The impact factor is an indicator of journal quality. A measure of the frequency with which the "average article" in a journal has been cited in a particular year, the impactor factor is calculated by dividing the number of current citation to articles published in the two previous years by the total number of articles published in the two previous years. Ultimately, the impact factor helps researchers evaluate a journal's relative importance, especially when compared to others in the same field.
Keep in mind that not all journals are ranked. For example, in literary studies, few impact factors exist for specific journals, largely because literary scholars tend to cite primary texts more than secondary ones. This makes it crucial to determine a journal's overall impact by its availability and presence. Try using the following sources.
View this tutorial on determining impact factors of journals in Journal Citation Reports for additional information.
Some content courtesy of Susan Ariew, Academic Services Librarian for Education at University of South Florida Library, and her Library and Information Resources Related to Promotion and Tenure guide.