SJR is a prestige metric based on the methodology inspired by the Google PageRank algorithm and the idea that not all citations are the same. With SJR, the subject field, quality and reputation of the journal have a direct effect on the value of a citation and the impact that journal makes.
Detailed information about the calculation of this metric is available in an overview page and in a paper presenting the detailed formula.
This metric is based on citation data from the Scopus database. It is freely obtainable on the Web from the Scopus Journal Metrics website and Journal Rankings by Subject categories are also available.