where n is the number of grains per in2 at 100X and G is the ASTM grain size number. and M is the magnification and a is the area (5000 mm2 is the standard size). If the test area is different than 5000 mm2 (from a circle 79.8 mm diameter), then the alternate area used is divided into the magnification squared.Nov 12, 2012read more >>