Public Health

There are a number different methods and metrics used to rate a journal's impact. The following information is not exhaustive, but will give you a place to start.

Eigenfactor

eigenfactor.org

Citations to articles from the most recent five full years, divided by the total number of articles from the most recent five full years, but weeds out journal self-citations.  It also, unlike the Journal Citation Reports impact factor, cuts across both the hard sciences and the social sciences.

Source Normalized Impact per Paper (SNIP)
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SNIP measures contextual citation impact by weighting citations based on the total number of citations in a subject field. The impact of a single citation is given higher value in subject areas where citations are less likely, and vice versa. It is defined as the ratio of a journal's citation count per paper and the citation potential in its subject field. It aims to allow direct comparison of sources in different subject fields.

Impact per Publication (IPP)
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The IPP measures the ratio of citations in a year (Y) to scholarly papers published in the three previous years (Y-1, Y-2, Y-3) divided by the number of scholarly papers published in those same years (Y-1, Y-2, Y-3). The IPP metric is using a citation window of three years which is considered to be the optimal time period to accurately measure citations in most subject fields. Taking into account the same peer-reviewed scholarly papers only in both the numerator and denominator of the equation provides a fair impact measurement of the journal and diminishes the chance of manipulation.

SCImago Journal Rank (SJR)
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SJR is a prestige metric based on the idea that 'all citations are not created equal'. With SJR, the subject field, quality and reputation of the journal have a direct effect on the value of a citation. SJR is a measure of scientific influence of scholarly journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from.