Understanding ISO 4406 Cleanliness Codes: Setting and Verifying Contamination Targets
An ISO 4406 cleanliness code is a three-number string — such as 22/18/13 — that reports the particle contamination level of a hydraulic or lubricating fluid sample by expressing, on a logarithmic scale, how many particles per millilitre are present at three reference sizes: ≥4µm(c), ≥6µm(c), and ≥14µm(c). Each number is a separate range code, read independently, for its own particle-size channel. The standard is a coding and reporting method — it tells a buyer or engineer how contaminated a fluid sample is, but it does not by itself define what level of cleanliness a given system needs.
Last verified: 2026-07-16. Standards are periodically revised — always confirm the current edition and exact wording with the issuing standards body (ISO, SAE) before using this content for procurement or compliance decisions.
This guide is written for procurement managers, importers/distributors, and maintenance engineers who need to specify, request, or interpret ISO 4406 cleanliness data when sourcing hydraulic and industrial fluids. It assumes familiarity with basic hydraulic fluid categories; for HM/HV fluid chemistry and classification, see the platform's hydraulic fluids category rather than this article.
What is an ISO 4406 cleanliness code?
ISO 4406:2021 (4th edition, published January 2021, replacing the 2017 edition) defines the current method for reporting fluid cleanliness as a three-part code. The three positions always report, in order, the range number for particles counted at ≥4µm(c), ≥6µm(c), and ≥14µm(c) per millilitre of fluid. A code of 22/18/13, for example, means the ≥4µm(c) count falls in one range, the ≥6µm(c) count in a lower range, and the ≥14µm(c) count in a still lower range — because larger particles are naturally less numerous than smaller ones in the same fluid sample.
The scale is logarithmic: increasing a range number by one step roughly doubles the maximum particle count in that band, and decreasing it by one step roughly halves it. This means the difference between a code of 20 and a code of 22 at the same size channel is not a small increment — it represents roughly a fourfold change in maximum particle count. Understanding this logarithmic structure is essential before comparing two cleanliness reports or setting a target.
How is the range-number table structured?
The table below illustrates how ISO 4406 range numbers correspond to particle-count bands (particles per millilitre). It is reproduced from third-party technical references and should be cross-checked against the purchased ISO 4406:2021 text before being cited as an exact or authoritative source — it is included here to illustrate the logarithmic structure of the coding system, not as a substitute for the standard itself.
| Range number | Particle count per mL |
|---|---|
| 24 | >80,000 to 160,000 |
| 23 | >40,000 to 80,000 |
| 22 | >20,000 to 40,000 |
| 21 | >10,000 to 20,000 |
| 20 | >5,000 to 10,000 |
| 19 | >2,500 to 5,000 |
| 18 | >1,300 to 2,500 |
| 17 | >640 to 1,300 |
| 16 | >320 to 640 |
| 15 | >160 to 320 |
| 14 | >80 to 160 |
| 13 | >40 to 80 |
| 12 | >20 to 40 |
| 11 | >10 to 20 |
| 10 | >5 to 10 |
| 9 | >2.5 to 5 |
| 8 | >1.3 to 2.5 |
| 7 | >0.64 to 1.3 |
| 6 | >0.32 to 0.64 |
A critical structural point: each of the three positions in a code (≥4µm(c), ≥6µm(c), ≥14µm(c)) is a separate lookup on this same scale, applied to that size channel's own particle count. A code is never a single lookup applied once — it is three independent range-number determinations reported together.
Why does the (c) notation matter?
The "(c)" appended to each particle size in an ISO 4406 code (e.g., ≥4µm(c)) indicates that the particle sizing was calibrated according to the current calibration standard for automatic particle counters. This matters because particle-counting instruments do not measure size directly — they infer it from an optical or electrical signal, and that inference must be calibrated against a traceable reference to be comparable across laboratories and over time.
The current calibration standard is ISO 11171:2022 (5th edition, published March 2022), which supersedes the 4th edition (2020), the 3rd edition (2016), the 2nd edition (2010), and the 1st edition (1999). It specifies NIST-traceable calibration of automatic particle counters using primary sizing methods down to ≥1µm(c). A cleanliness code is only meaningful, and only comparable between two reports, if both were generated using instruments calibrated to the current edition of ISO 11171.
It is important to note that ISO 4402 — an older particle-sizing calibration reference sometimes still referenced in legacy documentation — is withdrawn and should never be cited as a current standard. Any cleanliness report or RFQ specification that still references ISO 4402 should be treated as outdated and clarified with the issuing party.
How does ISO 4406 relate to older or parallel classification systems?
Buyers sourcing internationally, or reviewing older equipment documentation, may encounter two other contamination classification systems:
NAS 1638 is a legacy classification system of US aerospace origin, first issued in 1966, and is now inactive for new designs. It reports differential particle counts per 100 mL of sample, which is a fundamentally different counting method from ISO 4406's cumulative counts per mL. Because the underlying methodologies differ, any cross-reference table converting between NAS 1638 and ISO 4406 codes should be treated as an approximation only, not an exact equivalence.
