Yes. Class 100 is generally considered equivalent to ISO Class 5 when comparing airborne particles at ≥0.5 µm. However, Class 100 and ISO 5 are not the same standard. Class 100 comes from the older U.S. Federal Standard 209E, while ISO 5 is part of the ISO 14644-1 classification system.
The difference matters because the numbers can look very different. A Class 100 cleanroom is described as allowing no more than 100 particles per cubic foot at ≥0.5 µm, while ISO 5 allows no more than 3,520 particles per cubic metre at the same particle-size threshold. The two values represent essentially the same cleanliness level when the units are converted.
Class 100 vs. ISO 5 at a Glance
|
Requirement |
Class 100 |
ISO Class 5 |
|
Classification system |
Federal Standard 209E |
ISO 14644-1 |
|
Status |
Legacy standard |
Current ISO classification framework |
|
Main unit |
Particles/ft³ |
Particles/m³ |
|
Key comparison |
≥0.5 µm |
≥0.5 µm |
|
Maximum concentration |
100 particles/ft³ |
3,520 particles/m³ |
|
General relationship |
Corresponds to ISO 5 |
Corresponds to Class 100 |
Federal Standard 209E was officially cancelled in 2001, while ISO 14644-1 became the modern international framework for cleanroom air classification. The older terminology, however, is still found in equipment specifications, facility documents and purchasing requirements.
Why Does Class 100 Allow 100 Particles but ISO 5 Allows 3,520?
This is the question that causes the most confusion.
The answer is volume.
Class 100 uses cubic feet. ISO 5 uses cubic metres.
One cubic metre contains approximately 35.3 cubic feet.
So:
100 particles/ft³ × 35.3 ft³/m³ ≈ 3,530 particles/m³
The ISO 5 limit is 3,520 particles/m³ at ≥0.5 µm. The small numerical difference should not be interpreted as a meaningful difference in cleanliness. The two classifications correspond at this particle-size threshold.
The simple rule
100 particles/ft³ and 3,520 particles/m³ look different because the units are different, not because ISO 5 is 35 times cleaner than Class 100.
This is why comparing the raw numbers without checking the unit can lead to the wrong conclusion.
What Does "≥0.5 µm" Mean?
The particle-size threshold is just as important as the unit.
The symbol ≥ means equal to or larger than.
Therefore:
100 particles/ft³ at ≥0.5 µm
means that no more than 100 particles of 0.5 µm or larger should be present in one cubic foot of air under the applicable classification condition.
Likewise:
3,520 particles/m³ at ≥0.5 µm
means no more than 3,520 particles of 0.5 µm or larger per cubic metre.
ISO 14644-1 uses cumulative particle concentrations at specified threshold sizes ranging from 0.1 µm to 5 µm and determines particle concentration using airborne particle counters at designated sampling locations.
This is important because a cleanroom class is not a count of particles exactly 0.5 µm in size. It includes particles larger than that threshold.
Are Class 100 and ISO 5 Actually the Same Standard?
No. They are equivalent classifications at the relevant particle-size comparison, but they come from different standards.
This distinction is easy to miss.
Class 100
Class 100 is a designation from Federal Standard 209E.
Its traditional definition is based on a maximum of 100 particles per cubic foot at ≥0.5 µm. The original Federal Standard 209E also used the familiar Class 10, Class 1,000, Class 10,000 and Class 100,000 terminology.
ISO 5
ISO 5 is a classification under ISO 14644-1:2015.
The ISO system expresses particle concentration in cubic metres and provides limits at several particle-size thresholds. For ISO 5, the limits include:
|
Particle size |
ISO 5 maximum concentration |
|
≥0.1 µm |
100,000 particles/m³ |
|
≥0.2 µm |
23,700 particles/m³ |
|
≥0.3 µm |
10,200 particles/m³ |
|
≥0.5 µm |
3,520 particles/m³ |
|
≥1 µm |
832 particles/m³ |
|
≥5 µm |
29 particles/m³ |
This broader classification structure is one reason current specifications generally use ISO terminology rather than relying only on the old Class 100 label.
Why Is Class 100 Still Used?
Even though Federal Standard 209E is a legacy standard, Class 100 remains common in older drawings, equipment specifications and industry conversations.
A buyer may therefore receive documents that say:
Class 100
Class 10,000
Class 100,000
while a newer supplier document says:
ISO 5
ISO 7
ISO 8
The commonly used comparison is:
|
Former FS 209E |
ISO 14644-1 |
|
Class 1 |
ISO 3 |
|
Class 10 |
ISO 4 |
|
Class 100 |
ISO 5 |
|
Class 1,000 |
ISO 6 |
|
Class 10,000 |
ISO 7 |
|
Class 100,000 |
ISO 8 |
The relationship is useful when reading legacy documents, but a current project specification should identify the applicable ISO standard rather than relying only on the older class name.
