What Is a DOI? How to Find and Cite It — Complete Guide

What Is a DOI?

A DOI, or Digital Object Identifier, is a permanent, unique alphanumeric string assigned to a digital object — most commonly a journal article, book chapter, conference paper, or dataset. Think of it as a permanent address for a piece of academic content on the internet. Unlike regular URLs, which can change when publishers redesign their websites, reorganise their content, or go out of business, a DOI is designed to remain stable indefinitely. The publisher or owner of the content can change the web address all they want; the DOI always points to the correct, current location.

DOIs are managed through the Crossref registry, a non-profit organisation that serves as the official registration agency for academic DOIs. When a publisher assigns a DOI to an article, Crossref records the metadata — author, title, journal, year, volume, issue, pages — and stores the URL where the content currently lives. The DOI resolver at https://doi.org/ then acts as a redirect service: enter a valid DOI into that resolver, and it takes you to the article's current landing page, even if the URL has changed a hundred times since the DOI was first assigned.

The system was developed in the early 2000s by the International DOI Foundation (IDF) and is now used by virtually every major academic publisher — Elsevier, Springer, Wiley, Taylor & Francis, the American Psychological Association, Nature Portfolio, PLOS, and thousands more. If you have been reading peer-reviewed research, you have almost certainly seen DOIs without necessarily knowing what they were. They appear at the bottom of article PDFs, on journal website landing pages, and in the metadata of databases like PubMed, Scopus, and Web of Science.

Why DOIs Matter for Academic Citations

Before DOIs became standard practice, citing a journal article meant providing a URL that might break within months or years — a phenomenon known as link rot. Studies have found that a substantial proportion of URLs cited in academic papers become inaccessible within five to ten years. For a researcher trying to verify a source, a broken URL is nearly useless. DOIs solve this problem entirely.

When you include a DOI in your citation, you are giving your reader a permanent pathway directly to the source. If the article is open access, the DOI takes them straight to the full text. If it is behind a paywall, the DOI takes them to the publisher's landing page, where they can access the article through their institutional subscription, request it via interlibrary loan, or find a legal open-access version. This is far more useful than a raw URL, which may work today but not tomorrow.

For instructors and peer reviewers, DOIs make verification effortless. Rather than searching for an article by title or navigating publisher websites, they can click a single link. This is why major citation style authorities — including the APA, MLA, and Chicago Manual of Style — have made DOIs the preferred form of electronic location information for journal articles and other content that carries one.

From a research integrity standpoint, DOIs also help with deduplication and citation tracking. Because each article has a single, stable DOI, databases can accurately count how many times a paper has been cited across the literature — a metric that matters enormously for academic careers and journal impact factors.

Anatomy of a DOI

Every DOI has the same two-part structure: a prefix and a suffix, separated by a forward slash.

10.1037/a0012345
 ^prefix^ ^suffix^

The prefix always begins with 10. followed by a four- to nine-digit registrant code. The registrant code identifies the organisation that registered the DOI — for example, 10.1037 belongs to the American Psychological Association, 10.1016 belongs to Elsevier, and 10.1136 belongs to the BMJ Publishing Group. The prefix is assigned once to a publisher or organisation and never changes.

The suffix is whatever the registrant chooses to use as a unique identifier within their namespace. It could be a journal abbreviation followed by article ID (a0012345), a year and sequence number, or any other scheme the publisher prefers. Suffixes are case-insensitive — A0012345 and a0012345 refer to the same article.

When a DOI is used as a hyperlink — which is the required format in modern citation styles — it is prefixed with the resolver URL https://doi.org/:

https://doi.org/10.1037/a0012345

This is the format you should include in your reference list. Clicking or following this link will take the reader to the article's current landing page. Note that older DOI format conventions used http://dx.doi.org/ as the resolver — this still works, but current APA 7, MLA 9, and Chicago guidelines specify https://doi.org/.

