Question 1

What is the ISO 4406 cleanliness code?

Accepted Answer

ISO 4406:2021 communicates the solid-particle contamination level of a hydraulic or lubricating fluid as three scale numbers separated by oblique strokes (e.g. 22/18/13). The numbers map cumulative particle counts ≥ 4 µm(c), ≥ 6 µm(c), and ≥ 14 µm(c) per millilitre to a scale defined in Table 1 of the standard.

Question 2

What does "µm(c)" mean?

Accepted Answer

The "(c)" suffix denotes calibration per ISO 11171 (or PCM calibration per ISO 21018-3 / 21018-4). It distinguishes automatic-particle-counter (APC) sizes from optical-microscope sizes per ISO 4407, which are reported as 5 µm and 15 µm without the suffix.

Question 3

How do I read 22/18/13?

Accepted Answer

Per Clause 4.4.5: more than 20 000 and up to 40 000 particles ≥ 4 µm(c) per mL, more than 1 300 and up to 2 500 ≥ 6 µm(c) per mL, and more than 40 and up to 80 ≥ 14 µm(c) per mL. Each "step" in the scale roughly doubles the population.

Question 4

What if my particle count is too high to count or too low to be statistically valid?

Accepted Answer

Per Clauses 4.4.6–4.4.7: "*" replaces the scale number when too many particles are present to count; "—" indicates no requirement to count that channel; and "≥N" prefixes a class when fewer than 20 particles were counted in that size range, indicating reduced statistical confidence.

Question 5

Where does the data for the calculator come from?

Accepted Answer

The implementation reproduces Table 1 of ISO 4406:2021 verbatim. Class boundaries use "up to and including" semantics — a count exactly at the upper bound maps to that scale number, not to the next one up. Counts above 2 500 000 / mL are reported as ">28" per the first row of Table 1.

ISO 4406 Cleanliness Code Classifier

Inputs

Result

Worked examples

Standard's reference example (22/18/13)

Typical in-service hydraulic oil (18/16/13)

Contaminated fluid (23/21/17)

Methodology