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IdentifiersMarch 20, 2025 · 9 min read

DOI vs ISBN: What They Are and How to Use Them in Citations

You are staring at a journal article that has both an ISSN and a DOI and you have no idea which one your professor wants. You type the ISBN from your textbook into the citation box and nothing happens. This guide explains what all of these actually are and which ones go into your citations.

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What is a DOI?

A DOI is a Digital Object Identifier. It is a permanent code assigned to academic articles, journal papers, book chapters, datasets, and other scholarly content. The key word is permanent: unlike a regular URL which can change when a website redesigns or moves things around, a DOI always points to the same content.

Every DOI starts with 10. followed by a publisher prefix and a unique suffix. Here is what one looks like:

10.1037/0022-3514.51.6.1173

And here is how it gets formatted as a full link, which is how it appears in APA 7th edition citations:

https://doi.org/10.1037/0022-3514.51.6.1173

DOIs are issued by CrossRef, a nonprofit that manages the system. Publishers register their articles and get DOIs assigned. When your reader clicks that link, it takes them directly to the article regardless of which database or journal website hosts it.

What is an ISBN?

An ISBN is an International Standard Book Number. It is a unique number assigned to each edition of a book. The current standard is 13 digits (ISBN-13), though older books have 10-digit ISBNs (ISBN-10). Here is what each looks like:

ISBN-13 (modern)978-0-14-028329-7
ISBN-10 (older books)0-14-028329-X

Each edition of a book gets a different ISBN. The hardcover, paperback, and ebook of the same book all have different ISBNs. This is actually really useful when you are citing because you want to make sure you are referencing the exact edition you used.

You can find the ISBN on the back cover of a physical book (usually under the barcode), on the copyright page inside the book, or on any bookshop or library listing.

The big difference: which one goes in your citation?

The short answer

DOIs appear in citations for journal articles. ISBNs do NOT appear in APA, MLA, or Chicago citations at all. You can use an ISBN to look up a book, but you do not write it in the citation itself.

DOI
Appears in citation: Yes, for journal articles in APA 7th (as https://doi.org/ link), and in Chicago
Not standard in MLA 9th (use URL instead)
ISBN
Appears in citation: No — in any major style guide
Use it to find and verify book details, then cite normally without it

How DOIs appear in APA 7th citations

APA 7th edition changed how DOIs are formatted compared to the 6th edition. In APA 7th, the DOI is always written as a full hyperlink:

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

A real, complete example:

Baumeister, R. F., Bratslavsky, E., Finkenauer, C., & Vohs, K. D. (2001). Bad is stronger than good. Review of General Psychology, 5(4), 323–370. https://doi.org/10.1037/1089-2680.5.4.323

Notice there is no period after the DOI link. APA formatting convention is to never put a period at the end of a URL or DOI because it might look like part of the address.

Where to find a DOI

Journal article page
Usually shown at the top of the article landing page, labeled 'DOI:' or 'https://doi.org/...'.
Article PDF
Often printed in the header or footer of the first page of the PDF, especially for journal articles.
Database abstract page
JSTOR, PubMed, Google Scholar, Scopus, and most databases list the DOI on the article details page.
CrossRef search
Go to search.crossref.org and enter the article title. It will return the DOI if one has been registered.

What if the article has no DOI?

Some older articles and some open-access publications do not have DOIs. In that case:

  • If the article is freely available online, use the URL of the journal article page instead of a DOI.
  • If you found it through a database that requires a subscription (like EBSCO or ProQuest), use the URL of the database home page rather than the article-specific URL, since that URL won't work for your reader.
  • If the article is in a print-only journal with no web version, you do not need a URL or DOI at all. End the citation with the page numbers.
APA 7TH: ARTICLE WITH NO DOI, AVAILABLE ONLINE
Schwartz, B. (2004). The tyranny of choice. Scientific American, 290(4), 70–75. https://www.scientificamerican.com/article/the-tyranny-of-choice/

Book citations: where ISBN fits in (and doesn't)

Let us be very clear about books. Here is a standard APA 7th book citation:

Kahneman, D. (2011). Thinking, fast and slow. Farrar, Straus and Giroux.

No ISBN. Not at the end, not in parentheses, nowhere. The citation just does not include it. The same is true for MLA 9th:

Kahneman, Daniel. Thinking, Fast and Slow. Farrar, Straus and Giroux, 2011.

Still no ISBN. The ISBN is useful for you as the researcher to make sure you are citing the right edition of a book, and it is how FreeCitation looks up book details when you search by ISBN. But once the citation is generated, it does not appear in the output.

ISSN: the third identifier you might see

While we are at it: an ISSN (International Standard Serial Number) identifies a journal or periodical as a whole, not an individual article. You might see it on a journal's website listed as ISSN or e-ISSN. Like the ISBN for books, the ISSN does not appear in your article citation. It is purely an identifier for the publication as a series.

What actually goes in citations: a quick summary
DOI
Yes, for journal articles in APA 7th and Chicago
ISBN
No — use it to find the book, not in the citation
ISSN
No — identifies the journal series, not your specific article
URL
Yes, when no DOI exists and article is publicly available

How FreeCitation uses DOIs and ISBNs

When you enter a DOI into FreeCitation, it queries CrossRef's database to retrieve the article metadata: authors, journal name, volume, issue, page numbers, and publication date. It then formats all of that into a complete APA, MLA, or Chicago citation with the DOI already formatted correctly.

When you enter an ISBN, it queries the Google Books or Open Library API to pull the book details: author names, full title, publisher, and year. It generates the book citation and, since ISBNs do not appear in citation formats, you get the complete reference without the ISBN included in the output. You can also check out our guide on how to write a bibliography to understand where these citations go in your final document.

Frequently asked questions

Can two different articles have the same DOI?
No. DOIs are unique by design. Every DOI points to exactly one piece of content. If you paste a DOI and it goes to the wrong article, you may have copied an incorrect DOI — double check the number.
Do book chapters have DOIs?
Sometimes. Academic book chapters, particularly in edited volumes published by major academic publishers, can have their own DOIs separate from the overall book's ISBN. If a chapter has a DOI, use it in your citation instead of the URL.
What does it mean when a DOI link says 'not found'?
It usually means the DOI has been entered incorrectly, or very rarely that the publisher has changed the registration. Try copying the DOI exactly from the article page rather than typing it. You can also paste it into doi.org to verify. If it still fails, use the direct URL to the journal article instead.

Related guides: how to cite a website in APA, how to write a bibliography, and how to cite a YouTube video.

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