What Is a DOI and How to Use It in Citations
Understand what a DOI is, how to find one, and how to include it correctly in APA, MLA, and IEEE citations.
A DOI (Digital Object Identifier) is a permanent link to a digital document. Think of it as an ISBN for online content – it never changes, even if the URL does. Most citation styles now require a DOI when one’s available.
What Does a DOI Look Like?
A DOI is a string that starts with 10. followed by a prefix (identifying the publisher) and a suffix (identifying the document):
10.1038/nature12373
When formatted as a URL (which most styles now require):
Every DOI resolves to the current location of the document. Even if a journal changes its website, the DOI link still works.
DOI vs URL vs Permalink
Students often treat “DOI,” “URL,” and “permalink” as interchangeable, but only one of the three is actually guaranteed to keep working. A regular URL describes only where a page happens to live right now – it breaks the moment a site is redesigned, a journal switches publishing platforms, or a domain gets retired. A “permalink” is a website’s own promise that one specific URL won’t change, but that promise is enforced by the website itself, not by any outside standard, so it fails the same way a URL does the moment the site migrates or shuts down. A DOI is different: it’s registered with a nonprofit agency such as Crossref or DataCite, and that agency – not the publisher – guarantees the link keeps resolving to wherever the content currently lives, even if the publisher’s own website disappears entirely.
| DOI | Permalink | Regular URL | |
|---|---|---|---|
| Format | 10.xxxx/xxxxx or https://doi.org/10.xxxx/xxxxx | Any URL a site labels “permanent” | Whatever’s in the address bar |
| Who guarantees it works | A registration agency (Crossref, DataCite) | The website itself | Nobody |
| Survives a site redesign | Yes | Usually | No |
| Survives the publisher changing platforms | Yes | No | No |
| Preferred in citations | Yes, when available | Only if no DOI exists | Last resort |
If a source has both a DOI and a “permalink,” cite the DOI. If it only has a permalink, treat it the same way you’d treat any other URL – there’s no special citation format tied to the word “permalink” itself.
How to Find a DOI
On the Article Page
Most journal articles display the DOI on the first page, usually near the title, abstract, or footer. Look for “DOI:” or “https://doi.org/” followed by a string.
In the Database
If you found the article through Google Scholar, PubMed, or a library database, the DOI is usually listed in the article metadata. If you have a PubMed ID instead, you can generate a citation directly from the PMID.
Using CrossRef
If you can’t find a DOI, search for the article on CrossRef. Enter the title, and CrossRef will return the DOI if one exists. Crossref is the largest DOI registration agency for scholarly content, and it’s the database that doi.org itself queries during content negotiation – when you paste a DOI into a browser, it’s Crossref’s (or another registration agency’s) records that resolve it to the publisher’s current URL.
Not Everything Has a DOI
DOIs are standard for journal articles, conference papers, and some books. They’re less common for:
- Websites and blog posts
- Government reports
- Older publications (pre-2000)
- Theses and dissertations (some have DOIs, many don’t)
If no DOI exists, use the URL instead. Don’t make one up or leave the field blank.
DOI Format by Citation Style
APA 7th Edition
Format the DOI as a full URL. No “Retrieved from” prefix. No period at the end.
Smith, J. D. (2025). Article title. Journal Name, 15(3), 45–62. https://doi.org/10.1234/example
MLA 9th Edition
MLA includes the DOI as a URL, prefixed with doi.org/:
Smith, John D. “Article Title.” Journal Name, vol. 15, no. 3, 2025, pp. 45–62, doi.org/10.1234/example.
Chicago 18th Edition
Chicago uses the full DOI URL:
Smith, John D. “Article Title.” Journal Name 15, no. 3 (2025): 45–62. https://doi.org/10.1234/example.
Harvard
Harvard (Cite Them Right) places the DOI at the end of the reference, either as a full URL or prefixed with “doi:” depending on your institution’s preference:
Smith, J.D. (2025) ‘Article title’, Journal Name, 15(3), pp. 45–62. doi:10.1234/example.
Because the DOI itself is a stable identifier, Cite Them Right doesn’t require an access date alongside it – that requirement only applies to sources you can locate solely through an ordinary URL.
IEEE
IEEE places the DOI at the end after “doi:”:
J. D. Smith, “Article title,” Journal Name, vol. 15, no. 3, pp. 45–62, 2025, doi: 10.1234/example.
Vancouver
Vancouver places the DOI at the end of the reference after the page range, using the same lowercase “doi:” prefix PubMed uses in its own records:
- Smith JD, Johnson MK. Article title. J Name. 2025;15(3):45-62. doi:10.1234/example.
For a full comparison of how these styles differ, see our citation style comparison.
Troubleshooting: When a DOI Doesn’t Resolve
Occasionally a DOI you’ve copied correctly still won’t resolve. Before assuming the citation itself is wrong, check these common causes:
- The DOI was copied with extra characters. A trailing period, a stray space, or a pasted line break at the end of the string is the most common cause – DOI resolution is case-insensitive but whitespace-sensitive. Retype the identifier by hand if pasting keeps failing.
- The article was only recently published. Publishers sometimes register a DOI days before the article’s metadata is fully indexed. Wait a day or two and try again, or search the title directly on CrossRef.
- The DOI belongs to a different version of the source. Preprints, published versions, and datasets tied to the same paper are often assigned separate DOIs. If one link is dead, check whether the record lists a related DOI for the specific version you actually used.
- You’re using the outdated
dx.doi.orgdomain. Older citations sometimes listdx.doi.orginstead of the currentdoi.orgdomain. Both should redirect correctly, but if a link is broken, try swapping the domain manually before giving up. - The publisher deprecated the DOI without reassigning it. This is rare, but it happens with retracted articles and discontinued journals. If a DOI genuinely dead-ends, cite the source using its last known stable URL and add an access date, the same way you would for a source with no DOI at all.
If none of these explain the failure, the identifier may be mistyped somewhere upstream – verify it against the original PDF or the journal’s own reference list rather than trusting a secondhand citation. Before you submit, run the rest of your reference list through our thesis bibliography checklist to catch this and similar issues.
Common Mistakes
- Using the old
dx.doi.orgformat – The current standard isdoi.org(withoutdx.) - Adding “Retrieved from” before the DOI – Not needed in APA 7th edition
- Putting a period after the DOI URL – This can break the link
- Using the DOI number without the URL prefix – Most styles now want the full
https://doi.org/URL - Omitting the DOI when it exists – Always include it if available
- Treating a permalink as if it were a DOI – A “permanent link” a website assigns itself isn’t registered with any outside agency; use the actual DOI when one exists, even if the permalink looks stable
Try It with CiteTools
The fastest way to get a perfect citation from a DOI: paste it directly into CiteTools.io. We’ll look up all the metadata automatically and format the citation in APA, MLA, Chicago, Harvard, IEEE, or Vancouver. Paste 10.1038/nature12373 or https://doi.org/10.1038/nature12373 – both work. If you already know your target style, open the dedicated DOI to APA converter – it loads with APA pre-selected, and its sibling pages cover MLA, Chicago, Harvard, IEEE, and Vancouver.