Open access is the part of the research literature anyone can read without a subscription. Piwowar et al. (2018) estimated that "at least 28% of the scholarly literature is OA (19M in total)", and found the share highest in the most recent year they studied, 2015, at 45%. For the rest, a paywall is rarely the end of the road: there is often a legal free copy, and a legal way to ask for one.
This guide explains the five kinds of open access (gold, green, hybrid, diamond and bronze), what the licences allow, the order in which to look for a free copy, and how to cite the version you read.
Two of the steps are ones EdCitation does free, with no account. Find sources covers about 300 million published works, with a filter that keeps only papers marked free to read. Once you have the paper, Cite a source builds its reference from the DOI.
What is open access?
Open access means a research work is free to read online, with no subscription, payment or login. The founding definition is the Budapest Open Access Initiative's, published on 14 February 2002. It asks for "free availability on the public internet", together with the right to read, download, copy, share, print, search and link to the full text, and to use it for any lawful purpose, with no financial, legal or technical barrier beyond getting online (Budapest Open Access Initiative, 2002).
That definition covers reuse as well as reading. Piwowar et al. (2018) draw the line with two older terms: libre open access, which grants reuse rights and is close to the Budapest definition, and gratis open access, which grants only the right to read. For reading and citing, gratis is enough; the difference matters when you reuse part of a paper, such as a figure in a thesis that will be published.
The Budapest statement proposed two routes: authors depositing articles in open archives, and journals that charge readers nothing. They became green and gold open access.
Gold vs green open access: what is the difference?
Gold open access means the publisher makes the article free on its own site, in a journal where every article is open. Green open access means the journal keeps the article behind a paywall, but a free copy sits in a repository. Piwowar et al. (2018) sort open papers into four exclusive kinds, gold, green, hybrid and bronze, and when a paper is both in an open journal and in a repository, it counts as gold. Diamond describes a journal by who pays, and the Open Access Diamond Journals Study uses it for journals free to readers and authors alike (Bosman et al., 2021).
| Kind | Where the free copy sits | How to find it | Source |
|---|---|---|---|
| Gold | The publisher's site, in a journal where all articles are open (the study counts journals listed in the Directory of Open Access Journals) | Follow the DOI; tick Free to read (open access) in Find sources | Piwowar et al. (2018) |
| Green | A repository, subject or university; the journal's own copy stays paywalled | Search the repository; a PDF link beside a Google Scholar result | Piwowar et al. (2018); University of Delaware Library (n.d.) |
| Hybrid | The publisher's site, one article opened under an open licence inside a subscription journal, usually for a charge paid by the authors | Follow the DOI; look for a Creative Commons licence on the article page | Piwowar et al. (2018) |
| Bronze | The publisher's site, free to read but with no clear licence | Follow the DOI; save a copy, as free access may not last | Piwowar et al. (2018) |
| Diamond | The publisher's site, in a journal that charges neither readers nor authors | Follow the DOI; many are small journals missing from the large indexes | Bosman et al. (2021) |
| Preprint (a version, not a kind) | A preprint server, before or during peer review | Search the server; the Preprint kind in Find sources | arXiv (n.d.) |
How much of the literature is open access?
In a sample of 100,000 articles with Crossref DOIs, from all years, Piwowar et al. (2018) found 27.9% open: 16.2% bronze, 4.8% green, 3.6% hybrid and 3.2% gold, with 72.0% closed. Among the articles that users of the Unpaywall browser extension looked at in one week of 2017, 47% were open, because readers favour recent papers and recent papers are more often open. Bronze, the kind with no licence, was the most common route of all.
Those figures describe the literature in 2017, and we did not find a newer study of the same scale to read, so treat them as a floor: the paper reports the share was growing.
Diamond journals: free to publish and free to read
Bosman et al. (2021) estimate that between about 17,000 and 29,000 diamond journals exist, with only a third registered in the Directory of Open Access Journals. Most publish fewer than 25 articles a year, so a diamond journal in your field may be missing from the database you usually search. For who pays for gold and hybrid, and how to tell a sound open access journal from a predatory one, see how to choose a journal and avoid predatory ones.
Where do free copies of papers live?
Besides the publisher's site, free copies live on preprint servers, in subject archives and in university repositories, and each describes itself differently.
arXiv
arXiv calls itself a curated research-sharing platform open to anyone, holding more than three million articles in eight subject areas: physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics. Volunteer moderators screen submissions, but arXiv states plainly that it does not peer review what it hosts (arXiv, n.d.).
PubMed Central
PMC describes itself as a free full-text archive of biomedical and life sciences journal literature at the US National Library of Medicine, and reports 11,327,616 articles for fiscal year 2025 (National Center for Biotechnology Information, n.d.). It holds three kinds of text: articles as journals published them, authors' manuscripts accepted after peer review, and preprints posted before review. The article page says which one you have.
