This document (EN ISO 643:2012) has been prepared by Technical Committee ISO/TC 17 "Steel" in collaboration with Technical Committee ECISS/TC 101 "Test methods for steel (other than chemical analysis)", the secretariat of which is held by AFNOR. The responsible German committee is Joint Working Committee NA 062-01-31 GA "Gemeinschaftsarbeitsausschuss NMP/FES: Metallographische Prüfverfahren" ("Joint working committee NMP/FES: Metallographical test methods") of the Materials Testing Standards Committee (NMP) at DIN. This International Standard specifies a micrographic method of determining apparent ferritic or austenitic grain size in steels. It describes the methods of revealing grain boundaries and of estimating the mean grain size of specimens with unimodal size distribution. Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional plane, even in a sample with a perfectly consistent grain size.