Where Ф (phi) size and d is the grain diameter in milimeters. For example, a grain 4 mm in diameter has a phi size of -2, which is the power required to raise the base (2) of the logarithm to 4 (i.e. 22) | Where Ф (phi) size and d is the grain diameter in milimeters. For example, a grain 4 mm in diameter has a phi size of -2, which is the power required to raise the base (2) of the logarithm to 4 (i.e. 22) |