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How to Calculate FFU Quantity for Different Cleanroom Classes?

Created on 04.15
In the design and construction of cleanrooms, the quantity of FFU (Fan Filter Units) directly determines the air cleanliness level. Whether designing a new modular cleanroom or upgrading an existing facility, accurately calculating the FFU count is critical to ensuring certification under ISO 14644-1.
For engineering teams, this is not just a technical parameter—it is the core of cost control regarding energy consumption and material investment. This article provides a standard calculation formula and a step-by-step guide.
Metal boxes with vented tops arranged on a blue floor.

1. The Core Formula (FFU Calculation Formula)

To determine the required number of FFUs, follow this logical formula:
$$N = \frac{V \times ACR}{v}$$
Parameter Descriptions:
$N$: Number of FFUs required (always round up to the nearest whole number).
$V$: Total volume of the cleanroom (Area $\times$ Ceiling Height, in $m^3$).
$ACR$ (Air Change Rate): Number of air changes per hour. This is the core variable determined by the target ISO class.
$v$: Air volume of a single FFU (usually measured in $m^3/h$). A standard $1175 \times 575$ mm FFU typically has an air volume between $1000$ and $1200\ m^3/h$ at full load.

2. Air Change Rate (ACR) Standards by ISO Class

Before applying the formula, select the appropriate ACR based on your target cleanliness level. Below are standard empirical values used in the industry:
Cleanroom Class (ISO)
Recommended ACR (Changes/Hour)
Suggested FFU Ceiling Coverage
ISO 5 (Class 100)
240 - 480
60% - 100%
ISO 6 (Class 1,000)
70 - 160
25% - 40%
ISO 7 (Class 10,000)
30 - 70
15% - 25%
ISO 8 (Class 100,000)
10 - 25
5% - 15%

3. Step-by-Step Calculation Example

Suppose you are designing an ISO 7 (Class 10,000) cleanroom for an electronics assembly workshop with an area of 50 $m^2$ and a height of 2.5 $m$.
Step 1: Calculate Total Volume ($V$)
$50\ m^2 \times 2.5\ m = 125\ m^3$
Step 2: Determine Air Change Rate ($ACR$)
Taking the median value for ISO 7, let's assume 50 changes/hour.
Step 3: Determine Single FFU Air Volume ($v$)
Assuming the use of a GCC standard FFU with a volume of $1000\ m^3/h$.
Step 4: Apply the Formula
$N = (125 \times 50) / 1000 = 6.25$
Conclusion: You will need at least 7 FFUs to ensure the room stably meets ISO 7 standards.

4. Additional Factors Affecting FFU Quantity

In real-world engineering, relying solely on a formula is insufficient. The following variables must be considered:
Personnel Density: More staff on the floor increases particle generation. It is recommended to increase the ACR slightly in high-traffic areas.
Equipment Heat Load: If production machinery generates significant heat, the FFUs must work in tandem with the HVAC system to increase circulation for heat dissipation.
Filter Resistance: Over time, HEPA filters accumulate dust, increasing resistance. We recommend pre-calculating a 10% - 20% margin in air volume to ensure compliance throughout the filter's lifecycle.

Get a Professional FFU Layout Design

Precise calculation is only the first step; a rational airflow pattern layout is equally vital. At gcccleanroom.com, we utilize advanced simulation software to provide optimized FFU distribution plans for large-scale projects, minimizing energy consumption while ensuring total compliance.
If you need a customized FFU selection guide or a cleanroom project quote:
Please contact our Engineering Director, Jim:
📧 Email: Jim@gzkunling.com
📱 WhatsApp: +86 15018770887
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