A number of organizations create lists of journal rankings, and each uses its own formula to do so. One most well-known is the Impact Factor from InCites Journal Citation Reports. After a comprehensive review, the Van Pelt and Opie Library has discontinued access to InCites Journal Citation Reports (JCR), effective April 1, 2018.
Authors in search of a journal's Impact Factor should consult the publisher's journal site as the Impact Factor score is frequently featured on the journal site. A Google search of the journal's name will lead you to the journal's homepage (usually on the publisher's site), and the Impact Factor may be listed either on the front page or in the "About" section for the journal.
Note that the publisher should cite JCR, as there are cases of imitation "impact factors."
Other sources for journal rankings are listed below.