ISO 4406 expresses solid-particle contamination in hydraulic fluid as three range numbers. The numbers correspond to particle counts at ≥4 µm(c), ≥6 µm(c) and ≥14 µm(c), in that order. This selected chart shows five range-code bands per millilitre; it helps decode the scale but does not recommend a target cleanliness level for a particular machine.

ISO 4406 hydraulic oil cleanliness codes

How the three numbers work

Each position in a code represents a particle-size channel. A result such as 19/17/14 therefore has three separate bands: code 19 for ≥4 µm(c), code 17 for ≥6 µm(c) and code 14 for ≥14 µm(c). The “(c)” suffix identifies the calibrated particle-size convention used for automatic particle counting. Keep that notation with the value; it is not interchangeable with an unqualified microscope size.

The range number is logarithmic. Moving one code higher represents approximately twice as many particles in that channel. The chart gives selected lower and upper boundaries, with the lower boundary exclusive and the upper boundary inclusive, as stated by the source table. It omits the codes between the displayed rows, so do not interpolate missing code numbers from the visible examples.

Worked decoding example

For an ISO 4406 result of 19/17/14, look up each number independently. Code 19 corresponds to more than 2,500 and up to 5,000 particles per millilitre. Code 17 corresponds to more than 640 and up to 1,300. Code 14 corresponds to more than 80 and up to 160. These are count ranges for the three size channels in order, not three exact particle counts.

To calculate a code from a laboratory report, use the counted particles per millilitre at each size threshold and find the applicable range for each channel. A count on a boundary must follow the source’s inclusive/exclusive rule. If one channel is unavailable, preserve the missing-channel notation and report the measurement method rather than inventing a number.

Do not confuse code with target

ISO 4406 describes how to code a measured contamination level. It does not state one cleanliness target that applies to every system. A suitable target depends on component clearances and sensitivity, pressure, duty, reliability requirements, sampling and filtration strategy. Use the equipment maker’s limits or the responsible hydraulic engineer’s specification to set a target.

Particle counts can be distorted by poor sampling, dirty bottles, unflushed sampling ports, air bubbles, water or an unsuitable test method. Confirm the laboratory’s calibration basis, sample handling and fluid condition. Use ISO 11171 context where particle-counter calibration is relevant, and compare samples collected using consistent procedures.

Current standard and limits

ISO lists ISO 4406:2021, Edition 4, as the current published edition and shows it was confirmed in 2026. The selected thresholds are reproduced from the range table in a HYDAC technical guide that describes the established 4/6/14 µm(c) system. This is a partial lookup for interpretation, not a reproduction of every range or a replacement for the standard.

A code does not identify particle composition, wear cause, water content, varnish, dissolved contaminants or component damage. It is one measurement used in condition monitoring. Investigate trends and operating context before deciding on corrective filtration or maintenance.

When comparing results from two laboratories or counters, check the reported size thresholds and calibration. A legacy table using uncalibrated 5 µm and 15 µm channels should not be mixed with a current 4/6/14 µm(c) report. Keep the measurement basis with the code so trend comparisons remain meaningful.

Sources

Edition status and scope were checked on the ISO 4406:2021 catalogue page. Selected count bands come from HYDAC’s Filters Overview; the three-channel interpretation is cross-checked against Pall’s ISO 4406 explanation. Consult the current standard and equipment limits when setting a cleanliness requirement.