ASTM grain size calculation method
The American Society for Testing and Materials (ASTM) grain size calculation method is a standard approach for determining the grain sizes for the metallic materials. This method is one of the mostly used metals in the fields of materials science and metallurgy, to quantify the sizes of the individual grains in a polycrystalline material. ASTM has its own guidelines to be followed for the calculation of the grain sizes with accurate results and the same is shown below:

Sample preparation
Start using the sample preparation route and create a polished cross section of the sample as well.

Micrograph acquisition
Capturing a good quality microstructure of the polished cross section using the microscope.

Selection of Fields
Now choose regions within the micrographs on a random basis and carefully try to choose the areas representative of the entire microstructure.

Identification of the grain boundaries
In each of the selected field of view, identify and mark the individual grain boundaries and this step requires distinction from one grain to another. Based on the differences in the crystallographic orientations.

Grain counting
Count the number of grains within each field of view. These counts should be made without grain overlap and each grain needs to be counted only once.

Determining the ASTM grain size numbers
Use the grain counts to determine the ASTM grain size number using a Table provided by ASTM. The ASTM E112 standard provides a chart that correlates the number of grains per square inch (or per square millimetre) to a specific grain size number. Use the standard formula from ASTM given below for calculating the average grain size:
n=2^(N-1)

Where, n = total number of grains visible in the microstructure per inch square. It is measured at 100X magnification.

N = ASTM grain size parameter
It is also important to note that the Grain Size ~ (1/ N)
Calculating the Average grain size: Repeat the above steps several times to ensure the statistical significances. Calculate the average grain size by taking the mean of the grain size numbers obtained from different fields.