It is a measure of the inclusion content of the steel, and usually focuses on those inclusion types deleterious to processing performance and final product properties. (It should be noted that cleanness measures can also include non-harmful inclusions).

Steel cleanness is determined by measurement of the number, size and morphology of inclusions in a steel sample. These measurements can be performed in a number of different ways, but two common approaches are optical microscopy and ultrasonic testing. These results are then rated against standard tables / charts to determine the steel cleanness.