How to Read an Impact Factor — What the Number Means and Where It Fails
The Impact Factor (IF) is a number that tells you how many times, on average, the papers published in a given journal have been cited. It is a useful metric, but if you use it without knowing what it does and does not measure, you will choose the wrong target journal.
The formula is simple
The Journal Impact Factor (JIF) in Clarivate's annual Journal Citation Reports (JCR) is calculated as follows.
JIF for 2025 = the number of citations received during 2025 by the papers the journal published in 2023–2024 ÷ the number of "citable items" the journal published in 2023–2024
In other words, it is the average number of citations that the last two years of papers received in the following year. An IF of 5.0 can be read as: the journal's papers are cited about five times each within one to two years of publication.
The "citable items" in the denominator are mainly original articles and review articles; editorials and letters are not counted. Citations to those item types, however, do count toward the numerator, so journals that publish large numbers of editorials and news pieces (such as the Lancet or NEJM) end up with a small denominator and a large numerator — a structural effect that pushes their IF up.
Released every June
The JCR is updated in mid-June each year. What appeared in June 2026 is the JIF for 2025, and that value remains the current one until June 2027. If a publisher's website says "Impact Factor: 5.4 (2025)", that is the figure released in June 2026. The listings on this site follow the same year.
Because of this one-year lag, it is not unusual for a journal's IF to have changed between submission and publication when a paper takes a long time to get through the process.
The numbers differ by an order of magnitude between fields
You cannot compare absolute IF values across fields, because citation cultures differ.
| Field | IF of the top journals | IF of a "good specialty journal" in that field |
|---|---|---|
| Oncology | Lancet Oncology 33.7, JCO 44.7 | 5–10 |
| Cardiology | European Heart Journal 45.3 | 4–8 |
| Gastroenterology and hepatology | Journal of Hepatology 40.1, Gut 24.6 | 3–6 |
| Emergency medicine | Annals of Emergency Medicine 6.5 | 2–4 |
| Orthopedics | American Journal of Sports Medicine 5.4 | 2–3 |
An IF of 4 in emergency medicine may well stand where an IF of 15 stands in oncology. That is exactly why the JCR also reports quartiles (Q1–Q4), which show a journal's relative standing within its own field; when you are explaining your work to someone outside your field, "in the top 25% of its category (Q1)" gets the point across better. This is covered in detail in "What Q1 and Q2 and open access actually mean".
What the IF does not measure
- The quality of an individual paper: the IF is a journal average, and citation counts for individual papers vary enormously. Even in a journal with an IF of 10, plenty of papers are never cited at all.
- Clinical impact: whether a paper is cited in a guideline, or actually changes practice, is a separate question from how many citations it accumulates.
- How rigorous the peer review is: a higher IF does not necessarily mean more careful peer review. Some journals raise their IF by publishing large numbers of review articles.
- Long-term value: because the window is only two years, fields in which citations accumulate slowly (epidemiology, surgical technique and the like) are at a disadvantage. The 5-year JIF sometimes reflects reality more closely.
Why an IF can swing so widely
A journal's IF can rise by half again or fall by half from one year to the next. The main reasons are as follows.
- In journals that publish few papers, a single hugely successful paper sends the IF jumping (small-journal noise)
- The flood of citations to COVID-19 papers (the JIFs for 2021–2022) and the rebound afterwards (2023 onward)
- From 2023, Clarivate began assigning IFs to ESCI-indexed journals as well, and the number of journals listed rose sharply
- Temporary exclusion from the JCR (suppression) for citation manipulation or excessive self-citation
For that reason, we recommend looking at the trend over several years rather than a single year's value. Publishers' websites often list the IFs for the past few years, and aggregator sites let you check the trend as well.
Summary
The IF is a rough guide to how widely a journal's papers are read and cited, and it is useful as a first filter when narrowing down candidate journals. The final decision, though, should be made together with other factors: relative standing within the field (Q), the journal's scope, review time, and the article processing charge. The next article, "How to choose a target journal", walks through the specific steps.