The impact factor measures the average number of citations received by articles published in a certain journal.
The impact factor is based on citations received in the current year by citable articles published in the two previous years. Journal self-citations are included.
Example:
A hypothetical journal publishes 42 articles in 2019 and 48 articles in 2018.
In 2020, there are 85 citations to this journal's 2019 articles and 59 citations to its 2018 articles.
The 2020 impact factor is (85 + 59)/(42+48) = 144/90 = 1.60. Articles in this journal were cited, on average, 1.60 times.
Impact factors are calculated based on Web of Science citation data and are published in Journal Citation Reports.
The five-year impact factor is identical to the standard impact factor except that it uses citation data from the past five years instead of the past two.
That is, a journal’s 2020 five-year impact factor uses the citations received in 2020 by articles published in 2019, 2018, 2017, 2016, and 2015.
The five-year impact factor is considered a more accurate measure in some fields whose citation patterns tend not to concentrate as much on the most recent literature.
Web of Science does not depend on the Journal Impact Factor alone in assessing the usefulness of a journal, and neither should anyone else. The Journal Impact Factor should not be used without careful attention to the many phenomena that influence citation rates – for example the average number of references cited in the average article. The Journal Impact Factor should be used with informed peer review. In the case of academic evaluation for tenure, it is sometimes inappropriate to use the impact of the source journal to estimate the expected frequency of a recently published article. Again, the Journal Impact Factor should be used with informed peer review. Citation frequencies for individual articles are quite varied.
Journal Citation Reports now includes more article-level data to provide a clearer understanding of the reciprocal relationship between the article and the journal. This level of transparency allows you to not only see the data, but also see through the data to a more nuanced consideration of journal value.
For more, see "Using Journal Citation Reports Wisely" from Web of Science.