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 some Humanities disciplines, few impact factors exist for specific journals, often because Humanities 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.
Another indicator of journal impact is the journal's acceptance/rejection rate. Many journals provide their acceptance/rejection rate on their website as do some databases. If acceptance/rejection rate is unavailable, try contacting the journal's editor for this information.
As an alternative to using impact factors from the Journal Citation Reports, a lot of new sites have cropped up on the Internet using other metrics.
Some content courtesy of University of South Florida Library's guide, Impact: Library Tools for Promotion and Tenure.