Grain Size and Formula Clarification

[yxj]

1) Hi just checking if the definition of grain size is the average size of grains in a metal expressed as average diameter. if it is not or if there is a more accurate definition could u define it for me.
2) and if the formula N=2n-1 or is it N=2^(n-1)
n being the number of grains per square inch at magnification of 100 diameters.
or if i am completely wrong please tell me.

thank you

 

[Astronuc]

Simple forumula is n = 2^(G - 1), or n = 2^(N - 1), where:
n = the number of grains per square inch at 100X magnification, and
G, N = the ASTM grain size number.

See Complications---Different Measures of Size and
Evolution of Test Methods E 112

in http://www.metallography.com/grain.htm

http://mse.iastate.edu/solidification/Courses/MatE443-Website/instructions/ASTM_grainsize.pdf