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 2015 and 48 articles in 2014.

In 2016, there are 85 citations to this journal's 2015 articles and 59 citations to its 2014 articles.

The 2016 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 Knowledge citation data and are published in Journal Citation Reports. Instructions for finding impact factors are on this page.

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 2016 five-year impact factor uses the citations received in 2016 by articles published in 2015, 2014, 2013, 2012, and 2011.

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.

Impact factors and five-year impact factors are available in Journal Citation Reports for approximately 12,000 journals.

To find a journal’s impact factor:

• Go to Journal Citation Reports through the library website.
• Search for the journal title or ISSN in the search box.

To view the impact factors of multiple journals in the same discipline:

• Go to Journal Citation Reports through the library website.
• Click Select Category and choose your category or categories.

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• Scroll to the bottom of the menu and click the Submit button.
• Journals for the selected categories will be displayed on the page. The list can be sorted by impact factor, total number of cites, or eigenfactor.
• If you wish to compare specific journals, or make comparisons based on other metrics, use the Select Journals search tool.

The impact factor is calculated using only citations that appear in journals that are indexed in Web of Knowledge. (In 2016, Web of Knowledge included approximately 12,000 journals.)

Only "citable items" are included in calculating the impact factor's denominator. Citable items include research articles and reviews. Other items published in journals (such as editorials, letters, news items, and meeting abstracts) are not included among the total of citable items, although any citations to them are included in the numerator. (The justification for this is that non-article items receive few citations, and ISI attempts to avoid depressing the impact factors of titles that include content like book reviews.)

As with any citation metric, the impact factor is not capable of accounting for the reasons for or significance of the counted citations—that is, it cannot distinguish between vital citations and incidental or critical ones. The impact factor of a publication outlets does not communicate any information about the quality or impact of any individual article.

There is some evidence that international journals are underrepresented, especially those that are written in languages besides English.

Because citation patterns differ dramatically among different disciplines, impact factors of journals in different subject areas should not be compared.