ISO 4406 is the language the fluid power world uses to talk about how dirty a fluid is. It defines a compact code that turns a raw particle count into a small set of numbers engineers, OEMs, and laboratories all read the same way, and it has become the common currency of contamination control well beyond hydraulics. The code is deceptively simple: three numbers, separated by slashes, each standing for the cumulative count of particles at or above one of three defined sizes per millilitre of fluid. A reading such as 18/16/13 is instantly comparable against a target, last quarter's sample, or a sister machine — and that comparability is the whole point.
What it covers
ISO 4406 specifies the coding method only. It defines how to convert a measured particle count into scale numbers and how to write and read the resulting code, including the special notations used when a size cannot be counted or too few particles were seen for a confident result. It deliberately stops there: it does not prescribe a counting technique, set a target level for any application, or say anything about what the particles are made of, their shape, or their origin. Those questions belong to companion standards and OEM specifications.
A second, smaller code form exists for microscope results, reflecting that a microscope and an automatic counter size a particle differently — one from its longest dimension, the other from its projected area — so the standard keeps the two distinct and never silently mixed.
Why it matters in practice
A cleanliness code is only as good as the decisions it drives. Read on its own it is a snapshot; read in sequence it becomes a trend, where the engineering judgment lives. The spacing between the three numbers hints at the size distribution — fine-dominated contamination often points to fluid degradation, coarse-dominated to ingress or active wear. A code that climbs at one size while the others stay flat is a question worth asking before a filter, a seal, or a bearing answers it for you.
What the code cannot do matters just as much: it cannot tell wear debris from ingressed dirt or oxidation products, and it cannot see dissolved water or chemical degradation at all. Treated as a complete health check it misleads; treated as one channel among several it is a cost-effective early indicator.
How we use it
We report an ISO 4406 code on every particle-count result we handle, and read it as a trend rather than a verdict. Our particle-counting tools interpret a code in context — comparing it against the relevant target, flagging shifts between samples, and relating the three numbers to the underlying distribution. Where a code looks abnormal, we pair it with morphology and elemental analysis so the question moves from "how many particles" to "what kind, and from where". The coding belongs to ISO 4406, the calibration to ISO 11171, and the sample's representativeness to ISO 4021 — a single chain, because a perfect code on a poorly drawn sample is precisely wrong.