Myth: Your ISO Class Decides the Layout of a Cleanroom

If you’re reading this, you’re probably wondering if the layout of your cleanroom requires ISO 7 or ISO 8 classification.
While your ISO class determines the level of cleanliness and the specifications you need for an HVAC system, it does not necessarily determine the layout of your cleanroom.
It may influence some factors. For example, you can enter an ISO 8 class cleanroom from an uncontrolled environment, but an ISO 6 would require one or two airlocks. But the ISO class does not define the layout, strictly speaking.

This is a clarification we often have to make during the initial planning phase with our clients. So in this post, we’re going to explore what’s involved in classification.

Your Cleanroom’s ISO Class

ISO 14644-1 determines your cleanroom’s class according to the concentration of particles of various sizes ranging from 0.1 microns to 0.5 microns.
Today’s common ISO standard 14644-1 (ISO 5, 6, 7, 8) replaced the Federal Standard 209 classification (Class 100; Class 10,000; Class 100,000) in 1999 before being updated in 2015.
That being said, many organizations continue to use and recognize the Class 100; Class 10,000; Class 100,000 terminology.

Comparing ISO standard 14644-1 to Federal Standard 209:

ISO Standard 14644-1 Federal Standard 209
ISO 3 1
ISO 4 10
ISO 5 100
ISO 6 1,000
ISO 7 10,000
ISO 8 100,000

Your ISO Class and Airflow

The lower your ISO class, the more frequently air needs to pass through the HEPA filter, which is measured in air change per hour (ACH).

A common HVAC airflow will change the air twice or so per hour. However, in a cleanroom setting, the air needs to be cycled at least 10 times per hour.

Air Changes Per Hour (ACH)

ISO Class Approximate air changes per hour needed
ISO 8 10 to 25
ISO 7 30 to 50
ISO 6 60 to 150
ISO 5 150 to 250+

Next, we use your ACH and the size of each room to calculate required airflow, measured in cubic feet per minute (CFM).

Cubic Feet Per Minute (CFM)

Room volume in cubic feet (ft3)

Length (ft) x

Width (ft) x

Height (ft)

Air changes per hour (ACH) Required Airflow

cubic feet

per minute


4 000 sq.ft x 8 ft =
32 000 ft3
25 ACH
(ISO 8)
= 13 333 CFM
150 sq.ft x 8 ft = 1
200 ft3
50 ACH
(ISO 7)
= 1 000 CFM

HEPA Filters

A HEPA filter generally supplies anywhere from 400 to 700 CFM.  Therefore, an ISO 7 room that is 10’ x 15’ x 8’ needing 50 ACH would require 1,000 CFM and 2 HEPA filters.

Required Airflow

Cubic Feet Per Minute (CFM)

Number of HEPA filters

(1 HEPA = 500 CFM)

13 333 CFM 27
1 000 CFM 2

Of Course, There Are More Factors to Consider

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