ISO 4406 Cleanliness Codes are used to express the cleanliness level of hydraulic oil. The particulate contamination level per milliliter of fluid is determined for 3 particle sizes: 4 micron, 6 micron and 14 micron, and the cleanliness level is defined by three numbers divided by slashes (/). To determine the cleanliness code, an oil sample is taken. The size and number of particles per milliliter are then determined by a particle counter. An ISO code number is then assigned for the number of particles of a given size. For example, if the sample contains 50 particles that are between 4 and 6 micron then the ISO code would be 13.