The Impact Factor of a journal is calculated once a year. For this purpose the publications from the previous two years and the number of citations from the current year are taken into account in order to calculate the IF for the current year.

For example: a journal has published 50 articles in the years 2012 and 2013 and these articles were cited in the year 2014 exactly 100 times. To calculate the IF: 100 / 50 = 2

There is also the alternative five-year impact factor which includes articles from the previous four instead of two years.

Dear Andreas Meier,

Thank you so much, Sir! I got the idea of how the IF is derived. Still one question.......

If my paper is published in the year 2012, then can I quote the IF written on the same issue or I have to wait for two / four years?

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Yojana

In my opinion you can quote the IF as I have seen some researchers do so. Like you said, you would have to wait at least two or four years otherwise.

This way you simply show which IF the journal currently had when you were published in it.

Thanks so much!!! 

Sorry I saw the answer today.

Thanks indeed, I am satisfied with your answer.