How to Find a DOI for a Source

Finding a DOI is usually straightforward, but the location varies depending on where you are accessing the article.

On the Publisher's Website

The most reliable place to find a DOI is the article's landing page on the publisher's website. Scroll to the abstract or citation information section — you will almost always see the DOI displayed, often as a hyperlink that begins with https://doi.org/. Some publishers display it prominently below the title; others tuck it into the article metadata sidebar.

On the Article PDF

Many journal PDFs include the DOI on the first page, either in the header or footer. It typically appears as "DOI: 10.xxxx/yyyy" or "https://doi.org/10.xxxx/yyyy". If the PDF is a scanned older article, it may not have a DOI at all — DOIs were not widely adopted until the early 2000s.

In PubMed

For biomedical and life science literature, PubMed (pubmed.ncbi.nlm.nih.gov) is an excellent DOI lookup tool. Search for the article title or PMID, click through to the article record, and the DOI will appear in the citation panel on the right side of the page.

Through Crossref Metadata Search

Crossref's free metadata search tool at search.crossref.org allows you to search by title, author, journal, or year and retrieve the DOI. This is particularly useful when you have an article in hand but the DOI is not printed on it. Enter a few words from the title and the author's last name, and Crossref will return matching records with their DOIs.

In Google Scholar

Google Scholar often displays DOIs in the citation information. Click the quotation mark icon below a search result to see the citation, or click through to the article page. The DOI may appear in the URL if the result links directly to the publisher.

In Database Records

Library databases — including Web of Science, Scopus, EBSCO, and ProQuest — display DOIs in their article records. When you locate an article in one of these databases, look in the "Source" or "Publication Details" section for the DOI field.

When a Source Doesn't Have a DOI

Not every source has a DOI. Older articles published before roughly 2000 may not have been assigned one. Some journals — particularly those from smaller or regional publishers — have never enrolled with Crossref. Books, book chapters, and other content types sometimes have DOIs, but not always. When a DOI is absent, citation style guides provide specific rules for what to include instead.

APA 7: No DOI Available

In APA 7, if a journal article does not have a DOI but was retrieved from a database that requires a subscription, you do not include a URL or database name — you simply omit the location element entirely. The citation ends after the page numbers. If the article was retrieved from a website accessible to all readers (not a subscription database), include the URL.

MLA 9: No DOI Available

MLA 9 treats URLs as a fallback when no DOI exists. If the source is online, provide the URL (without "https://") enclosed in angle brackets. MLA also requires an access date for online sources that may change. If the source was accessed through a library database, include the database name as the container.

Formatting DOIs in APA 7

APA 7 has clear, specific rules for DOI formatting that changed significantly from the 6th edition. Here is what you need to know.

The Hyperlink Format Is Required

APA 7 requires DOIs to be presented as hyperlinks using the https://doi.org/ format. Do not write "doi:" or "DOI:" as a prefix. The old format (doi:10.xxxx/yyyy) is no longer correct in APA 7. Always write:

https://doi.org/10.1037/a0012345

This applies whether or not your paper will be printed or read digitally. The hyperlink format is the standard in all cases.

Placement in the Reference

The DOI appears at the very end of a journal article reference, after the page numbers, with no period following the hyperlink. The general format for a journal article in APA 7 is:

Author, A. A., & Author, B. B. (Year). Title of article. Title of Journal, Volume(Issue), pages. https://doi.org/xxxxx

APA 7 Journal Article Examples with DOI

When There Are More Than 20 Authors

APA 7 lists up to 20 authors in the reference list. If there are 21 or more, list the first 19, insert an ellipsis (…), and then list the final author. The DOI still appears at the end regardless of how many authors are listed.

Formatting DOIs in MLA 9

MLA 9 introduced more flexibility in how electronic location information is presented, but DOIs remain the preferred location element when available. MLA treats sources as a series of "containers" (like a journal containing an article), and the DOI or URL appears as the final element in the citation — in the "location" slot.

