Oil & fuel analysis
ISO 4406 cleanliness codes, and what to target
An ISO 4406 cleanliness code comes back as three numbers separated by slashes, 18/16/13, say. Most reports print it without explanation, and it gets filed. It is the single most useful number on the page.
What an ISO 4406 cleanliness code counts
Under ISO 4406, a laboratory counts particles in a one-millilitre sample at three sizes and reports a range code for each:
| Position | Counts particles | Typically indicates |
|---|---|---|
| First | 4 µm(c) and larger | Total silt load, and the wear that follows |
| Second | 6 µm(c) and larger | The population most filters are rated against |
| Third | 14 µm(c) and larger | Larger debris, usually ingress or component failure |
The (c) matters. It means the counter was calibrated to ISO 11171 using certified dust, which is why counts from a modern instrument do not match numbers taken before 1999.
The scale doubles
Each code covers a range of particle counts per millilitre, and each step up roughly doubles the count. That is the part people miss.
Going from 18/16/13 to 21/19/16 is not "a bit dirtier". It is roughly eight times the particles at every size.
It also means a single code of improvement is a real, measurable win, and that a report drifting up by one code between samples is worth acting on before it drifts up by three.
Targets worth aiming at
Cleanliness targets belong to the component, not to the oil. The most sensitive item in the circuit sets the target for the whole system:
| System or component | Target code |
|---|---|
| Servo and high-response valves | 15/13/10 |
| Proportional valves | 16/14/11 |
| Turbine lubrication systems | 16/14/11 |
| Variable-displacement piston pumps | 17/15/12 |
| Fixed piston and vane pumps | 18/16/13 |
| Gear pumps and general hydraulics | 19/17/14 |
| Bearings and gearboxes, splash-fed | 19/17/14 |
Treat these as starting points. A manufacturer's own figure, where one exists, always wins.
Reading a trend rather than a number
One count tells you the state today. Three counts from the same sampling point tell you the direction, and the direction is what justifies spending money.
- Flat and on target, the filtration is matched to the ingress. Leave it alone.
- Rising steadily, ingress has increased, or filtration has stopped keeping up. Look at
breathers, seals and the last filter change before you look at the oil.
- A single spike, then a return, usually a sampling error or a disturbed deposit. Repeat
before acting.
- Rising with the 14 µm number rising fastest, something is generating debris. This one
is worth stopping for.
What it does not tell you
A particle count says nothing about water, acidity, additive depletion or varnish. A clean count on a badly oxidised oil is common, and reassuring for the wrong reasons. Pair it with a water figure and, on turbine oils, an MPC result.
Malina runs particle counts as part of oil and fuel analysis, and uses the result to size filtration under oil and lubrication. The size ranges themselves are defined in ISO 4406, with calibration under ISO 11171.
Sampling technique changes particle counts more than almost anything else. If the counts look wrong, suspect the sample before the system.
Last reviewed 6 August 2026.