What counts as common knowledge in academic writing?

Information that most readers in your audience already know, that can be verified in any general reference, that is not disputed, and that no single author discovered or phrased. Harvard’s examples are water as H2O and Andrew Jackson as the seventh U.S. president. MLA 9 §4.13 adds basic biographical facts and the dates of major historical events. Statistics, specific findings, arguments, and exact wording do not qualify, however familiar they feel.

Do I need to cite something my professor said in a lecture?

Yes, if you use their idea, framework, or example. In APA 7 an unrecorded lecture is a personal communication cited in the text only, such as (T. Nguyen, personal communication, February 24, 2020); posted slides use the official Mack and Spake Canvas form. Prefer the published research the lecture was based on when it exists. MLA 9 and Chicago 18 both allow a formal works-cited or note entry; Chicago’s current Quick Guide specimen is Eric Oliver’s 23 February 2022 virtual lecture.

If I find a fact in five different sources, is it common knowledge?

Probably, if the five sources are independent, reputable, and state the fact without citing anyone. That heuristic is the Purdue OWL classroom shortcut, not an APA, MLA, or Chicago rule. It fails when the five sources all copy one Census release, and it never covers borrowed wording or a borrowed interpretation, which need citations regardless. Purdue’s own fallback is still to cite when you are unsure.

Do I have to cite a paraphrase?

Yes. APA 7 §8.1, MLA 9 chapter 4, and Chicago all require a citation for paraphrased ideas; rewording changes the sentence, not the ownership. UNC’s handout answers the same question in one line: yes, you do. Whether the citation needs a page number depends on the style: APA makes it optional for paraphrases, MLA expects it when the source has pages, and Chicago notes give it.

Do dates and definitions need citations?

Widely known dates and standard definitions do not, as long as you are writing for readers who would accept them. A date that comes from one archival study, or a definition you quote word for word from a dictionary or a theorist, does. The official APA dictionary specimen is the 2018 APA Dictionary of Psychology entry for internet addiction. Quoted wording always needs quotation marks and a citation with a locator.

Do I need to cite a Census Bureau or BLS number?

Yes. MIT’s handbook lists U.S. Census Bureau and Bureau of Labor Statistics figures as examples of what is not common knowledge. The July 1, 2024 resident-population estimate of 340.1 million, published by Kristie Wilder on 19 December 2024, is a counted number with a vintage, not a proverb. Cite the release you actually opened.

Do I need to cite my own earlier paper?

Yes. Reusing your own submitted or published text or data without citation is self-plagiarism in APA 7 (§8.3). The MLA Style Center says the same: it is possible to plagiarize yourself, and many honor codes forbid reuse even with a self-citation. Cite the earlier work as you would any source, and check whether your course allows reuse at all.

When is a famous quotation common knowledge?

Almost never. Harvard’s rare exception is a line that has entered public memory, such as Kennedy’s "Ask not what your country can do for you—ask what you can do for your country," used as a passing comparison. Quote more of that speech, or analyze it, and you cite the speech. If you are not sure the line is that famous, cite it.

What if I am still not sure whether something needs a citation?

Cite it. Harvard’s worst-case extra citation is an instructor saying you did not need it; a missing one can cost the assignment. CMOS’s editors say to be generous without being silly. UNC’s moral is "When in doubt, give a citation." Keep the source in your notes so the citation is easy to add.

What Needs a Citation vs. Common Knowledge (APA, MLA, Chicago) [2026]

Quick Answer: What Needs a Citation?

A fact is common knowledge when an educated reader in your audience would accept it without asking who said so, and could confirm it in any general reference. APA 7 §8.1 requires a citation for every fact or figure that is not common knowledge, and for every paraphrase and quotation. MLA 9 §4.13 exempts information widely available in reference works. Harvard treats common knowledge as the only source material you may use without attribution. Chicago’s editors say to footnote what you borrowed that is not common knowledge, and to be generous when the line is fuzzy. The three styles agree on the decision and differ only in how the citation looks.

Decision templates (the filled-in lines are official handbook or style-center examples)

Common knowledge (leave uncited)

Widely known scientific and historical facts—such as the molecular structure of water (H2O), or that Andrew Jackson was the seventh president of the United States—generally count as common knowledge.

Statistic (always cite)

The U.S. population reached 340.1 million, up 0.98% from 336.8 million on July 1, 2023 — the highest year-over-year increase since a jump of 0.99% between 2000 and 2001.

