Impact Factor (IF) (or Journal Impact Factor = JIF):

IF reflects the mean citation number in the last two years.

The calculation of Impact Factor (IF) is little bit tricky. And It is published every year, usually in the following year. (2021-IFs will be published in June-2022)

Here is the formula:

\[ \begin{aligned} \textbf{IF 2021} = \frac { \text{Citations in }\textbf{2021} \text{ to items published in }{\textbf{2019 & 2020}} } { \text{Citable items} \text{ published in } {\textbf{2019 & 2020}}} \end{aligned} \]

Journal Citation Indicator (JCI):

JCI is the average category-normalized impact of citable items published over a recent three year period.

Some important notes for Academics:

Impact Factor is a metric specific to Web of Science. (Scopus use “Cite score”, SCImago uses SJR as performance metrics.)

Citable items (denominator of the formula) include only scholarly literature (articles, reviews, and case reports).
NOT non-scholarly literature (editorials, comments, letter, etc.)

Citations (numerator of the formula) include all citations.

Onle SCIE-indexed and SSCI-indexed journals have Impact Factor. AHCI- and ESCI-indexed journals do not have Impact Factor.

ESCI-indexed journals have only JCI, but not Impact Factor.

When the journals in each category are sorted from highest-to-lowest according to their performance metrics (JIF or JCI), and the list is divided into four groups, the top journal group is defined as first-quartile (Q1 journals in the category), the next is Q2, and the next ones Q3, Q4.

Journal IF and Journal CI are different metrics. Therefore, there are 2 definition for quartiles. One is for IF, one for JCI.

Because all categories have various number of journals, and some journals belong to more than one category, a journal can be classified in different quartiles (depends on the category).