Similar to h-index, researchers do not need to calculate journal impact factor (JIF) manually. Researchers can find JIF via Journal Citation Reports (JCR). However, if you'd like to see how IF is calculated for a given journal in 2018, see the following steps (source):
A: 2018 cites to articles published in 2016 & 2017 in the journal.
B: Number of articles published in 2016 & 2017.
C: A/B= 2018 Impact Factor
For instance, the current impact factor of Journal of the Academy of Marketing Science is 9.36. This means, on average, the papers published in the journal in 2016 and 2017 received roughly 9 citations each in 2018.
The Journal Citation Reports (JCR) “allows you to evaluate and compare journals using citation data drawn from approximately 12,000 scholarly and technical journals and conference proceedings from more than 3,300 publishers in over 60 countries/territories. Journal Citation Reports is the only source of citation data on journals, and includes virtually all specialties in the areas of science, technology, and social sciences.
Key features in Journal Citation Reports allow you to:
By typing and searching a journal name (e.g. International Journal of Production Economics) in the search box, you'll see the following information:
If you'd like to browse journals by category (e.g. BUSINESS), you can click "Browse by Category " in JCR, and from the left sidebar click "Select Categories" to choose the desired subject category. You'll see an page like the following one. Then, click the number below #Journals, which is 147. It shows that 147 journals exist in this category.
After clicking the number, 147, you'll see a list of journals in the category sorted by their impact factors. The highest impact factor in this category belongs to Academy of Management Annals (IF= 12.289).