Someone’s interpretation (always cite)

If you read in R.A.C. Parker’s history of World War II that British Prime Minister Neville Chamberlain did not have to agree to the 1938 Munich Pact with Hitler, and that he could have chosen an alternate path, you would need to acknowledge the source, since this judgment is Parker’s theory rather than a widely agreed upon fact.

Unrecorded lecture (APA 7 §8.9, in-text only)

(T. Nguyen, personal communication, February 24, 2020)

The handbook sentences this page treats as the rule are:

APA Publication Manual, 7th ed., §8.1

Cite the work of those individuals whose ideas, theories, or research have directly influenced your work.

In addition to crediting the ideas of others that you used to develop your thesis, provide documentation for all facts and figures that are not common knowledge.

Both paraphrases (see Sections 8.23-8.24) and direct quotations (see Sections 8.25-8.35) require citations.

Avoid both undercitation and overcitation. Undercitation can lead to plagiarism (see Section 8.2) and/or self-plagiarism (see Section 8.3).

Harvard Guide to Using Sources, "The Exception: Common Knowledge"

The only source material that you can use in an essay without attribution is material that is considered common knowledge and is therefore not attributable to one source.

Common knowledge is information generally known to an educated reader, such as widely known facts and dates, and, more rarely, ideas or language.

Facts, ideas, and language that are distinct and unique products of a particular individual's work do not count as common knowledge and must always be cited.

MLA Handbook, 9th ed., §4.13, as quoted by Washington and Lee

Common knowledge includes information widely available in reference works, such as basic biographical facts about prominent persons and the dates and circumstances of major historical events.

Chicago Manual of Style Q&A (Documentation)

You should footnote information that you borrow from someone else that isn’t common knowledge, whether you quote it or not; and if any of the information is disputed (for instance, if some sources say that Jefferson had seven sisters), it’s a good idea to footnote the version that supports your statement.

Everyone knows that George Washington was the first president, so even if you read it somewhere, there are so many sources that say so that it doesn’t make sense to give credit to a particular one.

It’s not always clear whether something should be cited or not, so until you develop some confidence, be generous in your citations, without being silly.

What this guide covers (and what it does not)

This is the decision page. It tells you whether a sentence in your draft needs a citation at all, which official definitions disagree at the edges, and which real examples the manuals and writing centers publish. It is not a quoting-mechanics lesson and not a second plagiarism policy.

Use this page when the only question is "is this common knowledge, or does it need a source?"
Use how to avoid plagiarism for the ethics overview: APA §§8.2–8.3, MLA §§4.1–4.16, Harvard, Princeton §2.4, and the official McDougall, Agar, Ward, Sandel, and Kernan examples.
Use paraphrasing vs. quoting once you have already decided to cite and need to choose a rewrite.
Use do you need a page number when paraphrasing? for the locator question after the citation is required.

What common knowledge actually means

Common knowledge is not "things I happen to know." It is information that is widely held, easy to verify in general sources, and not attributable to one person's work. MIT Academic Integrity puts the audience test in one sentence:

MIT Academic Integrity Handbook

Broadly speaking, common knowledge refers to information that the average, educated reader would accept as reliable without having to look it up.

Four tests have to be true at once. When any one fails, cite.

Widely known. Most educated readers, or most readers in your field, already accept it: water is H2O; Andrew Jackson was the seventh U.S. president; the Second World War ended in 1945.
Verifiable anywhere. You could confirm it in an encyclopedia, an atlas, or an introductory textbook without caring which one. MLA 9 §4.13 uses that "reference works" test.
Not contested. Nobody serious disputes it, and you are not using it to settle an argument. Chicago’s Jefferson-siblings example is the warning: if sources disagree about the number of sisters, footnote the version you used.
Not owned. No single author discovered it, measured it, or phrased it the way you are phrasing it. Harvard’s Franz Boas example is the other side of the same coin: that Boas held the first U.S. academic appointment in anthropology is common knowledge because no one person discovered it, even if you learned it last week.

A widely repeated claim can still be contested ("we only use 10% of our brains"), and a verifiable number can still be owned (a census total was counted by a specific agency on a specific date). MIT is blunt about statistics: figures from the U.S. Census Bureau and the Bureau of Labor Statistics are not common knowledge. That is why the 340.1 million July 1, 2024 estimate on this page is cited, even though it is easy to find.

