An impact factor is a measure of the frequency that the "average article" published in a scholarly journal has been cited in a two or five year period. It is often used to measure or describe the importance of a particular journal in an academic discipline. Authors can use impact factors to evaluate journals to decide which to publish in. Impact factors were initially developed by the Institute for Scientific Information (ISI).
The impact factor of a journal in a particular year is the number of citations received in the current year to articles published in the two preceding years divided by the number of articles published in the same two years. For example, Journal of Petrology has a 2012 impact factor of 4.714, which means that on average each of its 2010 and 2011 articles was cited 5.012 times in 2011.
Here's how that was calculated:
|Cites in 2012 to items published in:||2011||294||Number of items published in:||2011||84|
|Calculation:||Cites to recent items||825||=||4.714|
|Number of recent items||175|
The primary tool for locating impact factors is the Web of Science database Journal Citation Reports.
You can find impact factors for specific journals or journals within a field. New journals are tracked every year, but JCR by no means calculates impact factors for every journal published. Some journals not tracked by JCR calculate their own impact factors and post them on the journal's home page.