where the airflow divided by the room volume is the air change rate directly.
Time per change
The page also shows how long one air change takes: 60 minutes divided by the rate. At 6 air changes an hour, a room’s worth of air passes through every 10 minutes.
With your numbers
- Air changes per hour = CFM × 60 ÷ room volume in cubic feet
Quick reference
Assumes the air mixes evenly; real rooms have corners the airflow misses.
| Job | Result |
|---|---|
| 12 × 12 × 8 ft room, 100 cfm | Air changes per hour: 5.21 |
| 30 × 40 × 8 ft, 1,000 cfm | Air changes per hour: 6.25 |
| 10 × 8 × 8 ft bathroom, 50 cfm | Air changes per hour: 4.69 |
| 12 × 12 × 8 ft room, 6 air changes | Airflow: 115.2 cfm |
| 20 × 15 × 9 ft room, 4 air changes | Airflow: 180 cfm |
| 4 × 5 × 2.5 m room, 150 m³/h | Air changes per hour: 3 |
| 4 × 5 × 2.5 m room, 4 air changes | Airflow: 200 m³/h |
Common questions about the CFM Calculator
- How do I convert CFM to air changes per hour?
- Multiply the CFM by 60 to get cubic feet an hour, then divide by the room’s volume in cubic feet. A 100 CFM fan in a 12 by 12 foot room with an 8-foot ceiling, 1,152 cubic feet, gives 100 × 60 ÷ 1,152 = 5.21 air changes an hour.
- How many air changes per hour should a room have?
- It depends on the room and what it is for, and the figures come from building codes, ventilation standards and equipment guidance rather than from a formula. This page does not recommend one. Enter the number your code, standard or designer gives you and it works out the airflow.
- Does an air change replace all the air in the room?
- No. One air change moves a volume of air equal to the room, but incoming air mixes with the air already there, so some of the old air stays. That is why guidance on clearing a room of a contaminant quotes both an air-change rate and a time.
- Is the fan's rated CFM what it actually moves?
- Often not. Ratings are usually measured at a stated pressure, and ducts, grilles and filters add resistance that lowers the delivered airflow. A measured figure, with a flow hood or anemometer, gives a truer air-change rate.