ISO 4406 vs SAE AS4059: which oil cleanliness code to specify

ISO 4406 codes cumulative counts per mL at 4, 6 and 14 µm(c) (Pall, 2011); AS4059 uses NAS-based classes per 100 mL with size letters (Chevron, 2020). ISO 4406 suits industrial hydraulic and lube oil, AS4059 aerospace and NAS-era contracts. Both assume an ISO 11171 calibrated counter; ISO 4406:2021 and AS4059G are current as of 2026-10-13.

• ISO 4406 reports the number of particles greater than 4, 6 and 14 µm(c) in 1 mL of sample fluid (Pall, 2011).

• NAS 1638 counts particles per 100 mL in five differential size ranges from 5 µm to over 100 µm, in classes 00 to 12 (Chevron, 2020).

• SAE AS4059 was first issued in June 1988; revision G, published 22 November 2022, is the current issue (SAE International, 2022).

• ISO 11218:1993 adopted SAE AS4059A as a de facto ISO standard; ISO 11218:2017 is the current second edition (ISO, 2017).

The comparison covers particle counts in hydraulic and lubricating oils. It does not cover weight-based (gravimetric) contamination limits, water content, or the cleanliness of parts before assembly, which need their own methods.

How do ISO 4406 and SAE AS4059 differ?

ISO 4406 and SAE AS4059 differ in volume basis, counting mode and code format: ISO 4406 gives three cumulative counts per millilitre, while SAE AS4059 gives classes per 100 mL with size letters for up to six sizes. ISO 4406 references the number of particles greater than 4, 6 and 14 µm(c) in 1 mL of fluid (Pall, 2011). SAE AS4059 keeps the NAS 1638 class numbers and counts per 100 mL (Chevron, 2020; Pall, 2011). SAE AS4059 revision F also accepts either differential or cumulative counts, while NAS 1638 uses differential counts only (Chevron, 2020).

The table below compares ISO 4406, SAE AS4059 and NAS 1638 property by property and decides which code to write into a specification.

The row that most often causes misreading between codes is volume basis: an ISO 4406 code counts particles per 1 mL, while SAE AS4059 and NAS 1638 count per 100 mL (Pall, 2011), so their numbers can never be read across one for one.

When is ISO 4406 the better code?

ISO 4406 is the better cleanliness code for industrial hydraulic and lubricating oil systems, where labs, filter makers and equipment builders already work in ISO 4406 scale numbers. The ISO 4406 system has largely been adopted by original equipment manufacturers, filtration companies and the lubrication industry (Chevron, 2020). Its current edition, ISO 4406:2021, was confirmed by ISO in 2026 (ISO, 2026).

The ISO 4406 code is easy to trend. Three numbers cover the small sizes, below about 15 µm, that cause the most damage to machinery (Chevron, 2020), and each one-step increase in a scale number represents a doubling of the contamination level (Pall, 2011).

ISO 4406 also handles microscope data. Where the first size is not required or the method cannot measure it, as with microscope counts, the first number is replaced by a dash, as in ISO -/13/10 (Pall, 2011).

ISO 4406 codes from automatic particle counters are comparable between labs when each counter is calibrated to ISO 11171, whose current edition is ISO 11171:2022 (ISO, 2022).

When is SAE AS4059 the better code?

SAE AS4059 is the better cleanliness code for aircraft hydraulic systems and for any drawing, contract or supplier that still states cleanliness as a NAS 1638 class. SAE AS4059 is an SAE Aerospace Standard, and its contamination classes "are based on the widely accepted NAS 1638 cleanliness classes" (SAE International, 2011). The SAE abstract also states that "Conversion from NAS 1638 cleanliness class specifications to AS4059 class specifications is defined" (SAE International, 2011).

SAE AS4059 also gives the specifier more control than a single NAS class. The AS4059E abstract describes the versatility to name a maximum class in multiple size ranges, a total number of particles larger than a specific size, or a class for each size (SAE International, 2011).

SAE AS4059 is the weaker choice for a plant whose lab reports only ISO 4406 codes, and it does not carry over the NAS 1638 classes based on particle weight (SAE International, 2011).

Is NAS 1638 still a valid cleanliness code for new specifications?

