How Many Fan Filter Units Do You Need? ISO Class, ACH & a Worked Example

Buyers asking how many fan filter units a cleanroom needs can start with three inputs: room volume, a design air-change target, and published airflow per module. The fan filter unit family hub shows the ceiling FFU, slim, gel-seal, control, and grid options that may enter that decision.

An ISO class is a particle-concentration outcome, not an FFU count or an air-change table. For an FFU cleanroom, the method below provides an illustrative metric estimate. It does not replace the project user requirement specification (URS), HVAC design, airflow layout, or particle testing. Once the count is known, the separate ordering-parameters guide turns the selection into an RFQ.

ISO Class vs ACH — what each decides

ISO 14644-1 classifies cleanrooms by maximum airborne particle concentration at stated particle sizes. For orientation, commonly restated limits at particles ≥0.5 µm are 352,000 particles/m³ for ISO 7 and 3,520,000 particles/m³ for ISO 8. These values describe classification limits; they do not prescribe how much air a room needs.

The required occupancy state also matters. ISO 14644-1 distinguishes as-built, at-rest, and operational conditions. A room tested without process activity is not demonstrating the same condition as one tested during normal work. The project therefore needs to state both its target class and the applicable occupancy state and test protocol.

ACH means air changes per hour. An ACH target of 45 means supplying a volume of air equal to 45 room volumes each hour. It is a useful planning input, but ISO 14644-1 does not mandate ACH values or FFU quantities.

MECART’s industry learning article, which also states that ISO does not specify ACH, publishes the following ranges. They are reproduced here as illustrative industry rules of thumb, not as ISO requirements:

Target ISO class	Illustrative MECART ACH range
ISO 8	10–30
ISO 7	30–65
ISO 6	80–150
ISO 5	200–450

A process that sheds many particles may need a target toward the higher end of the selected range. The project HVAC engineer should confirm the design ACH against the URS, process emissions, occupancy, room layout, and pressure requirements.

Other industry guides publish different bands. Clean Air Products, for example, gives wider or higher ACH ranges for some classes and also publishes ceiling-coverage percentages. Those figures should be treated as a separate alternative reference or sanity check, not merged with the MECART ranges into a supposed “ISO ACH table.”

Metric calculation method

The basic FFU calculation converts the room volume and design ACH into total airflow, then divides that airflow by the published airflow of one module:

V (m³) = L × W × H
Q_total (m³/h) = ACH × V
N_FFU = ceil(Q_total / Q_unit)
ACH_check = (N_FFU × Q_unit) / V

Here, L, W, and H are room length, width, and height in metres. Q_unit is the assumed airflow of one FFU in m³/h. ceil means rounding the result up to the next whole unit whenever the division is not exact. Fractional FFUs cannot be installed, and rounding down would leave the design below the calculated airflow.

The final ACH_check is important because rounding up increases installed airflow. Report both the initial target and the resulting ACH rather than presenting the unit count alone.

For the arithmetic in this guide, use only the three airflow rows published on the AC fan filter unit page:

Module size (mm)	Reference airflow (m³/h)	Avg face velocity (m/s)	External static (Pa)	Supply on reference table
575 × 575 × 330	500	0.3–0.5	96	220 V / 50 Hz
575 × 1175 × 330	1000	0.3–0.5	96	220 V / 50 Hz
1175 × 1175 × 370	2000	0.3–0.5	96	220 V / 50 Hz

These are reference values; confirm with quotation before using them for an order. The quotation, rather than this catalogue-family summary, is binding.

The published 0.3–0.5 m/s face-velocity range describes the AC family reference. It is not a substitute for the stated m³/h rows in the main calculation. Deriving airflow from an assumed face velocity and an assumed active filter area would introduce another variable, so this guide uses the published module airflow directly.

Worked example (metric)

Every number in this section is an illustrative example, not a SENPEI performance guarantee or a guaranteed ISO design.

Set the example room and target

Assume an example room measuring 6.0 m long × 4.0 m wide × 2.7 m high:

V = 6.0 × 4.0 × 2.7 = 64.8 m³

Set a target of ISO 7, with the occupancy state still to be confirmed by the project. Choose an example design value of 45 ACH because it lies inside MECART’s illustrative ISO 7 range of 30–65 ACH.

Calculate total example airflow

Q_total = 45 × 64.8 = 2,916 m³/h

The example therefore needs 2,916 m³/h of total FFU airflow before rounding to whole modules.

Select an example AC module and round up

Choose the published AC reference row for a 575 × 1175 × 330 mm module at 1,000 m³/h:

N_FFU = ceil(2,916 / 1,000)
N_FFU = ceil(2.916)
N_FFU = 3

The result is three FFUs. Their combined reference airflow is 3 × 1,000 = 3,000 m³/h.

Check the resulting air-change rate:

ACH_check = 3,000 / 64.8 ≈ 46.3 ACH

The example result of approximately 46.3 ACH remains inside the selected MECART planning band of 30–65 ACH. This arithmetic does not establish that the room will pass ISO 7 particle testing.