University repositories
An institutional repository is a university's own archive of its researchers' work. Cambridge's, Apollo, describes itself as the University's institutional repository, run from Cambridge University Library to store and preserve research outputs from articles and chapters to theses and data (University of Cambridge, n.d.). The University of Delaware Library (n.d.) notes that most repositories are indexed by Google Scholar, but searching one directly can turn up recent deposits that have not reached it yet.
Which version have you found?
Three versions of a paper circulate. University of Waterloo Libraries (n.d.), following Sherpa Romeo, define the preprint as the paper before peer review, the accepted manuscript as the final manuscript after peer review with the revisions made, and the publisher's version as the typeset PDF. A repository copy can be any of the three. An accepted manuscript should say what the published article says, but its page numbers and layout will not match.
What do the Creative Commons licences on open access papers mean?
A Creative Commons licence tells you what you may do with a paper beyond reading it. Creative Commons (n.d.) publishes six licences, built from four elements:
| Element | What it asks of you |
|---|---|
| BY | Credit the creator. All six licences include it. |
| SA | Share any adaptation under the same terms. |
| NC | Use it only for noncommercial purposes. |
| ND | Share it unchanged only; no adaptations. |
CC BY, the broadest, lets anyone share and adapt the work, commercially too, with credit; CC BY-NC-ND, the narrowest, allows only sharing it unchanged, noncommercially, with credit. Hybrid articles carry an open licence by definition; bronze articles carry none, so they are free to read but not open for reuse (Piwowar et al., 2018).
For citing, the licence changes nothing: a CC BY paper and a paywalled one are cited in the same form. It matters when you reproduce a figure in published work, such as a thesis; see submitting a thesis to the repository.
How can I read a paywalled paper legally?
Work from the quickest route to the slowest, and stop when you have the text. These are the legal routes university libraries themselves list.
- Search where free copies are marked. In EdCitation's Find sources, open the filters and tick Free to read (open access) under Where it was published, or pick the quick set Open access only. Results that are free to read carry an Open access tag.
- Open the DOI. The publisher's page may be free after all, as bronze papers are. See what is a DOI if the link is new to you.
- Look for a green copy. Google Scholar shows a link to the right of a result when it has found a PDF (University of Delaware Library, n.d.). Browser extensions that look for legal copies from the article page are covered in our guide to finding sources for a literature review fast.
- Search the repositories directly. PMC for health and life sciences, arXiv for its eight subjects, and the author's university repository.
- Go through your library. Start from the library's website, not the journal's, so your university's subscription is recognised.
- Ask for interlibrary loan. Brown University Library (2026) says the service costs its students nothing and article requests are usually filled within a few days; the University of Delaware Library (n.d.) gives an average of one to two days, with no limit on requests.
- Email the author. The University of Delaware Library (n.d.) notes that most publishers let authors share their own publications responsibly, and suggests asking the author for a preprint. Give the full reference and say in a line why you need it.
- Cite what you read. Paste the DOI into Cite a source, and if your copy is a preprint or an accepted manuscript, cite that version, as the next section explains.
If nothing works by your deadline, choose another source: a paper you could not read is not one to cite.
How do I cite a preprint or an accepted manuscript in APA?
APA's rule is to cite the final published version where you can, and the version you actually used when it was the preprint (American Psychological Association, 2020). APA adds that a paper in a preprint archive may or may not have been peer reviewed, and may even be the author's accepted manuscript. APA's own example, for a preprint on PsyArXiv, reads:
Hampton, S., Rabagliati, H., Sorace, A., & Fletcher-Watson, S. (2017). Autism and bilingualism: A qualitative interview study of parents' perspectives and experiences. PsyArXiv. https://doi.org/10.31234/osf.io/76xfs
Unlike a journal article, the title is in italics, there is no journal, volume or page, and the archive stands where a publisher would.
PubMed Central: when to name it
For a published article found in PMC, leave PMC out and give the DOI, as for any research database. The exception is the authors' final peer-reviewed manuscripts in PMC: APA treats those as works of limited circulation, so the reference names PubMed Central and gives the PMC address (American Psychological Association, 2021).
When a preprint you cite is published
Before you hand in, check whether each preprint you cite has since appeared in a journal; if it has, read and cite the published version, because review may have changed the findings. Find sources labels each result by kind, Preprint or Article among them, so a title search can show both.
How does the open access filter in Find sources work?
EdCitation's Find sources is the best place to start a search for free research articles, because every result is a published record looked up in an index, never a reference written by a model, and the open access filter sits beside the others you need for an assignment: kind of source, years, field. It is free, with no account. Open the filters, go to Where it was published and tick Free to read (open access), or choose the quick set Open access only.
