ISO 4406 is a standard for classification of particle contamination in oils and fluids. It was developed by ISO ( International Organization of Standardization ) .

ISO4406 is the most commonly used standard in Europe and also widely used in Norway, but to a lesser degree in the oil-, offshore and subsea industry, where NAS1638 and SAE AS4059 are the dominant standards.

A cleanliness code is given based on the number of particles within a particle size range pr. 1 ml in an oil sample, with code 28 being the dirtiest and code 00 being the cleanest.

Cleanliness indications according to ISO4406 are given consisting of 3 codes, which indicate the number of particles in 3 different particle size ranges  : 

• 4 um(c)
• 6 um(c)
• 14 um(c) 

The cleanliness standard ISO 4406 was originally developed in 1987.