The Journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR).

The Journal Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. A Journal Impact Factor of 1.0 means that, on average, the articles published one or two years ago have been cited one time. A Journal Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. Citing articles may be from the same journal and most citing articles are from different journals.

The Journal Impact Factor was developed by Eugene Garfield at the Institute for Scientific Information (ISI), now owned by Thomson Reuters. Eugene Garfield wrote an article outlining its history in 2006 (Garfield E. The History and Meaning of the Journal Impact Factor. JAMA. 2006;295(1):90–93. doi:10.1001/jama.295.1.90).

Journal Impact Factor uses Thomson Reuters (ISI Web of Knowledge) citation data.

The only way to access Journal Impact Factor is by using Journal Citation Reports (ANU has subscription access).

An example from JCR of the journal with the highest Journal Impact Factor in the category of History is below: