Process of determining the opening area and opening length of seive

Let us consider we have to determine the opening area and opening length of #100 sieve.
#100 sieve means 100*100 openings are present in 1 square inch or 25.4*25.4= 645.16 square mm.

Hence 1 opening is present in =645.16/(100*100)
                                           =0.065 square mm
sieve openings are surrounded by threads which take some areas.Hence sieves actually get areas of around 36 to 40 percent(can vary) of its opening.
Let us assume sieve opening is 36 percentage.
Then the actual are of sieve opening is =0.065*(36/100)
                                                        =0.0234 square mm
As sieve openings are square size
Then length of the each sieve opening =square root of 0.0234
                                                       =0.153 mm