02/09/2026 by Joel Thompson
The non-governmental independent global group, the International Organization for Standardization (ISO), has established a cleanliness code for use in reports regarding industrial oil analysis.
The value of the code is designed to help determine a mechanical system’s overall cleanliness. Onsite, operators will set a target cleanliness value to meet, and this can give them a level of confidence providing that used oil samples meet the desired standard.
It is worth noting that, regardless of how high quality the lubricant used is and whether it is made by a respected brand like Mobil or Shell, achieving established cleanliness targets is a process that can take time. While it does have certain limitations, it can significantly extend both machine and lubricant service life to improve the bottom line of operations.
Here, we explore the ISO cleanliness code in detail.
Defining the ISO cleanliness code
Accepted as the global standard, the ISO cleanliness code (standardised as ISO 4406) measures and reports the number of solid-state dirt particles in a full range of industrial solutions, like fuel and lubricants such as hydraulic oil, using an established system. These microscopic fragments consist of the dry, organic and mineral pieces that make up dust and soil.
To describe contamination levels effectively, the code uses three numbers that are separated by slashes.
Understanding how to read numbers in the ISO cleanliness code
Each individual number that the ISO cleanliness code is comprised of represents a particle size and is measured using microns (μm) per one millilitre of fluid.
The code’s first number references extremely tiny particles. These include particles that are either equal to or greater than four microns.
The second number denotes medium-sized particles that are equal to or greater than six microns.
Finally, the third number represents larger particles. These are fragments that are equal in size or larger than 14 microns.
What do the numbers in the ISO cleanliness code mean?
End users should be aware that the numbers in the ISO code do not indicate the precise count of dirt particles in an oil sample. Instead, they represent range codes. Every increment in the code (for instance, an increase of 16 to 17) indicates that the number of dirt particles has doubled.
As a rule, when a code shows lower numbers, it is a sign that the fluid is cleaner, and when numbers are higher, the opposite.
While solid matter in systems should always be removed, different types of machinery have specific tolerances to dirt and debris. For instance, equipment that includes sensitive components, such as servo valves, requires extremely low numbers to reduce the risk of mechanical breakdowns.
What are the advantages of the ISO cleanliness code?
Industry professionals agree that the code is a valuable tool for setting target alarms and maintaining clear cleanliness goals for machine systems.
The code also offers the perfect value to employ when tracking recording and posting key performance indicators (KPIs). However, experts advise that in the realm of lubrication and lubricant analysis, in many cases, too great a value is stressed on the cleanliness code.
Studies show that some laboratories assessing lubricants through tests have started to report ISO code values only, and there is a much heavier reliance on the value by end-user analysts.
For an enhanced oversight, operators should remember that the ISO cleanliness code plays a secondary role in terms of evaluating data from used oil samples.
While it is often simple to understand and trackable for KPI reporting, users are advised to rely on the ISO cleanliness code only for targets. In terms of providing accurate machine condition support, it has limitations. The raw data provided from particle count testing enables end-users to confirm data from additional tests, like ferrous index and elemental analysis, but the ISO cleanliness code doesn’t permit this cross-confirmation. Reviewing raw data at all levels, not just ISO 4406, is critical for more accurate analysis.
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