Understanding the ISO 4406:1999 Cleanliness Guide

The current standard for measuring particulate in oils is ISO 4406:1999.

What is ISO 4406?

Essentially ISO 4406:1999 represents a standardized method of counting and reporting the number of particles of contamination found in an oil.

Method used

Oil is pumped past a fine laser beam. The laser looks for particles that are 4µm, 6µm & 14µm (4,6 & 14 microns). The particles are added up over a set time for each size. These totals can be referenced in the table below to produce a Cleanliness Code. Lower numbers in the table represent fewer particles in the oil. Examples of codes and their corresponding applications are:

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For example: The industry standard code for diesel used in engines is 18/16/13.

• 18 = More than 1300 and up to 2500 particles larger than 4µm exist in the sample.
• 16 = More than 320 and up to 640 particles larger than 6µm exist in the sample.
• 13 = More than 40 and up to 80 particles larger than 14µm exist in the sample.

	Number of Particles per mL		Typical Applications
4µm	6µm	14µm
14	12	9	High precision & laboratory servos
17	15	11	Robotic & servo systems
18	16	13	Very sensitive & high reliability systems
20	18	14	Sensitive & reliable systems
21	19	16	General equipment of limited reliability
23	21	18	Low pressure equipment – not much use

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Number of Particles per mL
More than	Up to and Including	Scale No.
2.5M	–	> 28
1.3M	2.5M	28
640k	1.3M	27
320k	640k	26
160k	320k	25
80k	160k	24
40k	80k	23
20k	40k	22
10k	20k	21
5000	10k	20
2500	5000	19
1300	2500	18
640	1300	17
320	640	16
160	320	15
80	160	14
40	80	13
20	40	12
10	20	11
5	10	10
2.5	5	9
1.3	2.5	8
0.64	1.3	7
0.32	0.64	6
0.16	0.32	5
0.08	0.16	4
0.04	0.08	3
0.02	0.04	2
0.01	0.02	1
0.0	0.01	0

This code has been superseded by ISO 4406:2017