The Journal Impact Factor also known as JIF or IF, is a journal-level metric. It measures the average number of recent citations published in a specific journal. The JIF was developed by Eugene Garfield and is now a commercially owned metric connected to Web of Science.

Definition: In a given year, the JIF is equal to the number of citations received in that year (y), for anything published in the journal during the two preceding years (y - 1 and y - 2), divided by the total number of citable items (articles and reviews) published in the journal during the two preceding years (y - 1 and y - 2).

Expressed:

Example:

To calculate the JIF for 2018 of a specific journal:
1. Count the number of citations from 2018 for everything in that journal in 2017 and 2016. Add them together.

2. Count the number of citable items (articles and reviews) published in that journal in 2017 and 2016. Add them together.

3. Divide the total number of citations by the total number of citable items. The result is the JIF for the specific journal in 2018.

Limitations of JIF:

JIF is not equal to the quality of the journal, nor is it a quality index of the individual papers in it.
It only looks at a two-year period.
Sensitive to outliers. Papers with many citations or no citations will alter the score.
Easily manipulated through, for example, self-citations.

Because research subjects produce new results at different paces, a two year period might be too short of a time span. If you want to look at a wider time span, Web of Science also offers an indicator called 5-year JIF. The calculation is the same as the regular JIF except it includes five years instead of two.

Find the JIF in Web of Science