How is the impact factor of a scientific journal calculated?
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The impact factor is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. The annual JCR impact factor is a ratio between citations and recent citable items published. Thus, the impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years.
The 2018 impact factor of a journal would be calculated as follows:
2018 impact factor = A/B.
where:
A = the number of times that all items published in that journal in 2016 and 2017 were cited by indexed publications during 2018.
B = the total number of "citable items" published by that journal in 2016 and 2017. ("Citable items" for this calculation are usually articles, reviews, proceedings, or notes; not editorials or letters to the editor).
http://thomsonreuters.com/products_services/science/free/essays/impact_factor/
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