ISO 4406 standard: what it requires for oil cleanliness codes
요약
ISO 4406:2021 specifies a code of scale numbers for particles per mL at 4, 6 and 14 µm(c) (as of 2026-10-14). Microscope counts use two numbers, at 5 and 15 µm. It is voluntary, not a regulation. Automatic counts need counters calibrated to ISO 11171; the 2021 edition also admits particle contamination monitors.
ISO 4406:2021 is edition 4, a 6-page standard published on 2021-01-07; ISO confirmed it on 2026-06-29 after a systematic review, so it remains current (ISO, 2026).
Each step in the ISO 4406 scale is a step ratio of generally two, so a code one number higher allows up to about twice as many particles per mL (Ayalytical, accessed 2026).
A size range in which fewer than 20 particles were actually counted carries the symbol ≥, as in 14/12/≥7 (Petro-Canada Lubricants, accessed 2026).
ISO 4407:2025, edition 3 with 25 pages, published in December 2025, replaced the 2002 microscope counting method that ISO 4406 points to (ISO, 2025).
This answer covers the ISO 4406 code for solid particles in hydraulic and lubricating fluids. ISO 4406 does not set a target code for any machine; component makers and buyers set those. It says nothing about what the particles are made of, and it does not cover water, varnish or other cleanliness codes.
Whom and what does ISO 4406 cover?
ISO 4406:2021 covers the code used to state how many solid particles are in the fluid of a hydraulic fluid power system, and it serves component makers, fluid suppliers, oil analysis labs and equipment users who need one shared way to report cleanliness. The ISO abstract reads: "This document specifies the code to be used in defining the quantity of solid particles in the fluid used in a given hydraulic fluid power system." (ISO, 2026). The standard is 6 pages long and belongs to technical committee ISO/TC 131/SC 6 (ISO, 2026).
The ISO 4406 code is a reporting language, not a limit. The standard does not tell you which code a pump, valve or bearing needs. Component makers set that, and BSI notes that most hydraulic component manufacturers require a certain ISO cleanliness level for warranty purposes (BSI, 2021). Donaldson, a filter maker, describes the same codes as quantifying particle contamination in oils and fuels (Donaldson, accessed 2026).
ISO 4406 is a voluntary standard, not a regulation; ISO standards are not legally mandated (SafetyCulture, 2026). It carries weight when a warranty, contract or purchase specification names a target code. In that case the lab report, and the method behind it, decides whether the target was met.
What does ISO 4406 require?
ISO 4406:2021 requires a fixed way of turning particle counts into a code: counts per mL at set sizes, a counter calibrated to ISO 11171, a scale number from the standard's table for each size, and set symbols for low or missing counts. The full text is a paid document, so the wording below comes from public summaries of the standard's clauses.
The table below decides whether a reported ISO 4406 code is valid, row by row, and what to look for on the lab report.
Requirement | What ISO 4406 says | How to check it on a lab report |
|---|---|---|
Size channels, automatic counts | Three scale numbers for particles ≥4, ≥6 and ≥14 µm(c) per mL (Ayalytical, accessed 2026) | Three numbers separated by slashes, sizes marked µm(c) |
Calibration | µm(c) means an automatic particle counter calibrated to ISO 11171 (Ayalytical, accessed 2026) | The report or lab accreditation names ISO 11171 |
Counting method | ISO 11500 or another recognized method (Ayalytical, accessed 2026); the 2021 edition adds particle contamination monitors (BSI, 2021) | The report says bottle sample, online monitor or microscope |
Microscope counts | Two scale numbers at ≥5 and ≥15 µm, written with "-" as the first part, as in -/18/13 (Ayalytical, accessed 2026) | A dash in first place means a microscope count or no requirement to count ≥4 µm(c); the method line tells which |
Low raw counts | Fewer than 20 particles counted in a range: label that scale number with ≥ (Ayalytical, accessed 2026) | A ≥ flag means the true code can be higher |
Notation | "*" means too numerous to count; "-" means no requirement to count (Ayalytical, accessed 2026) | */19/14 means too many ≥4 µm(c) particles to count |
The row that most often invalidates a report is calibration: a code at 4, 6 and 14 µm(c) only means something if the counter was calibrated to ISO 11171 (Ayalytical, accessed 2026). The microscope row matters when you compare results: a code such as -/18/13 is complete, and its sizes are measured differently from an automatic counter code, so the two should not be compared number for number (Ayalytical, accessed 2026).
