Air Scrubber & Negative-Air CFM Sizing

Work out the airflow and number of HEPA air scrubbers a containment needs, from the room dimensions and your target air changes per hour (ACH) — an IICRC/AHAM sizing rule of thumb.

Planning typicals: equipment counts are planning typicals from IICRC/AHAM rules of thumb. Actual counts depend on the class of water, materials and airflow on site — a certified technician must confirm the drying/containment plan.

Calculator

ft
Interior length of the contained space.
ft
Interior width of the contained space.
ft
Floor-to-ceiling height.
IAQ/clearance ~4 ACH; active containment ~6 ACH.
CFM/unit
Rated airflow per HEPA unit. Typical ~500 CFM.
Required airflow180 CFM
Negative-air / scrubber units1 unit
Room volume2,700 cu ft (15 × 20 × 9)
Target4 ACH · ~500 CFM/unit

A 2,700 cu ft room at 4 air changes per hour needs about 180 CFM (ACH × volume ÷ 60), so 1 scrubber at ~500 CFM each. Containment usually targets 4–6 ACH. These are labeled planning typicals — a certified technician confirms the setup.

During a mold, sewage or fire remediation, the work area is sealed off and put under negative pressure so contaminated air cannot drift into clean rooms. HEPA air scrubbers pull air through the containment and filter it, and the number you need depends on one thing: how many times per hour you want to exchange all the air in the room — the air changes per hour (ACH) target.

This is a sizing helper, not a cost tool. It turns the room dimensions and your ACH target into a required airflow in CFM and a scrubber count, using the same identity restoration technicians use in the field.

Formula

The math is two clean identities:

volume = length × width × height
required CFM = ACH × volume ÷ 60
scrubbers = ceil(required CFM ÷ scrubber CFM)

  • volume — the room in cubic feet.
  • ACH — target air changes per hour (this is the inverse of the ACH = CFM × 60 ÷ volume identity used to verify a setup). Containment usually targets 4–6 ACH.
  • ÷ 60 — converts changes-per-hour to airflow-per-minute (CFM).
  • ceil() — you cannot deploy a fraction of a scrubber, so round up.

The ACH target and the per-unit CFM are labeled planning typicals you can override to match the machines on the truck.

Worked example

Take a room 15 ft × 20 ft × 9 ft and a target of 4 ACH for general clearance, with 500 CFM scrubbers:

volume = 15 × 20 × 9 = 2,700 cu ft
required CFM = 4 × 2,700 ÷ 60 = 180 CFM
scrubbers = ceil(180 ÷ 500) = 1 unit

Raise the goal to 6 ACH for active containment and the requirement rises to 270 CFM — still one 500-CFM scrubber, but with less headroom. A larger or taller room, or a higher ACH, is what tips you into a second unit.

Air changes, negative pressure and what a scrubber is not

Why 4–6 ACH? Restoration practice targets enough air movement to keep airborne particulate low and to hold the containment under negative pressure, without over-drying or creating so much turbulence that settled spores are re-suspended. Lower targets (around 4 ACH) suit general indoor-air clearance and post-remediation verification; higher targets (around 6 ACH, and sometimes more) suit active containment during aggressive removal. The right number is a professional judgment based on the contaminant, the materials and how the space is sealed — which is exactly why this is a labeled planning helper, not a specification.

Two practical notes. First, a HEPA scrubber’s rated CFM drops as its filters load, so techs size with a margin and monitor pressure with a manometer rather than trusting the nameplate number all day. Second, an air scrubber is not the same as a dehumidifier or an air mover: scrubbers clean and exchange air, air movers push evaporation off wet surfaces, and dehumidifiers pull moisture out of the air. A drying job needs the first two sized together — use the structural drying equipment tool for the air-mover and dehumidifier counts.

This helper deliberately stays inside the restoration lane: it sizes negative-air / scrubber CFM for containment and indoor-air-quality work, not HVAC ventilation, furnace CFM or a mechanical ventilation design (that is a different trade). To price the whole air-quality phase — duct cleaning, scrubber rental days and clearance testing — use the indoor air quality cost tool. Equipment counts here are IICRC/AHAM planning typicals; a certified technician must confirm the containment and airflow plan on site.

Reference table

The airflow you need scales with room volume and your target air changes per hour (ACH). These are labeled IICRC/AHAM planning typicals — a certified technician confirms the setup on site.

Room volume4 ACH (IAQ / clearance)6 ACH (active containment)
1,000 cu ft67 CFM100 CFM
2,000 cu ft133 CFM200 CFM
2,700 cu ft180 CFM270 CFM
4,000 cu ft267 CFM400 CFM

Identity: required CFM = ACH × volume ÷ 60; units = ceil(CFM ÷ scrubber CFM). See the air-changes / CFM table and sources.

Frequently asked questions

How many air scrubbers do I need?
Compute the required airflow — ACH × room volume ÷ 60 — then divide by each unit’s CFM and round up. A 2,700 cu ft room at 4 ACH needs about 180 CFM, so a single 500-CFM scrubber. Bigger rooms or higher ACH targets add units.
What ACH should I target for containment?
Restoration practice commonly uses about 4 ACH for indoor-air-quality clearance and around 6 ACH for active containment during removal. The exact target is a professional call based on the contaminant and how the space is sealed.
How do I calculate room volume?
Multiply length × width × ceiling height in feet to get cubic feet. A 15 × 20 room with a 9 ft ceiling is 2,700 cu ft. For odd shapes, add up the volume of each rectangular section.
What is the difference between ACH and CFM?
CFM is airflow — cubic feet of air moved per minute. ACH is how many times per hour that airflow replaces the whole room’s volume. They are linked by ACH = CFM × 60 ÷ volume, which this tool rearranges to size the airflow you need.
Is an air scrubber the same as a dehumidifier or air mover?
No. An air scrubber filters and exchanges air (HEPA), an air mover pushes evaporation off wet surfaces, and a dehumidifier removes moisture from the air. A drying job sizes air movers and dehumidifiers separately — use the structural drying equipment calculator for those.
Why round the scrubber count up?
Because you cannot run a fraction of a machine, and under-sizing means you never reach the target air changes. Rounding up guarantees the containment holds its ACH — with a little headroom for filter loading, which lowers real-world CFM.

Related calculators

Structural Drying EquipmentSize structural drying: how many air movers and LGR dehumidifiers to dry a room from the affected square footage, using labeled IICRC/AHAM coverage typicals.Indoor Air Quality CostEstimate post-remediation indoor air quality cost from duct cleaning, air-scrubber days and clearance testing — with a formula, a worked example and a FAQ.Air Duct Cleaning CostEstimate air duct cleaning cost from a base fee, the number of vents and the per-vent rate you enter — with the formula, a worked example and a homeowner FAQ.