NAS 1638 is not a valid cleanliness code for new specifications: Chevron calls NAS 1638 a legacy system and reports that the NAS system has not been used for components designed after May 2001 (Chevron, 2020). Its calibration basis is also gone. Automatic particle counters reporting NAS 1638 were usually calibrated to ISO 4402, the ISO 4402 calibration dust (ACFTD) stopped production in 1999, and ISO 4402 calibrations are no longer effective since then (PAMAS, accessed 2026). A NAS 1638 class on a new purchase specification therefore conflicts with any lab that tests the oil on a counter calibrated to ISO 11171.

SAE AS4059 was developed to succeed NAS 1638 and to remove its disadvantages, most importantly that NAS 1638 has no traceable calibration standard (PAMAS, accessed 2026). For existing equipment whose drawings still state a NAS 1638 class, SAE AS4059 defines the conversion from NAS 1638 class specifications to AS4059 class specifications (SAE International, 2011).

No source from the Aerospace Industries Association (AIA), which publishes NAS 1638, confirmed a cancellation date as of 2026-10-13.

A NAS 1638 class in an inherited specification is a signal to rewrite it in SAE AS4059 or ISO 4406.

How does SAE AS4059 report cumulative and differential counts?

SAE AS4059, as Chevron (2020) summarizes revision F, reports either differential counts in the five NAS 1638 size ranges with classes 00 to 12, or cumulative counts in six size codes, A to F, with classes 000 to 12 (Chevron, 2020). The differential classes and their particle count limits are identical to NAS 1638, so SAE AS4059 class 8 equals NAS 1638 class 8 (Chevron, 2020).

A differential count gives particles from one size up to the next size; a cumulative count gives all particles at that size and larger (Chevron, 2020). Cumulative SAE AS4059 sizes A, B and C are greater than 4, 6 and 14 µm(c) on an ISO 11171 calibrated counter (Chevron, 2020).

A cumulative SAE AS4059 result is reported with the prefix "cpc", and the overall class equals the highest level found across the reported sizes (Chevron, 2020). A sample at levels 9, 9, 8, 9, 8 and 8 across sizes A to F is reported as cpc [9/9/8/9/8/8], which is cpc Class 9 (Chevron, 2020).

Specifications can name a class in three ways: one size such as 5B, several sizes such as 7B/6C/5D, or a complete size range from A through F (Chevron, 2020).

Which cleanliness code should a new specification use?

A new specification for industrial hydraulic or lube oil should use an ISO 4406:2021 three-number code measured on a counter calibrated to ISO 11171:2022, and SAE AS4059G only when an aerospace customer or contract requires NAS-style classes. This is editorial guidance, not a requirement of either standard.

A clear specification line states five things: the standard and its edition, the code, the counting method, the calibration standard, and the sample point. An illustrative ISO line reads "ISO 4406:2021 code 17/15/12 or cleaner, automatic particle counter calibrated to ISO 11171:2022, sampled at the return line before the filter." An illustrative SAE AS4059 line reads "SAE AS4059G cpc Class 7B-F." Always state the SAE AS4059 revision, because the reporting format changes between revisions: the AS4059F abstract states that a contamination code has been added to describe the contamination levels at the specified particle size ranges (SAE International, 2020).

After the move to ISO 11171, particle sizes were redefined and are reported in "µm(c)", so old and new measurements can be identified and told apart (PAMAS, accessed 2026). A specification that names no calibration standard likely invites two labs to report different first numbers for the same oil, because 1 µm under the older ISO 4402 calibration equals about 4 µm(c) under ISO 11171 (PAMAS, accessed 2026).

The required ISO 4406 level depends on how sensitive the system is to contaminant and the reliability the user requires (Pall, 2011).

How do you move a specification from NAS 1638 to ISO 4406 or AS4059?

A specification moves from NAS 1638 to SAE AS4059 almost one for one, but a move to ISO 4406 needs new measured data, because ISO 4406 counts per 1 mL at different sizes. For SAE AS4059, the differential classes and particle count limits are identical to NAS 1638, so SAE AS4059 Class 8 is the same as NAS 1638 Class 8 (Chevron, 2020).

For ISO 4406, avoid copying a number from a conversion chart into the new specification. One published example shows why: a sample at ISO 4406 code 19/16/11 was reported as SAE AS4059 classes 10A, 8B and 9C, a different class at each size (Pall, 2011). The ISO 4406 to NAS 1638 conversion chart is approximate and is covered in a separate article.

A practical sequence has four steps: take baseline samples on a counter calibrated to ISO 11171; set the new target from that baseline and the component needs; report the old NAS 1638 class and the new code side by side for a transition period; then update drawings, purchase orders and lab contracts together. Working a change like this from your own documents and site history is the kind of case Lubinpla structures into one cited document for your review.