SAE AS4059 (current revision AS4059G, issued 2022-11-22, titled "Contamination Classification for Hydraulic Fluids") superseded NAS 1638 for aerospace and fluid-power applications and also reports counts per 100 mL. SAE AS4059 remains relevant where an aerospace or fluid-power specification calls for it, but ISO 4406 is the dominant classification system for general industrial and mobile hydraulic applications, and is the system most cleanliness reports and RFQs in this sector will reference.
How are cleanliness targets set?
There is no single, universal ISO 4406 target code that applies across all hydraulic or lubrication systems. ISO 4406 is a reporting and coding method — it describes how clean a fluid sample is, not how clean a particular system needs its fluid to be.
Target cleanliness codes are instead determined by the sensitivity of the most contamination-sensitive component in a given circuit. Servo valves and proportional valves, for example, operate with far tighter internal clearances than gear pumps or general-purpose cylinders, and are correspondingly more sensitive to particle contamination. The applicable target code for a given system is set by the manufacturer of that most sensitive component, based on its own published tolerances — it is a component-specific or OEM-specific recommendation, not a value defined by ISO 4406 itself.
ISO 4413:2010, which covers general rules and safety requirements for hydraulic fluid power systems, addresses contamination control as part of its broader scope, but it is not a source of specific target cleanliness numbers either. Because target codes are manufacturer- and application-specific, and no specific OEM target figures are verified for inclusion here, this guide does not publish a target-code table. Any target code a buyer works to should be sourced directly from the relevant component or equipment manufacturer's own published documentation.
How is cleanliness verified?
Verifying an ISO 4406 code involves a chain of standards, each covering one stage of the process from sample extraction to reported result.
| Stage | Standard | Edition |
|---|---|---|
| Extracting the fluid sample from an operating system | ISO 4021 | 1992 |
| Calibrating the automatic particle counter (APC) | ISO 11171 | 2022, 5th edition |
| Gravimetric contamination measurement | ISO 4405 | 2022 |
| Optical-microscope counting on a membrane filter ("patch test") | ISO 4407 | 2025 |
| Reporting the resulting cleanliness code | ISO 4406 | 2021, 4th edition |
ISO 4021:1992 governs how a representative fluid sample is extracted from the lines of an operating system — the preferred point is the main flow line, with the reservoir as an alternative when a live flow line sample is not practical. Sample bottles used in this process should be pre-cleaned and certified for particle-count sampling, though this guide does not specify a particular bottle-cleanliness standard.
ISO 4405:2022 defines the gravimetric method — determining contamination by mass rather than particle count. ISO 4407:2025 defines the optical-microscope "patch test," in which a fluid sample is passed through a membrane filter and the retained particles are counted under a microscope, using a sizing threshold of ≥2µm.
In current practice, automatic particle counters using light-obscuration technology are the dominant verification method, and any APC used to generate an ISO 4406 code must be calibrated per ISO 11171:2022. ISO 4406:2021 itself also references Particle Contamination Monitors (PCMs) as an accepted measurement approach — PCMs are calibrated to a separate standard from APCs, so a report should note which instrument type and calibration route was used. Filtration effectiveness — expressed through filter micron rating and beta ratio — is the system-side mechanism used to reach and maintain a target cleanliness code, though this guide does not cite specific beta-ratio figures.
What should a buyer request in an RFQ?
When sourcing hydraulic or industrial fluids where cleanliness matters — particularly for systems containing servo valves, proportional valves, or other sensitive components — a buyer can structure a Request for Quote to address contamination control explicitly:
- State a target ISO 4406 code, referencing the relevant component manufacturer's own published target rather than a generic figure, since no universal ISO 4406 target exists.
- Request that a batch cleanliness report accompany delivery, showing the ISO 4406 code as measured by an ISO 11171-calibrated automatic particle counter or by the ISO 4407 patch-test method — functioning analogously to a certificate of analysis that reports viscosity (KV) and viscosity index (VI) alongside cleanliness data.
- Address filtration as the system-side control, noting the filter micron rating and beta ratio the supplier's fluid-handling or delivery process uses to protect against contamination in transit and storage.
This is offered as general guidance on how to frame a cleanliness specification within an RFQ; it does not represent a claim that any particular supplier on the platform currently provides ISO 4406 reports as a standard offering. Buyers should confirm cleanliness-reporting capability directly with individual suppliers as part of the RFQ exchange. Procurement teams working through the platform's RFQ Center can include these cleanliness-code and reporting requirements directly in their inquiry when contacting a supplier, and can browse the hydraulic fluids category for relevant product listings before submitting an RFQ.
Last verified: 2026-07-16. Standards content changes with each revision cycle — verify current editions, exact wording, and numeric tables directly with ISO, SAE, or the relevant issuing body before relying on this guide for a compliance or procurement decision.