ISO 4406 Cleanliness Codes Explained: A Practical Tool in Oil Maintenance
Steffen D. Nyman
7 min
4 March 2026
ISO 4406 is the universal language of oil cleanliness - but many maintenance engineers find the three-number code system confusing. This guide demystifies the standard and shows you how to set cleanliness targets for your specific equipment.
What the Three Numbers Mean
An ISO 4406 cleanliness code is using a table to convert a number of particles into a specific code, consisting of three numbers separated by slashes, for example 17/15/12. Each number is a range code that represents a particle count per 100 mL of oil (or 1 mL) at a specific particle size threshold:
- The first number reverts to particles ≥ 4 micrometres (µm)
- The second number are particles ≥ 6 µm
- The third number are particles ≥ 14 µm
Each ISO range code represents a band of particle counts that doubles with each step - the scale is logarithmic. This means that a single-digit improvement in ISO code will likely represent a halving of the particle count - a significant change in system cleanliness. New, unused oil is typically contaminated to around ISO 19/17/14 straight from the drum, which is why pre-filtration of a lubricant is so important.
Setting Target Cleanliness Levels
The appropriate target cleanliness level depends on the most critical component in the system.
As a general rule, the target should be set based on the component with the tightest clearances and highest sensitivity to contamination.
Component Type | Recommended ISO Target |
|---|---|
High-pressure hydraulic incl. servo valves and piston pumps | 16/14/11 |
Standard hydraulic | 17/15/12 |
Gears , non-critical lubrication systems | 18/16/13 |
Engine lube oil (4-stroke trunk engines) | 20/18/15 |
Turbine lubrication incl. hydraulic control systems | 16/14/11 |
Rolling element bearings | 16/14/11 |
High pressure common rail diesel systems (HPCR) | 16/14/11 |
How to Use ISO Codes in Practice
The most effective way to use ISO 4406 codes is as part of a condition monitoring programme. Establish a baseline cleanliness level for each system depending on criticality and sensitive components. Then take regular oil samples - at least quarterly for critical systems, monthly for high-value or high-risk equipment - and track the trend over time.
A single high reading outlier may be an anomaly or a sampling error. A consistent trend of rising ISO codes is a reliable indicator of a developing contamination problem that requires investigation. Common causes include a failing breather, worn seals, a deteriorating filter element, change in operating conditions (more load) or worse, increasing machine wear.
The Business Case for Cleanliness Targets
Setting and achieving cleanliness targets has a direct and measurable impact on maintenance costs and equipment availability. Documented case studies from large component manufacturers show that improving oil cleanliness from ISO 22/20/17 to 16/14/11 will extend hydraulic component life approximately 5 times and gearbox life by 2.5 times.
For a large industrial facility with multiple hydraulic and lubrication systems, this translates to significant savings in avoided component replacement costs and unplanned downtime.
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