Air Scrubber Size Calculator: CFM, ACH & Number of Units
Calculate the nominal airflow needed to reach a target air-change rate in a room or containment area. Enter length, width, height and the target ACH supplied by your project plan. You can stop at required CFM, or select a current normalized air scrubber to estimate the number of whole air-scrubber units, resulting nominal ACH and planning electrical load.
The arithmetic is fixed: room volume × target ACH ÷ 60. The appropriate target depends on the project, containment and professional requirements. The estimated number of units is a nominal planning result, not a guarantee of field airflow through ducting or loaded filters.
Room and air-change inputs
The air-scrubber CFM formula
The basic relationship is Required CFM = room volume × target ACH ÷ 60. Room volume is length × width × height. ACH means how many room-volumes of air are nominally moved through the filtration system per hour. Dividing by 60 converts the hourly airflow requirement into cubic feet per minute.
For example, a 20 × 15 × 8 ft containment has 2,400 cubic feet of volume. At a user-entered target of 6 ACH, the nominal requirement is 240 CFM. At 10 ACH, it is 400 CFM. The calculator is intentionally neutral about which target is appropriate; it only translates the target you provide into airflow.
Why square-foot rules are too crude
A “one scrubber per X square feet” rule ignores ceiling height. A 500 ft² room with an 8 ft ceiling contains 4,000 ft³ of air; the same floor area with a 14 ft ceiling contains 7,000 ft³. At the same target ACH, the taller room requires 75% more nominal airflow.
Square-foot rules also hide equipment airflow. One current machine may publish 400 CFM, another 550, another 700 and another 2,000. Converting the room to CFM first lets you compare unlike machines on the same airflow requirement.
How the number of units is calculated
When a model is selected, the tool divides required CFM by the planning airflow for that exact normalized product and rounds up. If required airflow is 720 CFM and the selected model’s planning value is 550 CFM, one machine is not enough by the nominal arithmetic, so the result becomes two units.
For current models with a published rated airflow, the tool uses that value. If the normalized record has a variable range or only a maximum published airflow, the tool uses the current maximum value and labels that basis in the result. That makes the assumption visible instead of quietly treating “maximum” as though it were a fixed operating point.
Why resulting ACH can be higher than the target
The number of units must be a whole number, so rounding up usually produces more nominal airflow than the minimum arithmetic. If a room requires 720 CFM and two 550 CFM units are selected, nominal combined airflow is 1,100 CFM. The resulting nominal ACH is therefore higher than the target used to determine the minimum number of units.
That does not mean the job is “over-remediated.” It simply shows the arithmetic consequence of discrete machine sizes. Variable-speed equipment may allow the operator to reduce airflow where appropriate, subject to the exact equipment instructions and project plan.
Field airflow can be lower than a free-air or maximum rating
Ducting, filter loading and system resistance can reduce actual airflow. Dri-Eaz publishes 3.0 inches H₂O static pressure for the HEPA 700 and documents 14-inch ducting options for containment work. XPOWER documents a ductable outlet on the X-2480A. Those details show why a model’s maximum CFM is useful but not a complete prediction of ducted field performance.
The calculator therefore calls its outputs nominal. It does not invent an airflow-loss percentage because that loss depends on the exact machine, filter condition, duct length, bends and installed setup.
Electrical output is a planning aid
If the selected model has verified amp data, the calculator multiplies its planning amp value by the rounded number of units. Rated amps are used when published; otherwise the maximum published amps are used. This is intentionally conservative for variable-speed units such as the HEPA 700, which currently publishes 1.5–3A.
The output does not decide how many units can be placed on a circuit. Manufacturer daisy-chain limits, other connected equipment, circuit rating and jobsite electrical conditions remain separate checks. A later restoration-equipment power calculator will own that broader circuit-planning workflow.
Current airflow examples from the normalized catalog
| Model | Planning airflow used here | Planning amps | Unit-estimate implication |
|---|---|---|---|
| Dri-Eaz DefendAir 400 | 400 CFM | 2.2 A | Useful for distributing more smaller units across rooms or containments. |
| XPOWER X-2480A | 550 CFM rated | 2.8 A | Compact 550-CFM planning point with low transport weight. |
| Dri-Eaz HEPA 700 | 700 CFM maximum published | 3.0 A maximum | Variable 275–700 CFM; calculator uses the maximum as the unit-estimate planning value. |
| AlorAir HEPA Max 970 | 750 CFM maximum published | 2.8 A | Higher-flow option where fewer units may meet the nominal target. |
| Dri-Eaz HEPA 2000 | 2,000 CFM maximum published | 7.7 A | Large-airflow machine with a very different weight/power profile. |
Turn the airflow requirement into a product shortlist
The calculator answers “how much nominal airflow?” The Finder answers a different question: which current exact machines also fit your negative-air, carbon, amp, weight, daisy-chain and monitoring requirements.
Do not use the number of units as proof of remediation adequacy
A correct airflow calculation does not inspect a building, classify contamination, measure pressure differential, determine source removal or verify that surfaces and cavities are dry. It is one equipment-planning input. For mold-remediation context, use Air Scrubber for Mold; for ducted containment, use Negative Air Machine.
Primary references for this page
- Legend Brands air-change guidance ↗Manufacturer technical guidance for the volume × ACH ÷ 60 relationship and project-dependent target air changes.
- Dri-Eaz HEPA 700 ↗Current manufacturer airflow, amp range and static-pressure specifications used by the normalized product record.
- Dri-Eaz HEPA 700 ducting ↗Current manufacturer duct-size and duct-length information showing why ducted field airflow is setup-dependent.
- XPOWER X-2480A ↗Current rated airflow, electrical load and ductable-outlet information.
Need to combine this load with other restoration equipment?
Use the power calculator to add the planned scrubber quantity to dehumidifiers, air movers or custom loads.
Frequently asked questions
How do I calculate the CFM needed for an air scrubber?
Multiply room volume in cubic feet by the target air changes per hour, then divide by 60. Room volume is length × width × height. The calculator performs that arithmetic but does not choose the target ACH for your project.
Why does the calculator ask for ceiling height?
Square footage alone does not describe the amount of air in the space. Two rooms with the same floor area but different ceiling heights have different air volumes and therefore different CFM requirements for the same target ACH.
How does the calculator determine the number of air scrubbers?
When you select a current model or enter a custom airflow value, the calculator divides required CFM by the planning CFM per unit and rounds up to the next whole machine.
Does the calculated number of air scrubbers include ducting and filter losses?
No. The unit estimate is nominal arithmetic based on the published or custom airflow value. Field airflow can change with ducting, filter loading, containment leakage and operating conditions.
What amp value is used for total electrical planning?
For a normalized model, the calculator uses the published rated amp value when available, otherwise the published maximum amp value. For variable-speed equipment this is a planning value, not a prediction of exact real-time current draw.
What ACH should I use for mold remediation?
Restoration Select does not prescribe a universal remediation ACH. Use a target established by the remediation plan, applicable professional requirements or qualified professional guidance. Manufacturer technical examples can provide context but are not universal rules.