What is an ISO 4406 cleanliness level?
An ISO 4406 cleanliness level is a code such as 18/16/13 in which each number is a scale number standing for a range of particle counts per mL of fluid at one particle size. With an automatic particle counter, the first number covers particles equal to or larger than 4 µm(c), the second 6 µm(c) and the third 14 µm(c) (Ayalytical, accessed 2026). A lower number means fewer particles and a cleaner fluid.
The scale numbers come from a table in which the upper limit of each range is about twice the lower limit, a step ratio of generally two chosen to keep the number of scale numbers manageable (Ayalytical, accessed 2026). The table below shows the per-mL range behind four scale numbers used in the standard's examples. The ranges come from the ISO 4406:1999 scale table (Petro-Canada Lubricants, accessed 2026) and match the coding examples in the current text (Ayalytical, accessed 2026).
Scale number | More than (particles per mL) | Up to and including (particles per mL) |
|---|---|---|
22 | 20 000 | 40 000 (Petro-Canada Lubricants, accessed 2026) |
18 | 1 300 | 2 500 (Petro-Canada Lubricants, accessed 2026) |
13 | 40 | 80 (Petro-Canada Lubricants, accessed 2026) |
7 | 0.64 | 1.3 (Petro-Canada Lubricants, accessed 2026) |
Read together, the code 22/18/13 means more than 20 000 and up to 40 000 particles ≥4 µm(c), more than 1 300 and up to 2 500 particles ≥6 µm(c), and more than 40 and up to 80 particles ≥14 µm(c) in 1 mL of sample (Ayalytical, accessed 2026). Because the scale is logarithmic, a drop from 18 to 15 in the second number means roughly 8 times fewer particles of that size (illustrative calculation from the step ratio of two).
How do you check a lab report against ISO 4406?
You check a lab report against ISO 4406:2021 by confirming the method and calibration, then recomputing each scale number from the raw counts per mL and reading every flag. A code printed without counts, sizes or method cannot be audited.
Find the method. The report should name ISO 4406 and the counting method: an automatic particle counter on a bottle sample to ISO 11500, an online particle contamination monitor, or a microscope count to ISO 4407. ISO 11500:2022 applies to bottle samples of "clear, homogeneous, single-phase liquids" (ISO, 2022), so a hazy or wet oil needs a note on how it was handled.
Check sizes and units. Automatic counts should read µm(c). Counts should be per mL; some filter makers chart counts per 100 mL, which makes their ranges 100 times larger (Donaldson, accessed 2026).
Recompute the code. Take each cumulative count per mL and look it up in the scale table. For example, 3 000 particles ≥4 µm(c) per mL falls in scale number 19, more than 2 500 and up to 5 000 (illustrative count; range from Petro-Canada Lubricants, accessed 2026).
Check the order. Counts are cumulative, so the ≥4 µm(c) count includes the larger particles and should be the largest of the three (Mobius Institute, 2018).
Read the flags. A ≥, "*" or "-" changes what the code proves (Ayalytical, accessed 2026).
How does ISO 4406 relate to ISO 11171 and ISO 4407?
ISO 4406 defines the code, ISO 11171 defines what a particle size means on an automatic counter, and ISO 4407 defines how to count particles under a microscope; a valid code needs ISO 4406 plus one of the other two. ISO 11171:2022, edition 5, published in March 2022, specifies primary particle-sizing calibration for particle sizes 1 µm(c) and larger, and it applies to hydraulic fluids, aviation and diesel fuels, engine oil and other petroleum-based fluids (ISO, 2022).
