Google Scholar is the only openly available database suitable for journal metric calculation. It has a wide coverage and is a meaningful source. For this reason, BluePen Journals is calculating its own Impact Factor based on Google Scholar's citation counts. Scientists are used to Thomson Reuters' way of calculating an impact factor. For this reason, BluePen Journals applies Thomson Reuters'(TR) algorithm as published on http://wokinfo.com/essays/impact-factor in Figure 1. This algorithm is not protected and can be used by anyone. In short: BluePen Journals calculates a 2-year Google-based Journal Impact Factor (2-GJIF).

Thomson Reuters Impact Factor
The impact factor was devised by ‘Eugene Garfield’, the founder of the ‘Institute for Scientific Information’ (ISI), now part of ‘Thomson Reuters’. Impact factors are calculated yearly for those journals that are indexed in ‘Thomson Reuters’ ‘Journal Citation Reports’ (JCR). JCR provides quantitative tools for evaluating, categorizing and comparing journals. The impact factor (IF) measures the average numbers of citations of articles published in scientific and social science journals. 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 annual JCR impact factor is a ratio between citations and recent citable items published. 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. In general high impact factor journals are recognized as the most influential as compared to the other journals published in the same field. The IF is used to compare different journals within a certain field. The ‘Institute for Scientific Information’ indexes more than 11,000 science and social science journals.

A. Calculation of yearly impact factor based on three year period.
X=Total cites in 2013
Y= 2013 cites to articles published in 2011-12 (This is subset of X)
Z= Number of articles published in 2011-12
IF=Y/Z=2013 impact factor

Actual Calculation of impact factor in 2013.
Cites in 2009 to articles published in:
2012 = 456
2011 = 468
Total = 924

Number of articles published in:
2012 = 80
2011 = 74
Total = 154
Impact factor = cites to recent articles/number of recent articles = 924/154=6.0
B. Calculation of five year impact factor.
X= Citations in 2011 over the 2006-2010
Y= Articles published over the years 2006-2010
Z= X/Y = Five year impact factor

(Source: ISI Web of Knowledge)