Use the Journal Citation Reports (JCR) to locate impact factors. The impact factor is a measure of the frequency with which the average article in a journal has been cited in a particular year. The JCR also lists journals and their impact factors and ranking in the context of their specific field(s).
Working on promotion and tenure documents? Need Times Cited information as well? You can also first search for an article in Web of Science using the citation's bibliographic information, then link to the impact factor and rank for the article's journal. Start with the steps under Alternative Method below.