scholarly-metadata

OpenCitations

An open index of scholarly citation links, with more than two billion DOI-to-DOI connections as of its February 2026 refresh. Citation counts, incoming citations, and reference lists for any DOI are available through a free API; the website's search page sits behind a CAPTCHA, but the API answers cleanly. Use it to see who cites a paper or to build citation trails from fully open data.

API

Search this archive

Why it’s useful & how it works

OpenCitations is the primary open, freely reusable source for citation link data. Use it to retrieve citation counts, the full list of papers that cite a given work, or the references a paper makes, all identified by DOI. Because the data is fully open and the API requires no key, it is reliable for systematic citation-trail work at scale. The website's search page requires completing a CAPTCHA, but the API itself answers all queries cleanly. Citation counts and incoming-citation lists are the most common use cases.

What’s inside

More than 2.2 billion DOI-to-DOI citation links as of the July 2025 data dump, drawn from open references deposited with Crossref and DataCite and refreshed several times per year.

API access

https://api.opencitations.net/index/v2/citation-count/doi: <doi> ; citations/references endpoints

What we measured

Our own probes, not the archive’s own claims. Re-run periodically; every reading below is dated.

Reachability

Direct request: CAPTCHA HTTP 200 636 ms (hcaptcha.com)
Through a datacenter proxy: CAPTCHA HTTP 200 854 ms (hcaptcha.com)
API, direct: Responded HTTP 200 758 ms
API, through a proxy: Responded HTTP 200 3.3 s

Did a real search return anything?

Returned results · 217 results for the sample query · median 1.6 s · stable across runs

Sample query: omid:br/0632074494 doi:10.3389/fphar.2022.1071114 openalex:W4312223440

Benchmark coverage

Found 0 of 1 test items (0%) in the Academic papers cluster. Median response 4.1 s.

By how well known the item is: 0/1 well-known .

Reachability measured 2026-08-22. Search checked 2026-08-22. Benchmark run 2026-08-22.