INDUSTRIAL STANDARD · 2021 (4th edition; confirmed 2026)
ISO 4406
Hydraulic Fluid Power — Method for Coding the Level of Contamination by Solid Particles
Coding the level of solid-particle contamination in fluid used in hydraulic fluid-power systems.
KEY PARAMETERS
Code solid-particle contamination level in hydraulic fluid
Purpose
ISO cleanliness code / range numbers
Output
Equipment, component or application requirement
Target ownership
ISO 16889
Separate filter test
01 /
What ISO 4406 Defines
ISO 4406:2021 specifies the code used to define the quantity of solid particles in hydraulic fluid. The code provides a common reporting language for contamination level so measurements can be communicated consistently across laboratories, equipment owners and maintenance programs.
02 /
What ISO 4406 Does Not Define
ISO 4406 does not prescribe one universal cleanliness target for servo valves, pumps, cylinders, mobile equipment or industrial systems. The acceptable code for an application must come from the equipment or component requirement, validated engineering guidance or an approved contamination-control program.
03 /
Measurement and Trending
Particle-count data are converted into ISO 4406 range numbers for reporting and trending. Sampling quality, measurement method, calibration and contamination introduced during sample handling can materially affect the reported result, so the measurement procedure must be controlled.
04 /
Relationship to Filter Testing
ISO 4406 describes fluid cleanliness. It does not evaluate filter-element efficiency. ISO 16889 is the separate multi-pass method used to evaluate hydraulic filter-element filtration performance. The two standards are complementary but must not be presented as interchangeable.
COMMON ENGINEERING MISTAKES
Treating an example cleanliness code as a universal target mandated by ISO 4406.
Treating ISO 4406 as a filter-efficiency or multi-pass performance test. That role belongs to ISO 16889.
Converting historical or alternate cleanliness classifications into ISO 4406 as though the conversion were exact without application evidence.
Using a particle-count result without controlling sampling, handling and measurement quality.
ENGINEERING REFERENCES
ISO 4406:2021 — Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles (3rd edition)
Primary standard defining the Range Number cleanliness code system, particle size channels (≥4, ≥6, ≥14 µm), count unit (particles per mL), and measurement procedures for hydraulic and lubricating fluid particle contamination classification.
ISO 16889:2022 — Hydraulic fluid power — Filters — Multi-pass method for evaluating filtration performance of a filter element (4th edition)
Defines the multi-pass test methodology and ISO 11171-calibrated particle counter requirements that produce ISO 4406-compatible cleanliness codes with improved inter-laboratory reproducibility compared to earlier manual counting methods.
ISO 11171:2016 — Hydraulic fluid power — Calibration of automatic particle counters for liquids
Mandatory calibration protocol for APCs used in ISO 4406 cleanliness code measurement; the "(c)" suffix on particle size channels confirms ISO 11171-compliant NIST-traceable calibration.
ISO 11500:2008 — Hydraulic fluid power — Determination of the particulate contamination level of a liquid sample by automatic particle counting using the light-extinction principle
Specifies the light extinction particle counting method applicable to ISO 4406 measurement as an alternative for fluid samples not suitable for standard light obscuration APC measurement.
FREQUENTLY ASKED QUESTIONS — ISO 4406
ENGINEERING ARTICLES REFERENCING ISO 4406
Asset Protection Engineering
10 min read
Beta Ratio
9 min read
Contamination Control
12 min read
Contamination Ingression Rate Modelling: Cleanliness Budget Methodology for Fluid Systems
13 min read
Contamination Sensitivity of Hydraulic and Lubrication Components: Clearance Data and ISO 4406 Targets
13 min read
Failure Analysis
8 min read
Related Engineering Content
CITE THIS PAGE
ELIMFILTERS. (2021). ISO 4406: Hydraulic Fluid Power — Method for Coding the Level of Contamination by Solid Particles. ELIMFILTERS Engineering Knowledge Platform. https://elimfilters.com/knowledge-center/standards/iso-4406/