Explainer · Plain-language
What is an h-index?
The h-index is an author-level metric that combines productivity and citation impact in a single number. A researcher has an h-index of h if h of their publications have each been cited at least h times. It was proposed by the physicist Jorge E. Hirsch in 2005.
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How the h-index is calculated
Rank an author's papers by citation count, highest first. The h-index is the highest number h such that the top h papers each have at least h citations. If a researcher has 10 papers with at least 10 citations each, but the 11th has fewer than 11, their h-index is 10. The measure balances quantity (number of papers) against impact (citations per paper) in one figure.
Where it came from
The physicist Jorge E. Hirsch proposed the index in a 2005 paper in PNAS, as a way to characterise an individual's scientific output more robustly than raw paper or citation counts. It was quickly adopted by citation databases and has become one of the most widely cited author-level metrics.
Why it is database-dependent
The h-index is only as complete as the citations the underlying database has indexed. The same researcher typically has a higher h-index in Google Scholar — which indexes preprints, theses, and grey literature — than in the more selective Scopus or Web of Science. Always state which source an h-index came from, because values are not directly comparable across databases.
Known limitations
The h-index favours long careers (it can only ever grow), ignores author position and contribution, is insensitive to a few exceptionally highly-cited papers, and is not comparable across disciplines with different citation cultures. DORA and CoARA caution against using it as a proxy for the quality of a researcher's work, and the Leiden Manifesto urges responsible, context-aware use of such indicators.
Key facts
At a glance
- Definition: h papers each cited at least h times
- Proposed: Jorge E. Hirsch, 2005 (PNAS)
- Level: author (also applied to groups, journals)
- Reported by: Scopus, Web of Science, Google Scholar
- Behaviour: cumulative — never decreases over a career
- Caveats: field-dependent; ignores author position; DORA / CoARA urge caution
Common misconceptions
What people often get wrong
Often heard: An h-index is comparable across disciplines.
Actually: No — citation rates vary greatly by field, so a strong h-index in mathematics looks very different from one in molecular biology. Raw h-index values should not be compared across disciplines without field normalisation.
Often heard: There is one correct h-index for each researcher.
Actually: No — the value depends on the database. Google Scholar typically returns a higher h-index than Scopus or Web of Science because it indexes more sources, so the figure is only meaningful alongside its source.
Going deeper
