The ASTM standard E112 was adopted in 1961 for rating grain size. The value of the grain size number, G, is dependent upon the number of grains per square inch, observed on a polished and etched metallographic specimen at a magnification of 100X. As the value of the number of grains per square inch increases, so does the grain size number.

ASTM grain size The equivalent, G, ASTM grain size number can be calculated from, L, the mean intercept length as; G = - 3.2877 - 6.6439 log_10 {L}, where L is given in mm.

The ASTM grain size number, N, is related to the number of grains per square inch at 100 X magnification, n, by the relationship, n = 2 N-1 … (2.44) Where, ADVERTISEMENTS: N is the ASTM grain size number, and n is the number of grains per square inch at 100 X (ASTM E 112 – 63 or IS 4748 – 1968).