A DOI, or Digital Object Identifier, is a permanent code that identifies a journal article, book chapter, report or dataset online. Unlike a web address, it keeps working even when the publisher moves the content.
Most citation styles now ask you to include a DOI whenever a source has one. This guide explains how DOIs are built, where to find them, what to do when there is none, and how each major style wants them written.
A DOI in One Paragraph
A DOI always starts with "10." followed by a registrant code, a slash and a suffix, for example 10.1000/xyz123. Adding https://doi.org/ in front turns it into a link that resolves to the source's current location.
APA 7 writes it as https://doi.org/10.1000/xyz123 at the end of the reference with no full stop. Other styles vary slightly, as the table below shows.
What Is a DOI?
A DOI is a persistent identifier assigned by a registration agency on behalf of a publisher. Crossref registers most journal articles and books; DataCite registers many datasets and research outputs.
The publisher keeps the DOI's record pointed at the current location of the content. If a journal changes platforms, the old web address breaks, but the DOI simply points to the new page. That reliability is why citation styles prefer DOIs to URLs.
Why DOIs Matter for Citations
- Permanence: readers can find the source years later.
- Precision: a DOI identifies one specific item, not a search page or journal home page.
- Verification: markers and editors can check a source in seconds.
- Metadata: a DOI links to the official author, title and publication details.
Anatomy of a DOI
| Part | Example | Meaning |
|---|---|---|
| Directory indicator | 10. | Every DOI begins with 10. |
| Registrant code | 1000 | Identifies the publisher or organization |
| Separator | / | Divides the prefix from the suffix |
| Suffix | xyz123 | Chosen by the publisher to identify the item |
| Resolver | https://doi.org/ | Turns the DOI into a working link |
Suffixes can contain letters, numbers and punctuation such as full stops, hyphens and brackets. Copy them exactly. DOIs are not case-sensitive, but copying character by character avoids errors.
You may see older forms such as http://dx.doi.org/ or a "doi:" label. They still resolve, but APA 7 and many other styles now use the https://doi.org/ form.
How to Find the DOI for a Source
- The article's first page: most PDFs print the DOI in the header, footer or near the abstract.
- The publisher's web page: look near the title, author list or "cite this article" panel.
- Your library database record: CINAHL, PsycINFO, Scopus and others list the DOI in the full record.
- PubMed: the DOI appears below the abstract and in the citation details.
- Crossref search: Crossref's free search lets you look up a DOI by title and author.
- Google Scholar: follow the title link to the publisher page, where the DOI is usually shown.
Once you have a DOI, the free Bibloq citation generator can pull the article's details from it and build the reference in your style.
Checking a DOI works. Paste https://doi.org/ followed by the DOI into your browser. If it opens the article's landing page, the DOI is correct. If you get a "DOI not found" message, check for a missing character or a stray full stop copied from the end of a sentence.
When a Source Does Not Have a DOI
Many sources have no DOI: older articles, most web pages, many reports and print-only books. That is normal, and you should never invent one.
| Style | What to do without a DOI |
|---|---|
| APA 7 | For articles from most academic databases, end the reference after the page range. Add a URL only if the source is freely available online or from a database with original content. |
| MLA 9 | Give a URL or a permalink if the source is online, or the database name as a second container. |
| Chicago | Use a URL or the database name for online sources; print sources need no locator. |
| Harvard | Give the URL with an accessed date for online sources, following your university's guide. |
How to Format a DOI in Each Citation Style
| Style | DOI format | Full stop after? |
|---|---|---|
| APA 7 | https://doi.org/10.1000/xyz123 | No |
| MLA 9 | https://doi.org/10.1000/xyz123 preferred; the older doi:10.1000/xyz123 form is still seen | Yes, the entry ends with a full stop |
| Chicago | https://doi.org/10.1000/xyz123 | Yes |
| Harvard | Varies: often "Available at: https://doi.org/..." or "doi:..." | Follow your guide |
| IEEE | doi: 10.1000/xyz123 | Yes |
| AMA 11 | doi:10.1000/xyz123 | No |
APA 7 Examples
Moreno, J., & Clarke, P. (2022). Sleep timing and exam performance in first-year students. Journal of Learning Science, 11(3), 145–160. https://doi.org/10.0000/jls.2022.113
In APA 7, present the DOI as a hyperlink, live or plain text depending on the paper's purpose, and do not add "Retrieved from" or a full stop after it.
