Calculate air changes per hour (ACH) from airflow in CFM and room size, or solve for the required CFM or the largest room a fan can ventilate at a target ACH.
Air Changes Per Hour (ACH) Formula
Air changes per hour is the number of times the full volume of air in a room is replaced in one hour. The calculator works in three directions, so it uses three arrangements of the same relationship.
To find ACH from a known airflow and room volume:
ACH = (CFM * 60) / V
To find the airflow you need to hit a target ACH:
CFM = (ACH * V) / 60
To find the largest room a given airflow can serve at a target ACH:
V_max = (CFM * 60) / ACH
Variables:
- ACH is the air changes per hour (how many times the room air is fully replaced each hour)
- CFM is the airflow of the fan, HVAC unit, or air purifier in cubic feet per minute
- V is the room volume in cubic feet, found from length times width times height
- 60 converts airflow per minute into airflow per hour
The airflow term is multiplied by 60 because CFM is measured per minute and ACH is measured per hour. Dividing that hourly airflow by the room volume tells you how many whole-room volumes pass through in an hour. A related output the calculator shows is the air change-out time in minutes, which is 60 divided by ACH. That value tells you how long one complete air change takes, which is often easier to picture than the raw ACH number. If your airflow is given in metric units, the calculator converts cubic meters per hour and liters per second to the same basis before dividing, so the result is the same dimensionless number.
Recommended Air Changes Per Hour By Room
Recommended ACH depends on how a space is used. Wet, crowded, or contaminant-heavy rooms need more changes; quiet storage needs fewer. The table below pairs common targets with the equivalent change-out time so you can see how fast the air actually turns over. The change-out time column is the practical detail most ACH pages leave out.
| Space | Typical ACH | Time per change |
| Bedroom (ASHRAE 62.2 minimum) | 0.35 to 1 | 60 to 171 min |
| Home or living space | 1 to 2 | 30 to 60 min |
| Office, private | 4 to 6 | 10 to 15 min |
| Open office or retail | 6 to 10 | 6 to 10 min |
| Kitchen, home | 7 to 8 | 7.5 to 8.6 min |
| Bathroom | 8 to 10 | 6 to 7.5 min |
| Restaurant dining room | 8 to 12 | 5 to 7.5 min |
| Commercial kitchen | 15 to 60 | 1 to 4 min |
| Bar or nightclub | 20 to 30 | 2 to 3 min |
| Warehouse or storage | 2 to 6 | 10 to 30 min |
The next table shows the airflow you would need to reach common ACH targets in a 2,700 cubic foot room, which is roughly a 20 by 15 foot office with a 9 foot ceiling. Use it as a quick sanity check against the calculator.
| Target ACH | Required airflow (2,700 ft³) |
| 2 | 90 CFM |
| 4 | 180 CFM |
| 6 | 270 CFM |
| 8 | 360 CFM |
| 10 | 450 CFM |
Example Problems
Example 1: Finding ACH. An office is 20 feet long, 15 feet wide, and 9 feet high, so the volume is 20 times 15 times 9, which is 2,700 cubic feet. The supply register delivers 400 CFM. The ACH is (400 times 60) divided by 2,700, which equals 8.89 air changes per hour. Dividing 60 by 8.89 gives a change-out time of about 6.7 minutes, so the room gets a full change of air roughly every seven minutes.
Example 2: Finding required airflow. A bathroom is 8 feet by 5 feet with an 8 foot ceiling, giving a volume of 320 cubic feet. To reach the recommended 8 ACH, the required airflow is (8 times 320) divided by 60, which equals about 42.7 CFM. A 50 CFM exhaust fan would comfortably meet the target.
FAQ
What is a good number of air changes per hour?
There is no single number, because the right ACH depends on the room. A bedroom may only need about 1 change per hour of outdoor air under ASHRAE 62.2, while a bathroom usually wants 8 to 10, and a commercial kitchen can need 15 to 60. Use the room table above to pick a target, then use the calculator to check whether your fan or HVAC unit actually delivers it.
How do I convert an air purifier’s CADR into ACH?
A purifier’s Clean Air Delivery Rate is reported in CFM, so it drops straight into the ACH formula. Multiply the CADR by 60 and divide by the room volume in cubic feet. A 250 CADR purifier in a 1,200 cubic foot bedroom gives (250 times 60) divided by 1,200, which is 12.5 ACH. Most purifier makers rate coverage at 2 to 5 ACH, so a higher number means faster cleaning for allergies or smoke.
Is ACH the same as CFM?
No. CFM measures the raw volume of air moving per minute, while ACH measures that airflow relative to the size of the room. The same 400 CFM produces a high ACH in a small room and a low ACH in a large one, which is why ACH is the better measure of how well a space is actually ventilated.
