The index is calculated based on the distribution of citations received by a given researcher's publications, such that given a set of articles ranked in decreasing order of the number of citations that they received, the g-index is the unique largest number such that the top g articles received together at least g2 citations.
The index is derived from the distribution of citations across a researcher’s publications. Specifically, if we list these publications in decreasing order by their citation counts, the g-index represents the highest value for which the sum of the top g publications' citations totals at least g squared.