The "five sources" heuristic

The Purdue OWL page on common knowledge and attribution teaches a classroom shortcut: treat a fact as common knowledge when you find it stated without a citation in at least five credible sources. The number is not from APA, MLA, or Chicago. It is a threshold that separates "this is in every reference book" from "this is in one article that every other article is quoting." Use it as a first pass, not a verdict. Purdue’s own golden rule on that page is still "when in doubt, just cite."

The heuristic has two known failure modes. First, five websites copying one Census Bureau press release are one source, not five; independence matters more than the count. Second, it says nothing about wording or interpretation. A fact can be common knowledge while the sentence you found it in is not, and while the conclusion the author draws from it is definitely not. If you borrow the sentence, quote it and cite it. If you borrow the argument, cite it.

The student-versus-expert test

UNC’s plagiarism handout adds a test the manuals leave implicit: you are usually not yet a professional in the field you are writing for. Ask two questions before you leave a sentence uncited:

UNC Writing Center, "Plagiarism"

Did I know this information before I took this course?

Did this information/idea come from my own brain?

If you answer “no” to either or both of these questions, then the information is not “common knowledge” to you.

The moral of this handout: When in doubt, give a citation.

What always needs a citation

Statistics and measurements. Percentages, counts, rates, survey results, experimental values. MIT’s list names Census Bureau and BLS figures by name. The July 1, 2024 U.S. resident-population estimate of 340.1 million is a counted number, not a proverb.
Specific research findings. "A 2025 study found that…" is a citation waiting to happen, however famous the journal. The official APA journal specimen is Niepel, Hausen, Weber, and Möller (2025) in Journal of Educational Psychology.
Interpretations, arguments, and theories. That the 1938 Munich Pact was signed is common knowledge. Parker’s judgment that Chamberlain could have chosen another path is not. Harvard’s other owned-conclusion example is Anthony Abraham Jack’s The Privileged Poor (Harvard University Press, 2019).
Exact wording. Any phrase you reproduce verbatim, even a short one, needs quotation marks and a citation with a locator — unless it is the rare public-memory line Harvard carves out (Kennedy’s "Ask not…" phrase, used as a passing comparison). Quote more of that speech, or analyze it, and you cite the speech. See paraphrasing vs. quoting.
Paraphrases and summaries. New words do not transfer ownership. APA §8.1, MLA chapter 4, and Chicago all require a citation for a paraphrase. UNC’s wording is the classroom version: "So, just to be clear—do you need to cite when you paraphrase? Yes, you do!" Whether the citation needs a page number is do you need a page number when paraphrasing?.
Tables, figures, images, and data sets. Anything you reproduce or adapt is cited in the caption or note, and reproducing it may also need permission. The walkthrough is citing a reproduced figure or table.
Ideas from lectures, seminars, and conversations. Your instructor’s framework is their work. Cite the lecture, the posted slides, or the person, in the form your style uses.
Anything you learned from one particular source while researching. If you did not know it before you opened the source, the reader probably did not either. APA’s Avoiding Plagiarism Guide calls the "I heard it somewhere, so it must be common knowledge" move a common form of idea plagiarism.
Your own previously submitted or published work. Reusing it without citation is self-plagiarism in APA (§8.3). The MLA Style Center says the same: "It’s even possible to plagiarize yourself."

APA Style, Avoiding Plagiarism Guide (last updated 15 November 2021)

Always attribute every idea, fact, or finding you put in your paper when you write it.

Another common form of idea plagiarism is when students remember a fact they learned in class and put it in their paper without citing it or when they write about a fact they heard somewhere and mistakenly assume it is common knowledge.

Most important, always search the literature to find a source for any ideas, facts, or findings that you put in your paper.

Decision table: cite or not?

