It is now common to express grain sizes in terms of a simple exponential equation: (Equation 1)

n = 2 G - 1

where:

n = the number of grains per square inch at 100X magnification, and

G = the ASTM grain size number.

This approach was developed and introduced in 1951 with the premiere of ASTM standard E 91, Methods for Estimating the Average Grain Size of Non-Ferrous Metals, Other Than Copper and Their Alloys. Although the NA, d, or L3, values had been used for many years as measures of grain size, the G values were adopted readily due to their simplicity. As shown in Eq. 1, we can directly relate the number of grains per unit area to G, but the relationship between L3, and G, or NV and G are not as clearly defined. This problem is one of many being addressed by ASTM Committee E4 on Metallography.

Dear Oksana,

as Kadhim pointed out the grain size number G is related to the number n of grains per area of 254µm*254µm via n = 2(G-1)    (please pay attention: (G-1) is in the exponent of 2; please see the links).

So if all grains of total number n will fill the complete area of 254µm*254µm we will have an average area per grain of 254µm*254µm/n. For the case that these grains do not fill the specified area the average grain area will be smaller than 254µm*254µm/n. 

So the average grain width d will be roughly  equal or smaller than

  d

Dear Gerhard ,

 Thank you very march for your  details answer 

 Gerhard Martens, thank you. That's exactly what I needed.