What the impact factor measures
Clarivate’s Journal Impact Factor is an average. For a given year it counts the citations recorded that year to the items a journal published in the two preceding years, then divides by the number of citable items published in those two years. Published annually in the Journal Citation Reports, it describes a journal — never an individual paper, and never an author.
Because it is a two-year average, it moves. A single heavily cited review can lift a small journal’s figure noticeably, and a year with fewer publications changes the denominator. Year-on-year movement of a few tenths generally means nothing.
It is also field-bound. Molecular biology cites faster and more densely than mathematics or history, so impact factors across those fields are not comparable in any meaningful sense — which is precisely the problem the quartile was designed to solve.
The impact factor is a two-year citation average for a journal. It says nothing about any individual paper in it.
How a quartile is produced from it
Within each Web of Science subject category, journals are ordered by impact factor and the list is cut into four. The top quarter is Q1, the next Q2, and so on. Clarivate publishes the rule: with rank X out of Y journals in the category, Z = X/Y, and Q1 is 0 < Z ≤ 0.25.
So the quartile is a relative position and the impact factor is an absolute value, which produces the result that surprises people most: a journal with a higher impact factor can legitimately sit in a lower quartile than one with a lower figure, if it happens to compete in a more heavily-citing category.
This is why there is no impact-factor range that defines Q1. In one category the Q1 boundary may fall at a value that would place a journal mid-table in another. Asking "what impact factor do I need for Q1" has no answer until the category is named.
Which number to use, and when
Use the quartile when you are targeting a journal or reporting to an assessment framework that asks for one, since it accounts for the field. Name the category and the year alongside it — a journal in several categories holds a separate quartile in each, and "Q1" without a category is an incomplete claim.
Use the impact factor when you are comparing journals inside one category and want a sense of scale, or when a funder or institution asks for it specifically. Do not use it to compare across fields, and do not use it as a proxy for the quality of a particular paper.
Know which system is being asked for. Clarivate produces the impact factor and JCR quartiles from Web of Science; SCImago produces SJR quartiles and Scopus produces CiteScore quartiles from an entirely different journal population and category scheme. They disagree routinely and none of them is wrong.
Where both numbers stop being useful
Neither figure evaluates your paper. A paper in a Q1 journal can go uncited for a decade, and the most-cited paper in a field can sit in a modest title. Research-assessment reform — the San Francisco Declaration on Research Assessment and similar commitments adopted by many funders and universities — pushes explicitly against using journal metrics to judge individual work.
Both can also be gamed, which is why Clarivate suppresses journals from the JCR for abnormal citation behaviour each year, and why invented metrics with names resembling the Journal Impact Factor circulate on journal homepages. Verify any figure at the source that produces it.
The practical use of these numbers is narrow and real: they help you pick a realistic target in your own field. Beyond that, what matters to a reader, a reviewer or a hiring panel is the paper.