How can i calculate wood capacity?

Not sure I understand the question. It seems you are asking what is the total weight of the lumber. You don't state that you are using pine, but I have to assume so. In the last 20 years or so, pine dimensional lumber is quite "wet", so the density is probably around 55 to 60 lb/cu ft. When you figure the total volume of lumber, remember a 2 x 12 is actually closer to 1 1/2 x 11 1/2. Your spacing of 16 inch is standard. There are many ways this can be framed out, but it has a minimum of 430 linear feet. You have at least 51.6 cu ft of lumber.