MLA 9 Example
Moreno, Julia, and Peter Clarke. "Sleep Timing and Exam Performance in First-Year Students." Journal of Learning Science, vol. 11, no. 3, 2022, pp. 145–60, https://doi.org/10.0000/jls.2022.113.
Chicago Example (Notes-Bibliography)
Moreno, Julia, and Peter Clarke. "Sleep Timing and Exam Performance in First-Year Students." Journal of Learning Science 11, no. 3 (2022): 145–60. https://doi.org/10.0000/jls.2022.113.
IEEE Example
[1] J. Moreno and P. Clarke, "Sleep timing and exam performance in first-year students," J. Learn. Sci., vol. 11, no. 3, pp. 145–160, 2022, doi: 10.0000/jls.2022.113.
All examples are illustrative and show layout rather than a real article.
DOIs for Chapters, Preprints and Datasets
Journal articles are the most familiar DOI holders, but many other research outputs carry them too. Each needs a little care.
- Book chapters: in edited academic books, each chapter may have its own DOI as well as the book. Use the chapter's DOI when you cite the chapter.
- Preprints: a preprint server often gives the preprint its own DOI. If the paper was later published in a journal, cite the version you actually read, and prefer the published version where possible.
- Online-first articles: articles published ahead of an issue have a DOI before they have volume and page numbers. Cite them with the DOI and update the reference once the issue details exist.
- Datasets and software: repositories commonly assign DOIs, sometimes one per version. Cite the version you used.
- Reports and theses: some institutions and agencies register DOIs for their reports and theses. Include them when shown.
Does the DOI Go in the in-Text Citation?
No. In every major style the DOI belongs only in the reference list, bibliography or full note. The in-text citation stays short: an author and year, a page, or a number.
DOI Versus URL
| Feature | DOI | URL |
|---|---|---|
| Permanence | Designed to last; updated by the publisher | Can break when sites change |
| Points to | One specific item | Any page, including search results |
| Preferred by styles | Yes, when available | Fallback when there is no DOI |
| Access date needed? | No | Sometimes, for pages likely to change |
If a source has both, use the DOI. Never cite a database search-results URL or a link that only works when you are logged in to your library.
Common DOI Mistakes
- Adding a full stop after the DOI in APA 7, which can break the link.
- Mixing "doi:" and https://doi.org/ forms in one reference list.
- Copying a DOI with a missing character or a trailing punctuation mark.
- Citing the journal home page URL when the article has a DOI.
- Inventing a DOI or using the DOI of a different version, such as a preprint, without saying so.
How Bibloq Helps with DOIs and References
Send your paper and reference list with your style and edition. An editor finds missing DOIs where they exist, writes them in the correct form, completes other missing details where they can be verified and flags anything you need to supply.
You get tracked changes and a clean, consistent reference list. Free revisions within the scope of your original request are included.
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Frequently Asked Questions
No. Most articles published online in recent decades have one, but many older articles and some smaller journals do not.
Yes. Follow your style's rule for sources without a DOI, usually a URL for online sources or nothing extra for print sources.
No. A DOI is a permanent identifier; adding https://doi.org/ makes it a link that resolves to the current URL.
No. DOIs are persistent, so styles do not ask for an access date when you give one.
APA 7 allows live links in papers read online and plain text in printed ones. Follow your instructor's preference.
Check for typing errors or punctuation copied from the end of a sentence. If it still fails, confirm it on the publisher's page or in Crossref.
Some do, especially academic e-books and chapters. If the book or chapter has one, include it.
Many do, often registered through DataCite. Cite them with the DOI as you would an article.
Publishers and institutions register them through agencies such as Crossref, which handles most journal articles and books, and DataCite, which handles many datasets.