ISO 4406 Oil Cleanliness Code Table
ISO 4406 describes how clean a fluid is using three range numbers. This table converts each range number to the particle count per millilitre it stands for, so you can interpret a lab report or set a target.
| Range number | More than (particles per mL) | Up to and including (particles per mL) | Approx. particles per 100 mL (upper limit) |
|---|---|---|---|
6 | 0.32 | 0.64 | 64 |
7 | 0.64 | 1.3 | 130 |
8 | 1.3 | 2.5 | 250 |
9 | 2.5 | 5 | 500 |
10 | 5 | 10 | 1000 |
11 | 10 | 20 | 2000 |
12 | 20 | 40 | 4000 |
13 | 40 | 80 | 8000 |
14 | 80 | 160 | 16000 |
15 | 160 | 320 | 32000 |
16 | 320 | 640 | 64000 |
17 | 640 | 1300 | 130000 |
18 | 1300 | 2500 | 250000 |
19 | 2500 | 5000 | 500000 |
20 | 5000 | 10000 | 1000000 |
21 | 10000 | 20000 | 2000000 |
22 | 20000 | 40000 | 4000000 |
23 | 40000 | 80000 | 8000000 |
24 | 80000 | 160000 | 16000000 |
Each step up in range number roughly doubles the allowed particle count. The code is written as three numbers, for particles larger than 4, 6 and 14 microns (c), for example 18/16/13. Targets depend on the component and the manufacturer, so use the OEM figure first.
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How to read an ISO 4406 code
A code such as 18/16/13 has three numbers. They refer to the count of particles per millilitre larger than 4, 6 and 14 microns measured as (c) by an automatic particle counter calibrated to ISO 11171. Find the first number in the table: range 18 means more than 1,300 and up to 2,500 particles per millilitre larger than 4 microns. Do the same for the other two numbers. Because the scale is roughly doubling, one step is a factor of two and three steps is about a factor of eight.
- First number: particles larger than 4 microns
- Second number: particles larger than 6 microns
- Third number: particles larger than 14 microns
- Each range number step is about double the count
Setting a target
Pick the target from the most sensitive component in the system, for example a servo valve or a high pressure pump, using the manufacturer figure. Then choose filtration and breather options to hold that code in service. A common practice is to set a target, an alarm at one range number above and a danger limit at two above, but your own limits should come from OEM data and the results of your own trend. Re-sample after a filter change to confirm the code improved.
Worked example
A hydraulic press reservoir reads 20/18/15 against a target of 18/16/13. Each number is two steps over target, so roughly four times more particles in every size band. Look up range 20 (5,000 to 10,000 per mL) versus range 18 (1,300 to 2,500 per mL). The fix is to find the ingress source first, check the breather and cylinder rod seals, and then run an offline kidney loop filter until the sample is back on target.
Common mistakes
Sampling from the bottom of the tank or from a dead leg gives a distorted result. Always sample from a live line using a minimess port when the system is running at temperature. Another mistake is comparing codes from older lab reports that used NAS 1638 or older ISO 4406 versions with two numbers; the scales are not interchangeable. Finally, new oil is often not clean enough straight from the drum, so check it before filling.
Frequently Asked Questions
What does 18/16/13 mean?
It means the oil has 1,300 to 2,500 particles per millilitre larger than 4 microns, 320 to 640 larger than 6 microns, and 40 to 80 larger than 14 microns. Each number maps to a range in the table above.
Is a lower ISO 4406 code always better?
Lower is cleaner, but there is a cost. Chasing a much cleaner target than the component needs wastes money on filtration. Use the OEM requirement and the machine criticality to set a sensible figure.
How often should I sample hydraulic oil?
Many sites sample critical hydraulic systems monthly to quarterly and less critical ones twice a year, and always after a major repair or filter change. Increase frequency when the trend rises or when the machine is critical to production.
How is ISO 4406 different from NAS 1638?
ISO 4406 gives a code for three size thresholds based on cumulative counts. NAS 1638 uses differential counts in size bands and a single class number. They cannot be converted exactly, so quote the standard used on the report.
If lab results live in spreadsheets, we can help bring them into your CMMS or ERP so out-of-target samples raise a work request. Ask for a short expert review.
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