DOI Format in MLA 9

MLA 9 accepts two DOI formats. You may use the hyperlink format (recommended for digital documents) or the "doi:" prefix format. Both are considered correct:

https://doi.org/10.1037/a0012345
doi:10.1037/a0012345

The general MLA 9 format for a journal article is:

Last, First, and First Last. "Title of Article." Journal Name, vol. X, no. X, Year, pp. XX–XX. https://doi.org/xxxxx.

MLA 9 Journal Article Example with DOI

Note that in MLA 9, article titles are in quotation marks and journal names are italicised. Unlike APA, MLA gives the volume and issue as "vol." and "no." rather than numerals in parentheses. The DOI ends the citation, followed by a period.

Formatting DOIs in Chicago

Chicago style has two systems: the Notes-Bibliography system (used in humanities) and the Author-Date system (used in sciences and social sciences). Both systems handle DOIs similarly, but the citation structure differs.

Chicago Notes-Bibliography (Footnotes/Endnotes)

In the Notes-Bibliography system, the DOI appears at the end of both the footnote/endnote citation and the bibliography entry. Chicago also accepts the https://doi.org/ format as the preferred presentation.

Bibliography entry:

Footnote/endnote (short form after first citation):

Chicago Author-Date

In the Author-Date system, the citation structure resembles APA but the DOI appears at the end of the reference list entry:

Note the distinctive Chicago Author-Date feature: the year follows the author's name immediately, unlike APA where it appears in parentheses. The volume and issue numbers also follow a different format — "55 (1)" rather than APA's "55(1)".

Formatting DOIs in Harvard

Harvard referencing is not a single standardised system — it is a family of author-date styles used by many UK and Australian universities, each institution maintaining its own variant. However, most Harvard guides have converged on similar DOI formatting conventions.

Common Harvard DOI Format

Most UK institutional Harvard guides prefer the doi: prefix format (lowercase, no space) rather than the full hyperlink, though many also accept https://doi.org/:

doi:10.1037/0003-066X.55.1.5

A Harvard journal article citation with DOI typically looks like:

Key differences from APA: article titles are in single quotation marks (not italics), "pp." precedes the page numbers, and the year follows the author name in parentheses as part of the in-text author-date format. Always confirm the specific DOI format your institution's Harvard guide recommends, as conventions vary between the University of Leeds, Cite Them Right, APA Harvard, and other variants.

DOI vs URL — Key Differences

Common DOI Citation Mistakes to Avoid

Even students who understand what a DOI is often make formatting errors. Here are the most frequent mistakes — and how to avoid them.

Mistake 1: Using "doi:" Prefix in APA 7

APA 6 used "doi:" as a prefix. APA 7 replaced this with the full hyperlink format. Writing doi:10.1037/a0012345 in an APA 7 paper is technically incorrect — use https://doi.org/10.1037/a0012345 instead.

Mistake 2: Adding a Period After the DOI

In APA 7, there is no period after the DOI hyperlink. The DOI is treated as a URL, and ending URLs with periods can make them appear broken. Chicago and MLA do add a period after the DOI — know your style's rule.

Mistake 3: Using http:// Instead of https://

Modern DOI formatting uses https://doi.org/. Some older databases and citation managers still generate the old http://dx.doi.org/ format. Always update this when you see it.

Mistake 4: Copying a Broken DOI

If a DOI string has a line break in a PDF, copy-pasting can introduce a space or hyphen mid-string. Always verify your DOI resolves correctly by pasting https://doi.org/YOUR-DOI into a browser before submitting your paper.

Mistake 5: Omitting the DOI When It Exists

When a DOI exists, all major citation styles require you to include it. Omitting a DOI when it is available is a citation error. Use Crossref or the publisher's website to check before concluding that no DOI exists.

Mistake 6: Including Both a DOI and a Database URL

When a DOI is available, do not also include the database URL (e.g., the EBSCO or ProQuest link). The DOI alone is sufficient — and preferred.