The same test applied to statements you can actually check. Invented books and invented percentages are not on this table.
Statement in your draft	Cite?	Why
Water is H2O; it freezes at 32 °F.	No	Harvard’s molecular-structure example; MIT’s freezing-point example. Established, verifiable in any general reference.
Andrew Jackson was the seventh U.S. president. George Washington was the first.	No	Harvard (Jackson) and the CMOS Q&A (Washington). Widely known dates and offices.
The U.S. population reached 340.1 million on July 1, 2024.	Yes	A Census Bureau Vintage 2024 estimate, published 19 December 2024 by Kristie Wilder. MIT: Census figures are not common knowledge.
Historians disagree about whether the 1938 Munich Pact was inevitable.	Usually no	A general statement about a debate. Cite the moment you name a position or a historian.
Parker argues that Chamberlain did not have to sign the Munich Pact.	Yes	Harvard’s owned-interpretation example. The judgment is Parker’s, not a consensus fact.
Jack’s conclusions about economically disadvantaged students at elite colleges.	Yes	Harvard’s other owned-conclusion example: cite Anthony Abraham Jack, The Privileged Poor (Harvard University Press, 2019).
"Ask not what your country can do for you—ask what you can do for your country."	No, as a passing comparison	Harvard’s rare public-memory exception. Quote more of the speech, or analyze it, and you cite Kennedy’s inaugural.
Niepel, Hausen, Weber, and Möller (2025) measured state academic self-concept.	Yes	A specific finding from a specific study. Official APA §10.1 specimen in the 3 March 2026 Common Reference Examples Guide.
Our class survey of 40 students found that most prefer morning exams.	No citation; describe the method	Your own data has no external source, but readers need to know how you collected it. MIT still treats other people’s datasets as cited work.
An unrecorded lecture claim you cannot recover from a published source.	Yes, in-text only in APA	APA §8.9 personal communication: (T. Nguyen, personal communication, February 24, 2020). Prefer the published research the lecture was based on when it exists.
A definition copied from a dictionary, even of a common term.	Yes	Quoted wording. Official APA dictionary specimen: American Psychological Association. (2018). Internet addiction. In APA dictionary of psychology.
A map or table redrawn from a Census Bureau release.	Yes	Reproduced or adapted material is cited in the caption. See citing a reproduced figure or table.

Examples side by side

No citation needed

Paris is the capital of France.
Water is H2O.
Andrew Jackson was the seventh U.S. president.
Jane Austen wrote Pride and Prejudice.
The Big Bang theory says the universe began billions of years ago with an enormous explosion (MIT’s statement #1).
Your own observations, fieldwork, or experimental results, described in your methods.

Citation needed

The U.S. resident population reached 340.1 million on July 1, 2024 (Wilder, 2024).
Niepel et al. (2025) reported state academic self-concept in Journal of Educational Psychology.
Parker’s claim that Chamberlain could have refused the 1938 Munich Pact.
Jack’s conclusions in The Privileged Poor about disadvantaged students at elite colleges.
MIT’s statement #2: the phrase "Big Bang" was coined by Sir Fred Hoyle to mock the theory.
A theoretical model you borrowed to interpret your own results.

Common knowledge depends on the field and the audience

What counts as common knowledge is relative to the people who will read the paper. MIT’s handbook is the clearest official statement of that relativity, and it gives three worked cases rather than a slogan:

MIT Academic Integrity Handbook, audience cases

A description of the symptoms of Asperger’s Syndrome would need to be cited for a composition in a general writing class but probably not need citation for an audience of graduate students in psychology.

A reference to the practice of fair value accounting would be understood by a group of economists, but would need citation to an audience of non-experts.

A statement reporting that 24% of children under the age of 18 live in households headed by single mothers would need to be cited. This is information that would not be known to the average reader, who would want to know where the figure was obtained.

Harvard’s Boas sentence is the same idea from the other direction: a fact can be common knowledge inside a field even when you personally just learned it, provided no one person discovered it. UNC’s two questions then pull you back: if you learned the fact in this course, cite it for this paper. The two tests are not a contradiction. Boas tells you the fact is not owned. UNC tells you your instructor still wants to see how you know.

The same kind of statement, in five fields. The "still needs a citation" column is where students lose marks.
Field	Common knowledge for that audience	Still needs a citation
History	Dates of major events; names of rulers; that the 1938 Munich Pact was signed	Casualty figures; Parker’s claim about Chamberlain; a quotation from a primary source
Sciences	Textbook mechanisms; H2O; MIT’s Big Bang statement #1	Any measured value from a study; MIT’s Hoyle coinage; a contested hypothesis
Literature	Plot, characters, and publication dates of canonical works	Any critical reading; a passage you quote, with its page or line (CMOS Quick Guide: Yu, Interior Chinatown, 45)
Social sciences	Broad definitions (GDP, socialization); that Maslow’s hierarchy is one behavioral theory	Survey results; Jack’s conclusions; Niepel et al. (2025); a theory attached to a named author
Law	That a famous case exists and what it is generally known for	The holding, the pinpoint page, and any commentary on it

Two practical consequences. First, when you write outside your field, cite more. Second, when a grader tells you a fact "needs a source," they are usually telling you about the audience, not correcting the fact. Add the citation; do not argue that the fact is true.