The (c) after µm exists because calibration changed. The first edition of ISO 4406 reported at 5 and 15 µm; the current sizes of 6 and 14 µm(c) are equivalent to the 5 and 15 µm sizes obtained with the older ISO 4402:1991 calibration method (Ayalytical, accessed 2026).
ISO 4407:2025, edition 3, published in December 2025, counts particles captured on a filter membrane with an optical microscope, either by image analysis software (Annex A) or by manual counting (Annex B) (ISO, 2025). A microscope sizes a particle by its longest dimension, while an automatic counter derives an equivalent size from its cross-sectional area, a value different in most cases (Ayalytical, accessed 2026). That is why microscope codes keep the 5 and 15 µm sizes and leave the first part as a dash.
What changed in ISO 4406:2021, and what is coming?
ISO 4406:2021, the fourth edition, cancelled and replaced ISO 4406:2017 and was technically revised; the main change was to admit particle contamination monitors (PCMs) for determining the code. BSI describes the change in these words: "The use of Particle Contamination Monitors (PCMs) can also be utilized for the determination of the ISO 4406 contamination code and PCM is referenced throughout BS ISO 4406:2021." (BSI, 2021). The coding rules for automatic counters and microscopes are not listed among the main changes.
The status of ISO 4406:2021 is settled as of 2026-10-14. ISO opened a systematic review on 2026-01-15, closed it on 2026-06-05 and confirmed the standard on 2026-06-29, and the ISO page states that this version remains current (ISO, 2026). ISO reviews a standard every 5 years (ISO, 2026), so the next scheduled review falls around 2031. As of 2026-10-14, the ISO page lists no revision project for ISO 4406.
The methods around the code have moved more recently than the code itself. ISO 11171 was revised in 2022 (ISO, 2022). ISO 11500:2022, the bottle-sample counting method, gained Amendment 1 in 2026 (ISO, 2026). ISO 4407:2025 replaced the 2002 microscope method (ISO, 2025). A lab report written to the 2017 edition of ISO 4406 uses the same size channels, but check which editions of the counting and calibration methods the lab follows.
When should you ask a lab or specialist about an ISO 4406 code?
Ask the oil analysis lab, and if needed a contamination control specialist, about an ISO 4406 code when the report cannot be audited, when the method may not suit the fluid, or when the code decides a warranty or acceptance question. The ISO 4406 standard tells you how to write the code, not whether the sample or count was sound.
Raise it with the lab in these cases:
The report gives a code without counts per mL, without µm(c), or without naming ISO 11171 calibration.
A scale number carries ≥ or "*", and that size range is the one in your acceptance limit (Ayalytical, accessed 2026).
The fluid is hazy, wet or not single-phase, which is outside the scope of ISO 11500:2022 bottle counting (ISO, 2022).
You need to compare a microscope code with an automatic counter code, because the two size definitions differ (Ayalytical, accessed 2026).
A component maker's warranty names a target code and your result is near it (BSI, 2021).
Lubinpla, an applied AI company that structures technical reasoning for specialty chemicals, works such a case from your own lab reports, TDS and site history and returns one cited document for review.
자주 묻는 질문
Is ISO 4406:2017 still valid?
No. ISO lists ISO 4406:2017 as withdrawn and ISO 4406:2021 as the current edition (ISO, 2026). Reports that still cite 2017 use the same 4, 6 and 14 µm(c) channels, as Donaldson's 2017-based chart shows (Donaldson, accessed 2026), and BSI names monitor counting as the main 2021 change (BSI, 2021).
Does a lower ISO 4406 code always mean cleaner oil?
For the same size range, yes. Each code one higher is about double the particle quantity and each code lower is about half (Donaldson, accessed 2026). A lower code at 4 µm(c) says nothing about the 14 µm(c) range, so compare each number.
Does ISO 4406 tell you what the particles are?
No. ISO 4406 gives the quantity of particles, not their type (Donaldson, accessed 2026). To find out whether particles are wear metal, dirt or something else, labs use other tests such as elemental analysis or ferrography.