The impact factor (IF) is a measure of the frequency with which the average article in a journal has been cited in a particular year. It is used to measure the importance or rank of a journal by calculating the times it's articles are cited.

The calculation of IF is based on a two-year period and involves dividing the number of times articles were cited by the number of articles that are citable.

Example: Calculation of 2010 IF of a journal:

A = the number of times articles published in 2008 and 2009 were cited by indexed journals during 2010.B = the total number of "citable items" published in 2008 and 2009. A/B = 2010 impact factor 

The impact factor (IF) or journal impact factor (JIF) of an academic journal is a measure reflecting the yearly average number of citations to recent articles published in that journal. It is frequently used as a proxy for the relative importance of a journal within its field; journals with higher impact factors are often deemed to be more important than those with lower ones. The impact factor was devised by Eugene Garfield, the founder of the Institute for Scientific Information: Impact factor - Source Wikipedia

Impact Factors are used to measure the importance of a journal by calculating the number of times selected articles are cited within the last few years. The higher the impact factor, the more highly ranked the journal. It is one tool you can use to compare journals in a subject category.

Total no of citations in a year divided by no of publications

example for Current Science (India)

IF (2017) = [Citations in 2017 for items (mainly articles) published in 2016 and 2015] / [Number of citable items in 2016 and 2015] = [331(for 2016)+ 550 (for 2015)] / [493(for 2016)+ 505(for 2015)] = 881/998 = 0.883

Please note that the impact factor of a journal for 2018 can be published only in 2019, as it is impossible to assess the journal until all its issues for 2018 come out

It depends on the number of citations of a particular journal during the past two years. The large number of the citations of a journal the is the impact factor of that journal. It also depends on the number of publications of the same journal.

The impact of a journal is determined by its citations, reads, and downloads.