Compare a larger-module alternative

Using the other published AC rows, two 1175 × 1175 × 370 mm modules at 2,000 m³/h each would provide an example total of 4,000 m³/h:

ACH_check = 4,000 / 64.8 ≈ 61.7 ACH

That result also falls within the example’s 30–65 ACH band, but near its upper end. Fewer, larger modules are not automatically preferable. Air distribution, ceiling space, returns, control zones, and installation coordination still need review.

For a short ISO 8 alternative, keep the same example room but select 20 ACH from MECART’s ISO 8 range:

Example selection	Calculation	Result
Required airflow at 20 ACH	20 × 64.8	1,296 m³/h
Two 1,000 m³/h modules	2 × 1,000 = 2,000; 2,000 / 64.8	2 FFUs; approximately 30.9 ACH after rounding
Three 500 m³/h modules	3 × 500 = 1,500; 1,500 / 64.8	3 FFUs; approximately 23.1 ACH after rounding

Both options meet the example’s calculated 1,296 m³/h requirement. The two-module option lands at about 30.9 ACH, slightly above the top of MECART’s illustrative 10–30 ACH ISO 8 band, while the three-module option stays inside it. The selected module row should therefore be stated with the count, and the engineer should confirm whether the extra airflow is acceptable.

Use coverage only as a sanity check

The example ceiling area is 6.0 × 4.0 = 24 m². Using the plan dimensions of the three 575 × 1175 mm modules as a rough filter-face proxy:

3 × (0.575 × 1.175) ≈ 2.03 m²
2.03 / 24 ≈ 8.4% of ceiling area

This approximately 8.4% example result may sit below some published coverage-percentage tables. For comparison only, Clean Air Products separately publishes 15–25% coverage and 60–150 ACH for ISO 7. Those values belong to that source’s different guidance and should not be inserted into the MECART-based calculation above.

In short, an ACH-based count does not by itself demonstrate class compliance or meet another guide’s coverage recommendation. If coverage, airflow distribution, or particle-recovery concerns arise, the engineer should revisit the layout and design assumptions rather than treat the first whole-number result as final.

Choosing module family after the count

The AC range is the useful starting point for this calculation because it publishes the three reference airflow rows used above. Its H13 or H14 filter-grade options to EN 1822 remain buyer inputs, and the applicable configuration must be confirmed with quotation.

Slim fan filter units are relevant where a low ceiling void or constrained plenum makes depth important. The range has a reference depth of approximately 130 mm and AC or EC motor options. Its page does not publish an airflow table, so do not reuse the AC arithmetic as though it were a published Slim airflow rating.

Gel-seal FFUs use a gel edge seal and a separable filter, and an AC centrifugal fan. They may suit process-point or band layouts, with local, 0–10 V, or group control depending on configuration. This range likewise has no published airflow table for count arithmetic.

The FFU ceiling grid then translates the selected quantity into a coordinated ceiling. The modular aluminium system carries FFUs, blank panels, lights, and duct ends, with example pitches of 1200 × 600 mm or 1200 × 1200 mm. After the count is known, blank or close-out panels can complete positions not occupied by active units.

What the count does not settle

A calculated FFU quantity settles only the airflow arithmetic under the stated assumptions. It does not establish particle-class acceptance. Final validation depends on particle concentration testing under the required occupancy state and protocol.

Returns and airflow paths can create stagnant zones even when total airflow appears sufficient. The pressure cascade must also be coordinated with adjacent rooms, while filter loading can change operating conditions over time. Ceiling layout, process equipment, personnel activity, and commissioning all influence the finished installation.

Electrical and control details remain separate decisions. The AC reference table lists 220 V / 50 Hz, but the site supply must be confirmed. Motor type and the control interface are buyer inputs; group-control coordination may use the relevant AC or EC control arrangement where applicable. Filter grade, replacement direction, installation, electrical connection, and balancing also need project confirmation.

For the next stage, use the guide to fan filter unit ordering parameters. It covers the RFQ decisions that follow the count without replacing this calculation method.

Quote checklist

Copy this block into the project enquiry and replace the bracketed fields. Keep the design ACH source and per-unit airflow assumption visible so that the supplier can check the basis of the quantity.

Room internal dimensions L × W × H: [m]
Room volume: [m³]
Target ISO class: [class]
Required occupancy state: [as-built / at-rest / operational]
Design ACH: [ACH]
Design ACH source and engineer confirmation: [source / status]
Preferred module size: [mm]
Q_unit airflow assumption: [m³/h; reference or quoted value]
Calculated total airflow: [m³/h]
Calculated FFU count, rounded up: [units]
Resulting ACH_check: [ACH]
Filter grade: [H13 / H14 to EN 1822; confirm]
Site supply: [V / Hz]
Motor and control preference: [specify for confirmation]
Filter replacement direction: [specify]
Quantity: [units]
Destination: [country / city / delivery location]

Start with the AC fan filter unit reference page, or revisit the family hub linked above to compare construction options. When the room basis, module assumption, and specification are ready, request a quote and attach the completed checklist together with the ordering-parameters decisions.

Selected references

• MECART — HVAC cleanroom design calculations (ACH rule of thumb)
• Clean Air Products — Industry standards (ISO class guidance, ACH and coverage)

Questions buyers ask