There are a number of metrics that measure the impact of a journal. These metrics are generally a way to rank journals within a discipline. They can also help researchers make decisions about where to publish their research.
Journal Impact Factor, the most widespread, measures how frequently the average article in a given journal is cited over a period of time. Journal Impact Factor is widely reported and can be found in Journal Citation Reports, which is based on Web of Science data.
Eigenfactor, another metric, uses the same underlying data as Journal Impact Factor, but is calculated by weighing citations from highly ranked journals higher than those from lower ranked journals. It also calculates scores over a 5 year period and takes into account the differences in citation rates across fields. Eigenfactor is reported in Journal Citation Reports and is freely available 6 months after its initial release.
A third metric is CiteScore, which is based on data from Scopus. CiteScore's calculation differs from Journal Impact Factor by including all articles published by a journal rather than only original research in the number of total articles. CiteScore uses a 3 year window for its calculations, and it is freely available online.
Eigenfactor is updated annually and available for free six months after publication each year. Eigenfactor considers not only how often articles published in a given journal are cited, but how influential the citing journals are.
The Eigenfactor score is meant to measure a journal's total importance to the scientific community. All of the journals' scores add to 100, so a score of 2.0 has 2% of the total influence of all included journals.
Another measure is the Article Influence score - the average influence of a journal's articles over the first five years after publication. It is normalized so that the average article has an article influence of 1.00. Scores above 1.00 mean that the average article in that journal is cited that many times more than the average article in all of the included journals.
CiteScore is calculated annually each May and is freely available online.
CiteScore Tracker allows you to see calculations for journals in the current year. These numbers are updated every month, so you can track a journal's performance as time goes on.