What our test search returned
On 27 September 2026 we searched sleep deprivation academic performance, first without the filter and then with it. Without it, the heading read "3,059,312 results for sleep deprivation academic performance", and the first result was The Negative Impact of Sleep Deprivation on Students' Academic Performance (EDUCARE: Jurnal Pendidikan dan Kesehatan, 2025), with no open access tag. With Free to read ticked, the heading read "40,720 results for sleep deprivation academic performance", every one of the first 100 results carried the Open access tag, and the line below them said "Results from OpenAlex." The first was Effects of Sleep Deprivation on Performance: A Meta-Analysis, June J. Pilcher, Allen I. Huffcutt · SLEEP · 1996, cited 1,556 times.
The filter needs the full index, which records open access status. When that index is busy, search carries on from Crossref's index and names the filter it could not apply, with the notice "The full index is busy, so these filters are not applied right now: Open access.", rather than passing off an unfiltered list as filtered. If you meet the notice, run the search again later. A search reported in our journal-choice guide showed 79 of its first 100 results tagged open access.
Citing and checking what you find
Cite a source builds the reference from the DOI and screens it for retraction. For 10.7717/peerj.4375 it returned, in APA 7:
Piwowar, H., Priem, J., Larivière, V., Alperin, J. P., Matthias, L., Norlander, B., Farley, A., West, J., & Haustein, S. (2018). The state of OA: A large-scale analysis of the prevalence and impact of Open Access articles. PeerJ, 6, Article e4375. https://doi.org/10.7717/peerj.4375
The title keeps the capitals the publisher registered; APA's sentence case lowers "Open Access", so change those two words by hand. For 10.1371/journal.pone.0230416 it returned:
Colavizza, G., Hrynaszkiewicz, I., Staden, I., Whitaker, K., & McGillivray, B. (2020). The citation advantage of linking publications to research data. PLOS ONE, 15(4), Article e0230416. https://doi.org/10.1371/journal.pone.0230416
Neither carried a retraction or correction notice. We then pasted four entries into Verify references: the diamond study, the APA preprint example, the Piwowar paper, and the Colavizza paper with its year changed to 2019. Three, the preprint included, came back verified: "The DOI resolves to this record and the title matches." The fourth was marked Check this: "The work was published in 2020, not 2019; the record has no date in 2019."
Finding, citing and verifying cost nothing. For a finished paper, References from a file (Pro, $8 a month) reads the reference list out of the uploaded document, looks up each entry and ties every in-text citation to it.
Quick questions
Is open access the same as free?
Not quite. Every open access paper is free to read, but the Budapest definition also asks for the right to reuse, which bronze papers and most green copies do not grant (Piwowar et al., 2018).
Are open access journals peer reviewed?
Open access says who can read a paper, not how it was checked. The Directory of Open Access Journals states that its mission concerns quality, peer-reviewed open access journals (Directory of Open Access Journals, n.d.), while arXiv states that it does not peer review what it hosts, so check the journal or server itself.
Can I cite a preprint in my assignment?
APA allows it, provided you cite the version you read and name the archive. Check that your brief does not ask for peer-reviewed sources only.
How do I find only free articles in EdCitation?
In Find sources, tick Free to read (open access) under Where it was published, or choose the quick set Open access only. If a notice says the filter is not applied right now, run the search again later.
References
- American Psychological Association. (2020). Preprint article references. APA Style. https://apastyle.apa.org/style-grammar-guidelines/references/examples/preprint-article-references
- American Psychological Association. (2021). Database information in references. APA Style. https://apastyle.apa.org/style-grammar-guidelines/references/database-information
- arXiv. (n.d.). About arXiv. https://info.arxiv.org/about/index.html
- Bosman, J., Frantsvåg, J. E., Kramer, B., Langlais, P.-C., & Proudman, V. (2021). OA diamond journals study. Part 1: Findings. Zenodo. https://doi.org/10.5281/zenodo.4558704
- Brown University Library. (2026, April 30). Getting copies of articles and books you can't find. https://libguides.brown.edu/findinginformation/ill
- Budapest Open Access Initiative. (2002, February 14). Read the declaration. https://www.budapestopenaccessinitiative.org/read/
- Creative Commons. (n.d.). Sharing openly, sharing globally: The CC licenses. https://creativecommons.org/share-your-work/cclicenses/
- Directory of Open Access Journals. (n.d.). About DOAJ. https://doaj.org/about/
- National Center for Biotechnology Information. (n.d.). About PMC. https://pmc.ncbi.nlm.nih.gov/about/intro/
- Piwowar, H., Priem, J., Larivière, V., Alperin, J. P., Matthias, L., Norlander, B., Farley, A., West, J., & Haustein, S. (2018). The state of OA: A large-scale analysis of the prevalence and impact of open access articles. PeerJ, 6, Article e4375. https://doi.org/10.7717/peerj.4375
- University of Cambridge. (n.d.). Apollo. https://www.repository.cam.ac.uk/
- University of Delaware Library. (n.d.). Ways to access journal articles. https://library.udel.edu/collections/budget-information/ways-to-access-journal-articles/
- University of Waterloo Libraries. (n.d.). Publisher requirements: article versions. https://uwaterloo.ca/lib/publisher-requirements-article-versions