Can You Simply Replace "Class 100" with "ISO 5"?
For general cleanliness-level terminology, Class 100 maps to ISO 5. For a formal technical specification, however, do not change the wording without checking the original requirement.
Before accepting a Class 100 specification as ISO 5, verify four points:
1. Confirm the referenced standard.
Check whether "Class 100" refers specifically to Federal Standard 209E.
2. Confirm the particle size.
Make sure the requirement is based on particles ≥0.5 µm.
3. Confirm the measurement unit.
Class 100 uses particles per cubic foot; ISO 5 uses particles per cubic metre.
4. Check the actual test documentation.
A label such as "Class 100" is not the same as a complete qualification or classification report.
This last point matters in real purchasing situations. A classification name tells you the intended cleanliness level; the test record tells you whether the required condition was actually demonstrated.
Does Class 100 or ISO 5 Mean Zero Particles?
No. Neither classification means that the air is particle-free.
Class 100 allows up to 100 particles/ft³ at ≥0.5 µm.
ISO 5 allows up to 3,520 particles/m³ at ≥0.5 µm.
Both are therefore maximum concentration limits, not zero-particle conditions.
This distinction is important when discussing cleanroom performance. A cleaner room does not mean an absence of particles; it means the airborne particle concentration is controlled below a defined limit.
Does ISO 5 Mean the Room Is Sterile?
No. ISO 5 describes airborne particle concentration, not sterility.
ISO 14644-1 specifically states that the standard cannot be used to characterize the physical, chemical, radiological, viable or other nature of airborne particles. In other words, the classification tells you how many particles are present above specified size thresholds, but not whether those particles are microorganisms or what they contain.
This is why ISO 5, sterile, and microbiologically controlled should not be treated as interchangeable terms.
Does a Class 100 or ISO 5 Room Require a Specific Cleanroom Wipe?
Not automatically. A cleanroom classification describes the environment, while a wipe specification describes the cleaning material.
An ISO 5 room may require wipes with controlled particle generation, low lint, appropriate packaging or other application-specific characteristics. The correct wipe also depends on the surface and cleaning task.
For applications where electrostatic control is important, specialized products such as ESD Cleanroom Wipes may be considered alongside the cleanroom's particle-control requirements.
The important distinction is:
ISO 5 describes the cleanliness level of the environment. It does not mean that every product labeled "ISO 5" has identical performance.
What Should Buyers Use Today: Class 100 or ISO 5?
For new specifications, ISO 5 is the clearer modern terminology because it directly identifies the current ISO 14644-1 classification system.
For older documents, understanding Class 100 remains important because the term may still appear in:
- Legacy cleanroom drawings
- Older customer specifications
- Equipment documentation
- Facility qualification records
- Purchasing documents
- A practical way to write a modern specification is:
ISO Class 5 according to ISO 14644-1
When historical terminology needs to be preserved, you can state:
ISO Class 5, formerly referenced as Class 100 under Federal Standard 209E
This keeps the current classification clear while making the relationship understandable to anyone working with older documentation.
FAQ: Class 100 vs. ISO 5
Is Class 100 the same as ISO 5?
Yes, generally. Class 100 corresponds to ISO Class 5 when comparing airborne particles ≥0.5 µm. However, they originate from different cleanroom classification standards.
Why is ISO 5 written as 3,520 particles/m³ instead of 100 particles/ft³?
Because Class 100 uses cubic feet while ISO 5 uses cubic metres. One cubic metre is approximately 35.3 cubic feet.
Is ISO 5 cleaner than Class 100?
No. At the ≥0.5 µm comparison point, they represent essentially the same airborne cleanliness level.
Is Class 100 still a current standard?
Federal Standard 209E is a legacy standard that was cancelled in 2001. ISO 14644-1 is the current international framework used for cleanroom air classification.
Does ISO 5 mean there are no particles in the room?
No. ISO 5 defines a maximum airborne particle concentration; it does not mean zero particles.
What is the main difference between Class 100 and ISO 5?
The main difference is the classification system and unit of measurement. Class 100 uses the older FS 209E terminology and particles per cubic foot, while ISO 5 uses ISO 14644-1 and particles per cubic metre.
The Practical Answer
Class 100 and ISO 5 describe essentially the same airborne cleanliness level at ≥0.5 µm, but they are not two names for the same standard.
Remember these four points:
Class 100 = legacy FS 209E terminology
ISO 5 = current ISO 14644-1 terminology
100 particles/ft³ ≈ 3,520 particles/m³ at ≥0.5 µm
For current specifications, use the ISO classification and verify the actual test requirements
When a Class 100 requirement appears in an older document, translating it to ISO 5 is a useful first step. For a formal project or product specification, however, the safest approach is to look beyond the label and confirm the standard, particle size, concentration, units and test conditions.