Ideas from lectures and class discussion

An instructor’s framework, example, or argument is their intellectual work. APA’s Avoiding Plagiarism Guide names the uncited lecture-fact as a standard idea-plagiarism pattern. If the lecture was not recorded and the slides are not posted, APA treats it as a personal communication: cite it in the text only, with initials, surname, and the exact date, and give it no reference entry (APA personal communications, §8.9). If the slides or a recording live on the course site, APA cites that file as a classroom source (§8.8 / §10.14). The official Style Center specimens are T. Nguyen (unrecorded) and Mack and Spake (Canvas slides). MLA and Chicago both allow a formal entry for a live or recorded lecture; Chicago’s current Quick Guide specimen is Eric Oliver’s 23 February 2022 virtual lecture. The APA lecture guide and personal communications guide cover the variants.

One idea from a lecture, official forms only

APA 7, unrecorded lecture (in-text only):

(T. Nguyen, personal communication, February 24, 2020)

APA 7, narrative personal communication:

E.-M. Paradis (personal communication, August 8, 2019)

APA 7, slides posted on the course site (Common Reference Examples, 3 March 2026):

Mack, R., & Spake, G. (2018). Citing open source images and formatting references for presentations [PowerPoint slides]. Canvas@FNU. https://fnu.onelogin.com/login

APA 7 in-text for the slides:

(Mack & Spake, 2018)

Chicago 18 note (Quick Guide, recorded virtual lecture):

2. Eric Oliver, "Why So Many Americans Believe in So Many ‘Crazy’ Things," moderated by Andrew McCall, virtual lecture, February 23, 2022, posted March 21, 2022, by University of Chicago, YouTube, 1:01:45, https://youtu.be/hfq7AnCF5bg.

MLA 9 works-cited template (live lecture; fill in the real session):

Speaker Last, Speaker First. "Title of Lecture." Course or series, Day Mon. Year, Institution. Lecture.

APA’s recoverability warning, reprinted in the lecture guide, is the one students skip: if a published article already contains the claim, cite the article, not the talk. Use the personal-communication form only for original classroom content a reader cannot recover.

How each style cites a fact that is not common knowledge

Once you have decided that a fact needs a citation, the mechanics are the ordinary in-text citation for your style. The examples below use one real Census Bureau estimate (Wilder, 19 December 2024), one official APA journal specimen (Niepel et al., 2025), one official CMOS book specimen (Yu, Interior Chinatown), and Harvard’s owned-conclusion book (Jack, 2019). No invented publishers.

APA 7: author–date

APA cites the author and year every time a source contributes a fact, in parentheses or in the sentence. A page number is required for a quotation and recommended for a paraphrase of a long work. Full rules are on the APA basic principles page and in the APA in-text citation guide.

APA 7, a Census statistic and the official Niepel journal specimen

Reference (Census Bureau story):

Wilder, K. (2024, December 19). New 2024 population estimates show nation’s population grew by about 1% to 340.1 million since 2023. U.S. Census Bureau. https://www.census.gov/library/stories/2024/12/population-estimates.html

Parenthetical:

The resident population reached 340.1 million on July 1, 2024 (Wilder, 2024).

Reference (article; Common Reference Examples, 3 March 2026):

Niepel, C., Hausen, J. E., Weber, A. M., & Möller, J. (2025). Understanding mean-level and intraindividual variability in state academic self-concept: The role of students’ trait expectancies and values. Journal of Educational Psychology, 117(5), 772–788. https://doi.org/10.1037/edu0000946

Narrative:

Niepel et al. (2025) measured both mean-level and within-person variability in state academic self-concept.

Reference (Jack, Harvard’s owned-conclusion example):

Jack, A. A. (2019). The privileged poor: How elite colleges are failing disadvantaged students. Harvard University Press.

Parenthetical for the whole book:

Jack’s conclusions about economically disadvantaged students at elite colleges need a citation (Jack, 2019).

MLA 9: author–page

MLA cites the author's surname and the page where the fact appears; there is no year in the parenthetical. If you name the author in the sentence, only the page goes in parentheses. A fact drawn from a whole work rather than one passage is cited by author alone. The MLA in-text citations overview and our MLA in-text guide cover the variants.

