Journal Impact Factors

Journal Impact Factor is from Journal Citation Report (JCR), a product of Thomson ISI (Institute for Scientific Information). JCR provides quantitative tools for evaluating journals. The impact factor is one of these; it is a measure of the frequency with which the "average article" in a journal has been cited in a given period of time.

The impact factor for a journal is calculated based on a three-year period, and can be considered to be the average number of times published papers are cited up to two years after publication. For example, the journal impact factor 2019 for a journal would be calculated as follows:

A = the number of times articles published in 2017-2018 were cited in indexed journals during 2019

B = the number of articles, reviews, proceedings or notes published in 2017-2018

impact factor 2019 = A/B

(note that the impact factor 2018 will be published in 2019, because it could not be calculated until all of the 2018 publications had been received. Journal impact factor 2019 will be published in 2020)

Impact factor of Nature, Science and Cell journals can be found on their journal websites.


Science Watch provides ranking and impact factor for selective journals. The list is located here: - Biomedical Literature Search Engine
MIT Biology Hyertextbook
Cell Biology Protocols