ISO 4406

Understanding the ISO 4406:1999 Cleanliness Guide

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

A Simple Explanation

Essentially ISO 4406:1999 represents a standardised 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:


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 mLTypical Applications
4µm6µm14µm
14129High precision & laboratory servos
171511Robotic & servo systems
181613Very sensitive & high reliability systems
201814Sensitive & reliable systems
211916General equipment of limited reliability
232118Low pressure equipment – not much use

Number of Particles per mL 
More thanUp to and IncludingScale No.
2.5M> 28
1.3M2.5M28
640k 1.3M27
 320k640k26
 160k320k25
80k160k24
40k80k23
20k40k22
10k20k21
500010k20
2500500019
1300250018
640130017
32064016
16032015
8016014
408013
204012
102011
510 10
2.559
1.32.58
0.641.37
0.320.646
0.160.325
0.080.164
0.040.083
0.020.042
0.010.021
0.00.010

This code has been superseded by ISO 4406:2017