In a given period (Yearly/Half- yearly/ Quarterly/Monthly), the journal impact factor of a journal is the average number of citations received per paper published in that journal during the one or two (one or more) preceding periods.
For example (Calculation of Journal Impact Factor(JIF) Yearly) , if a journal has an impact factor of 5 in 2009, then its papers published in 2007 and 2008 received 5 citations each on average. The 2009 impact factor of a journal would be calculated as follows:
A = the number of times articles published in 2007 and 2008 were cited by journals, books, patent document, thesis, project reports, news papers, conference/ seminar proceedings, documents published in internet, notes and any other approved documents during 2009
B = the total number of “citable items” published by that journal in 2007 and 2008. (“Citable items” are usually articles, reviews, proceedings, notes or any other documents pre-reviewed before publishing it)
2009 impact factor = A/B.
New journals, which are indexed from their first published issue, will receive an impact factor after indexing it immediately.
For Example a journal published fist issue in June 2011, can get Journal Impact Factor for July 2011 onwards.
The journal impact factor relates to a specific time period; it is possible to calculate it for any desired period. The Journal Reference Reports (JRR) shows rankings of journals by journal impact factor, if desired by discipline, such as mechanical engineering or human resource management.