How is Impact Factor calculated?

In any given year, IF of a journal is the number of citations received in that year of articles published in that journal during the two preceding years, divided by the total number of articles published in that journal during the two preceding years.

Example:

Impact Factor = 1.173

Number of citations received in 2001 of articles published in:
1999 – 123
2000 – 67
Totally in 1999 and 2000 – 190

Number of articles published during the two preceding years:
1999 – 81
2000 – 81
Totally in 1999 and 2000 – 162
Impact Factor: 190:162 = 1.173

Impact Factor is used to evaluate the relative scientific importance of a journal in its field.

Each year Thomson Reuters publishes the impact factors of all journals indexed in the ISI Journal Citation Report. The IF can fluctuate considerably from year to year.

Impact Factor calculated for a given year should not be used for any other year.