Air Changes Per Hour (ACH) Calculator

What is an air change per hour?

An air change per hour is one complete replacement of the air volume inside a space in one hour. If a containment holds 2,400 cubic feet and your equipment moves 2,400 cubic feet of air through HEPA filtration every hour, that is one air change per hour. Ten air changes per hour means the equivalent of the whole enclosed volume passes through filtration ten times in that hour.

The word "equivalent" matters. Air does not queue up and file through the machine in tidy batches. Some air is pulled through twice in the first ten minutes while a pocket behind the stack of contents never moves at all. ACH is a design figure — it tells you whether the equipment on site is in the right league for the volume you enclosed, not that every cubic foot got filtered. That is why placement, discussed below, changes real-world results more than adding another machine usually does.

On a mold job the point of moving all that air is capture. Remediation disturbs contamination: cutting drywall, pulling insulation, HEPA-vacuuming framing. Airborne particulate that gets captured by a HEPA filter is particulate that never settles on a clean surface outside the work area, and never has to be cleaned twice. Combined with containment and negative pressure, filtration is what keeps a Condition 3 work area from turning the rest of the building into Condition 2. The ANSI/IICRC S520 treats these engineering controls as a package, not as independent line items.

How do you calculate ACH?

ACH = CFM × 60 ÷ volume in cubic feet. Airflow is measured in cubic feet per minute, so multiplying by 60 converts it to cubic feet per hour; dividing by the volume of the space gives the number of times that volume is turned over.

Worked example. A bedroom containment measures 20 ft × 15 ft with an 8 ft ceiling, so the volume is 20 × 15 × 8 = 2,400 cubic feet. You set a 500 CFM air scrubber in the room. ACH = 500 × 60 ÷ 2,400 = 12.5 air changes per hour on paper — before filter loading.

Run it the other way when you are planning instead of verifying. If the project design calls for 6 air changes per hour in that same 2,400 cubic feet, required CFM = 2,400 × 6 ÷ 60 = 240 CFM. Now derate for filter loading: a 500 CFM machine at a 20% derate is delivering about 400 CFM, so one unit covers the design, and the second machine you were about to put on the truck is better used somewhere else on the job.

The arithmetic is easy. The two places crews go wrong are measuring the wrong volume — using the whole floor instead of the sealed area, or forgetting a vaulted ceiling — and using clean-filter airflow ratings on day four of a demolition.

What ACH targets are commonly cited by space type?

Published air-change rates exist for healthcare and general ventilation design, but there is no published air-change requirement that applies to a mold remediation containment. Only rows with a source you can read are worth quoting on a job file:

• Healthcare airborne infection isolation and protective environment rooms — minimum air-change rates are published in the ASHRAE/ASHE Standard 170 ventilation tables and adopted in the CDC environmental infection-control guidelines, which also publish the time required to remove airborne contaminants at a given air-change rate.
• Healthcare construction and renovation barriers — the same CDC guidance and ASHE infection control risk assessment material set expectations for negative pressure and filtration in occupied facilities. Follow the facility's infection-control plan, not a rule of thumb.
• General occupied-building ventilation — minimum outdoor-air rates are set by ASHRAE Standard 62.1/62.2. These are ventilation rates for occupancy, not filtration rates for remediation, and they should not be borrowed as containment design figures.
• Mold remediation containment — four to six air changes per hour is commonly cited in the restoration industry as a starting point. No number is claimed here as a requirement: the S520 does not prescribe one, and the design belongs to the project, informed by the indoor environmental professional's scope, the extent of contamination and the equipment manufacturer's guidance.

Air scrubber vs negative air machine — what is the difference?

They are typically the same HEPA-filtered machine, distinguished by where the filtered air is discharged. Discharge into the work area and it is an air scrubber recirculating and cleaning the contained air. Duct the discharge out of the containment — ideally outdoors — and it becomes a negative air machine, because the air it removes must be replaced by makeup air pulled inward through the barrier.

That inward leak is the entire point of negative pressure. It means any gap in your poly moves clean air in rather than contaminated air out, and it is why the poly sucks visibly toward the work area on a properly set up containment. A manometer confirms the differential; a barrier that breathes outward tells you your exhaust path, makeup air or seal has a problem.

Both modes contribute filtration, and both count toward the airflow figure you calculate above. What they do not do is substitute for each other. A recirculating scrubber in a sealed room cleans air but creates no pressure differential, so a containment that must stay under negative pressure needs at least one machine ducted out.

Why does filter loading matter?

Rated CFM is measured with clean filters and drops as the filters load with debris. A machine badged at 500 CFM out of the box does not deliver 500 CFM after two days of drywall demolition, and the air changes you calculated quietly fall with it while the display on the machine says nothing.

The calculator defaults to a 20% derate, which is a conservative planning allowance rather than a measurement of your equipment. Where the manufacturer publishes a loaded-filter or "final resistance" airflow, use it — it beats any percentage guess. Then protect the HEPA the way it is meant to be protected: change pre-filters on schedule, because pre-filters are cheap and HEPA cartridges are not.

Document airflow the same way you document moisture. A note in the file that a machine was set at a given speed with fresh pre-filters on a given date is the difference between a defensible job file and an argument.

How should machines be placed, ducted and supplied with makeup air?

Place the intake where the work is and the discharge as far from it as possible. Short circuiting — filtered air going straight back into the intake — is the most common reason a containment with plenty of nameplate CFM still has visible dust drifting into corners.

Keep ducting short, straight and as large as the machine's collar. Every added foot of layflat, every elbow and every crushed section costs static pressure and therefore airflow. Exhaust outdoors where the structure allows, and be deliberate about the discharge point: not next to an open window, a fresh-air intake, a neighbor's door, or anywhere it re-enters the building.

Give the containment somewhere to draw makeup air from. A perfectly sealed enclosure with a negative air machine running is just a machine fighting the poly — airflow collapses, and the pressure differential you measure will not match the air changes you calculated. A defined makeup-air path, usually through the decontamination chamber, keeps the flow moving clean-to-dirty in the direction you want it.

What are the common mistakes with ACH calculations?

• Using the room instead of the containment. Only the sealed volume counts. Half a basement enclosed in poly is not the basement.
• Ignoring ceiling height. A 12 ft ceiling in an older home adds 50% to the volume of an 8 ft room and changes the machine count.
• Sizing on clean-filter CFM. Derate, or use published loaded-filter airflow.
• Treating recirculating scrubbers as negative pressure. They filter; they do not create a differential.
• Turning the machine down for noise and never turning it back up. Airflow varies with speed setting, so the ACH you calculated no longer applies.
• Quoting an ACH figure as a code requirement. Write the design figure and its basis in your scope instead — that is what holds up on review.
• Skipping documentation. Volume, target, equipment, speed and dates belong in the job file. Documentation questions show up on the AMRT exam as often as arithmetic does.

ACH calculator FAQ