DOIs and URLs in Citations: Format, Access Dates and Final Periods

The https://doi.org/10.… string at the end of a reference is now expected in almost every citation style. How the DOI is written, when a URL and an access date belong in the entry, and whether a period follows the link all vary by style. This guide covers DOI structure, where to find one, and the style-by-style rules.

What a DOI is

A DOI (Digital Object Identifier) is a persistent identifier for an article, book, dataset or other scholarly object. Web addresses change when publishers redesign their sites; a DOI does not. If the object moves, the registration agency updates the link behind the DOI, and the DOI you cite still works.

A DOI has a prefix and a suffix joined by a slash:

Put https://doi.org/ in front and it becomes a working link through the official resolver. DOIs are case-insensitive, but copy them exactly anyway, and never insert a hyphen when a DOI breaks across lines.

Crossref and DataCite DOIs

DOIs are issued by registration agencies under the International DOI Foundation. Two matter most for citations:

For citing, both look and behave the same. For datasets, check whether each version has its own DOI and cite the version you used.

Finding a DOI

1. The first page of the article, in the header, footer or under the abstract. Whatever the label, the DOI is the part starting with "10."
2. The publisher's article page or its "Cite this article" button.
3. Crossref Search (search.crossref.org): paste the title.
4. Korean databases such as KCI, DBpia and ScienceON often show the DOI on the article record. Many Korean journals have none; if so, leave it out.
5. Test it: open https://doi.org/ plus the DOI in a browser.

DOIs by style

APA's journal article example ends with the DOI and no period:

Grady, J. S., Her, M., Moreno, G., Perez, C., & Yelinek, J. (2019). Emotions in storybooks: A comparison of storybooks that represent ethnic and racial groups in the United States. Psychology of Popular Media Culture, 8(3), 207–217. https://doi.org/10.1037/ppm0000185

The IEEE Reference Guide uses "doi:" and ends with a period:

M. M. Chiampi and L. L. Zilberti, “Induction of electric field in human bodies moving near MRI: An efficient BEM computational procedure,” IEEE Trans. Biomed. Eng., vol. 58, no. 10, pp. 2787–2793, Oct. 2011, doi: 10.1109/TBME.2011.2158315.

NLM's sample for Vancouver style does the same:

Zhang M, Holman CD, Price SD, Sanfilippo FM, Preen DB, Bulsara MK. Comorbidity and repeat admission to hospital for adverse drug reactions in older adults: retrospective cohort study. BMJ. 2009 Jan 7;338:a2752. doi: 10.1136/bmj.a2752.

Crossref's display guidelines ask for the full https://doi.org/ link with no "doi:" label and no "dx" in the domain; APA, MLA and Chicago follow that pattern. Update legacy forms such as http://dx.doi.org/… when you use those styles. APA also accepts shortDOIs created at shortdoi.org when a DOI is long.

URL rules

• Cite the exact page you used, not the site's home page.
• Leave out database URLs. APA says not to include URLs from most academic research databases because they will not resolve for readers; give the DOI instead. Exceptions include works of limited circulation such as dissertations, preprints and ERIC documents, and content that exists only in one database, such as UpToDate. Library proxy links are never usable.
• Strip tracking parameters (?utm_source=) if the page still loads without them.
• Shortened URLs: APA accepts them, preferably from well-known services or the site itself, as long as you test the link. Chicago prefers the full URL because third-party shorteners are not always reliable or lasting. The full address is the safer default.
• MLA usually omits http:// and https:// from URLs.
• APA 7 no longer uses "Retrieved from" unless a retrieval date is needed.

Access dates

Record the date when you read the source; don't reconstruct it later.

The period after a link

• APA: no period after a DOI or URL, so the punctuation is not mistaken for part of the link.
• MLA and Chicago: entries end with a period, including after a URL.
• IEEE: entries end with a period, including those ending in a DOI, except entries that end with a URL. When an entry has a DOI or access date and a URL, the DOI or date comes first, then a period, then the URL.
• Vancouver (NLM): a closing "doi: …" takes a period; NLM samples ending in "Available from: URL" have none.

Common mistakes

• Writing "DOI: https://doi.org/…" in APA, which wants the link alone
• Keeping dx.doi.org or http:// forms
• Hyphens or spaces inserted into a DOI at a line break
• Proxy or database viewer URLs
• A publisher URL when a DOI exists
• The wrong closing punctuation for the style

The reference generator formats DOIs and final punctuation for each style automatically, and the BibTeX and RIS converter carries DOI fields across. See also journal articles and web pages.