The impact factor for a journal for a particular year is based on data from the previous two years, and is calculated in the following manner:
IF(year) = Number of citations (year - 1) + Number of citations (year - 2)/Number of Citable Items Published (year-1) + Number of Citable Items Published (year-2)
A "citable item", according to Clarivate Analytics, includes articles and reviews, but it does not include letters and editorials on the basis that those are not typically cited.
Impact Factor for the year 2017
For example the 2017 impact factor for a hypothetical journal Blue Rose would be calculated this way:
Number of citations to articles in Blue Rose in 2016 = 2000
Number of citations to articles in Blue Rose in 2015 = 1550
Total number of citations for the previous two years to articles in Blue Rose = 2000 + 1550 = 3550 (A)
Total number of citable items published in Blue Rose in 2016 = 350
Total number of citable items published in Blue Rose in 2015 =400
Total number of citable items published in Blue Rose in previous two years = 350 + 400 = 750 (B)
IF(2017) = A/B
3550/750 = 4.733
The 2017 Impact Factor for Blue Rose would be 4.733
There are many limitations to relying soley on the impact factor to determine the quality of a journal, not the least of which are:
Impact factor is a measure of the average number of times journal articles are cited by others. As such, it is used to rank journals as to their reach and importance relative to other journals in the field. It is a proprietary calculation, owned by Clarivate Analytics, and is often used in promotion and tenure considerations at universities.
For more information on impact factor and other publication metrics, see the Clarivate Analytics LibGuide, Journal Citation Reports,: Learn the Basics.
Journal Citation Reports is the most complete database the USA Libraries subscribes to for finding a journal's impact factor. Within the database, you can search for a journal by name, browse the journals by title, browse by category, or create custom reports.
Once you've found the journal you're interested in, click on the title to view information about it.
The JCR contains other information about a journal besides the impact factor. This information includes the citable items used in determining the impact factor, total cites. impact factor without self-cites, 5 year impact factor, immediacy index, the number of citable items, cited half-life, citing half-life, Eigenfactor score, article influence score, % articles in citable items, normalized Eigenfactor, and the average journal impact factor percentile. These can all be found beneath the impact factor graphs.