The 5-year journal Impact Factor is the average number of times articles from the journal published in the past five years have been cited in the JCR edition year. It is caclulated by dividing the number of citations in the JCR edition year by the total number of articles published in the five previous years.
The five-year Impact Factor is available only in JCR 2007 and later years. Using an earlier JCR year as a starting point the five-year Impact Factor can be calculated by following the instructions (http://admin-apps.isiknowledge.com/JCR/help/h_fiveyr_if.htm) on web pages.
A diagram of five-year Impact Factor calculation.
The calculation of five-year Impact Factor for Genome Research. Image source: Journal Citation Reports (Thomson Reuters)<http://www.isiknowledge.com/> 2.4.2009