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Air Changes per Hour (ACH) Calculator

Enter the room size and the air changes per hour you need. The calculator gives the airflow in CFM and how many air scrubbers or negative air machines it takes, from each machine's rated CFM.

What do you want to work out?

4 is the OSHA minimum inside an asbestos negative pressure enclosure (29 CFR 1926.1101). For other jobs, use the number in your job specification.

Our rule of thumb: plan on about 25% less than the rated CFM, because ducting, loaded filters and lower fan settings cut real airflow. Set 0 to use the maker's rating as it is.

You need

2 machines

Room volume
12,000 cu ft
Airflow needed
800 CFM
Usable CFM per machine
563 CFM
Air changes with those machines
5.6 per hour
Time to 99% removal at the target
69 min
Time to 99.9% removal
104 min

This is our calculation, not a job specification. Rated CFM is the maker's figure, and real airflow with ducting and loaded filters is lower, which is what the allowance covers. When a project spec, a standard or a regulator sets the air change rate, that number wins.

Air changes and clearing time

Minutes to remove 99% and 99.9% of an airborne contaminant, assuming perfect mixing. CDC Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B, Table B.1.
Air changes per hour99% removal99.9% removal
2138 min207 min
469 min104 min
646 min69 min
835 min52 min
1028 min41 min
1223 min35 min

How the ACH formula works

Air changes per hour is how many times the machines move a volume of air equal to the room's volume in one hour. Multiply length by width by height for the volume in cubic feet, then:

ACH = CFM × 60 ÷ room volume (cu ft)
CFM needed = room volume × target ACH ÷ 60
Machines = CFM needed ÷ (rated CFM × (1 - allowance)), rounded up

Example: a 40 by 30 by 10 ft space is 12,000 cu ft. At 4 air changes per hour it needs 12,000 × 4 ÷ 60 = 800 CFM. A machine rated at 750 CFM gives about 563 usable CFM after our 25% allowance, so the job takes 2 machines, which together give about 5.6 air changes per hour.

The clearing times use the CDC formula for a room with perfect mixing: minutes = ln(1 ÷ (1 - removal)) ÷ ACH × 60. At 4 ACH that is 69 minutes for 99% and 104 minutes for 99.9%, the same as the CDC's own table below the calculator. Real rooms mix less evenly, so treat these as the shortest likely times.

Sources

OSHA, 29 CFR 1926.1101(g)(5)(i)(A)(2): at least 4 air changes per hour in an asbestos negative pressure enclosure. CDC, Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: Table B.1 (air changes and clearing time) and Table B.2 (12 total air changes per hour for airborne infection isolation rooms). Machine airflow: the makers' rated CFM as listed on our product pages.

Air scrubbers and negative air machines

Frequently asked questions

How do you calculate air changes per hour?

Multiply the airflow in CFM by 60, then divide by the room volume in cubic feet. A 600 CFM machine in a 6,000 cu ft room gives 600 × 60 ÷ 6,000 = 6 air changes per hour.

How do you convert ACH to CFM?

Multiply the room volume in cubic feet by the ACH you want, then divide by 60. A 2,400 cu ft room at 4 ACH needs 160 CFM.

How many air changes per hour do I need?

It depends on the job and on who sets the rules for it. OSHA's asbestos standard calls for at least 4 air changes per hour inside a negative pressure enclosure, and the CDC's health-care ventilation table lists 12 total air changes per hour for airborne infection isolation rooms. For other work, use the number in the project specification.

Should I use the CFM printed on the machine?

Use it as the ceiling, not the working number. Makers often rate airflow at the top fan setting (the HEPA-AIRE H2KM is rated 2,000 CFM on high and 1,300 on low), and ducting and loaded filters cut it further on the job. That is why the calculator takes an allowance, 25% by default (our rule of thumb).

Does a higher air change rate clear the air faster?

Yes. With perfect mixing, the CDC's table gives 69 minutes to remove 99% of an airborne contaminant at 4 ACH and 23 minutes at 12 ACH. Corners, furniture and doorways slow it down in a real room.

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