What drives the cost of holding a cleanliness code?

The cost of holding a cleanliness code is driven mainly by how tight the code is, because each one-step lower ISO 4406 scale number halves the allowed particle count (Pall, 2011).

Five cost factors apply whichever code the specification uses. Filtration comes first: finer elements and offline filter loops cost more to run, so the code should not be tighter than the components need. New oil comes second: one example of new oil from a barrel measured ISO 21/19/16 (Pall, 2011), so new oil often needs filtering before it enters a clean system. Monitoring comes third: cleanliness levels typical of modern hydraulic systems, below about ISO 15/13/10, require on-line monitoring (Pall, 2011). Sealing and breathers come fourth, since ingress at reservoirs and seals replaces dirt the filters remove. Flushing after maintenance comes fifth.

The code format adds a smaller cost: an SAE AS4059 class over A through F asks the lab for six sizes instead of three, which likely adds rejection risk on sizes that are not critical; this is an inference, not a measured cost.

FAQ

What does µm(c) mean in a cleanliness code?

The "(c)" after a particle size, as in 4 µm(c), designates calibration per ISO 11171, traceable to NIST (Pall, 2011). The added "(c)" lets old and new measurements be identified and told apart (PAMAS, accessed 2026), and 1 µm under the older ISO 4402 calibration equals about 4 µm(c), so the two sets of numbers are not interchangeable.

Is ISO 11218 the same as SAE AS4059?

ISO 11218 is the ISO route to the same aerospace classification: AS4059 was adopted as ISO 11218, published in 2017 (Chevron, 2020). The withdrawn 1993 edition states: "Requirements are specified in SAE AS4059A which is adopted as a de facto ISO international Standard." (ISO, 1993). ISO 11218:2017, edition 2, specifies a cleanliness classification for hydraulic fluids in aeroplanes, helicopters and space vehicles (ISO, 2017).

Does ISO 4406:2021 replace the 2017 edition?

Yes. ISO lists ISO 4406:2017 as withdrawn and ISO 4406:2021, edition 4, as the published version, last reviewed and confirmed in 2026 (ISO, 2026).

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-13.

Sources

1. ISO. ISO 4406:2021 Hydraulic fluid power, Fluids, Method for coding the level of contamination by solid particles. Edition 4, 2021. https://committee.iso.org/standard/79716.html

2. SAE International. AS4059G Contamination Classification for Hydraulic Fluids. Revision G, 2022. https://saemobilus.sae.org/content/AS4059G

3. SAE International. AS4059E Aerospace Fluid Power, Cleanliness Classification for Hydraulic Fluids (abstract). Revision E, reaffirmed 2011. https://saemobilus.sae.org/standards/as4059e-aerospace-fluid-power-cleanliness-classification-hydraulic-fluids

4. ISO. ISO 11218:2017 Aerospace, Cleanliness classification for hydraulic fluids. Edition 2, 2017. https://committee.iso.org/standard/68318.html

5. ISO. ISO 11218:1993 Aerospace, Cleanliness classification for hydraulic fluids (withdrawn). Edition 1, 1993. https://committee.iso.org/standard/19207.html

6. ISO. ISO 11171:2022 Hydraulic fluid power, Calibration of automatic particle counters for liquids. Edition 5, 2022. https://committee.iso.org/standard/82850.html

7. PAMAS. Cleanliness classes for oils & fuels. Undated, accessed 2026-10-13. https://blog.pamas.de/en/cleanliness-classes-for-oils-fuels/

8. Pall Corporation. Industrial Pocket Book. 2011. https://www.pall.com/pdfs/Industrial-Manufacturing/IMPOCKETEN.pdf

9. Chevron. Technical Bulletin: Lubricant Cleanliness Classification Systems Explained. 2020. https://latinamerica.chevronlubricants.com/content/dam/external/isoclean/en_us/sales-material/all-other/Tech_Bulletin_Lube_Cleanliness_Alice_Hale.pdf

10. SAE International. AS4059F Aerospace Fluid Power, Contamination Classification for Hydraulic Fluids (abstract). Revision F, reaffirmed 2020. https://saemobilus.sae.org/standards/as4059f-aerospace-fluid-power-contamination-classification-hydraulic-fluids

Written by the Lubinpla team from the sources listed and checked against them before publication.