MLA 9, the same Census story and Jack book

Works cited (Census Bureau story):

Wilder, Kristie. "New 2024 Population Estimates Show Nation’s Population Grew by About 1% to 340.1 Million Since 2023." U.S. Census Bureau, 19 Dec. 2024, www.census.gov/library/stories/2024/12/population-estimates.html.

Parenthetical (unpaginated web source):

The resident population reached 340.1 million on 1 July 2024 (Wilder).

Works cited (Jack):

Jack, Anthony Abraham. The Privileged Poor: How Elite Colleges Are Failing Disadvantaged Students. Harvard UP, 2019.

Whole-work in-text:

The conclusions Jack drew from his study of economically disadvantaged students at elite colleges need a citation (Jack).

Chicago 18: note, bibliography, or author-date

In notes-bibliography style, a superscript number after the sentence points to a note that gives the source and the page; the first note is full and later notes are shortened. The bibliography lists the work without a pinpoint page (an article’s entry gives its full page range). In author-date style, the parenthetical looks like APA’s but drops the comma after the author and the "p." before the page. Both patterns are in the Chicago citation guide and our Chicago in-text guide.

Chicago 18, official Quick Guide book and journal forms plus the Census statistic

Sentence:

A sentence you took from Yu is cited with the official first-note page.¹

First note (CMOS Quick Guide):

1. Charles Yu, Interior Chinatown (Pantheon Books, 2020), 45.

Shortened note:

3. Yu, Interior Chinatown, 48.

Bibliography (book):

Yu, Charles. Interior Chinatown. Pantheon Books, 2020.

Note for a journal fact (CMOS Quick Guide):

1. Hyeyoung Kwon, "Inclusion Work: Children of Immigrants Claiming Membership in Everyday Life," American Journal of Sociology 127, no. 6 (2022): 1842–43, https://doi.org/10.1086/720277.

Note for the Census statistic:

4. Kristie Wilder, "New 2024 Population Estimates Show Nation’s Population Grew by About 1% to 340.1 Million Since 2023," U.S. Census Bureau, December 19, 2024, https://www.census.gov/library/stories/2024/12/population-estimates.html.

Author-date in-text (Quick Guide form):

(Yu 2020, 45)

Citing one paraphrased fact in the three styles.
Element	APA 7	MLA 9	Chicago 18
Where the citation goes	Parentheses, or author in the sentence with the year after it	Parentheses at the end of the sentence	Numbered note (N-B) or parentheses (author-date)
What it contains	Author, year	Author, page (or author alone for a whole work)	Full source and page in the first note; author and short title later
Page for a paraphrased fact	Optional; recommended for long works	Expected when the source has pages	Given in the note; given in author-date for a specific passage
Page for a quotation	Required	Required	Required
Fact from a whole work	(Jack, 2019)	(Jack)	Note without a pinpoint page; (Jack 2019)
Two authors	(Mack & Spake, 2018)	(Mack and Spake)	(Mack and Spake 2018)

When in doubt, cite

The cost of an unnecessary citation is a slightly cluttered sentence. The cost of a missing one is an integrity case. APA §8.1 warns against both undercitation and overcitation, but the two are not symmetrical: overcitation is a style problem, undercitation is plagiarism. Harvard’s version of the same advice is that the worst outcome of an extra citation is an instructor saying you did not need it. CMOS’s editors say to be generous without being silly. UNC’s moral is one sentence: when in doubt, give a citation.

APA Publication Manual, 7th ed., §8.1, on overcitation

Overcitation can be distracting and is unnecessary. For example, it is considered overcitation to repeat the same citation in every sentence when the source and topic have not changed. Instead, when paraphrasing a key point in more than one sentence within a paragraph, cite the source in the first sentence in which it is relevant and do not repeat the citation in subsequent sentences as long as the source remains clear and unchanged (see Section 8.24).

Ask where you learned it. If the answer is "from this article," cite the article.
Ask whether the reader could check it without you. If they would need your source to do so, cite it.
Ask whether anyone owns the phrasing. If you kept more than a few words, quote and cite.
Ask whether the statement would change if a different author had written the source. Parker’s Munich judgment would; the boiling point of water would not.
Ask your instructor. Course conventions decide the edge cases, and a question costs nothing.

Common mistakes

Treating a number as common knowledge because it is easy to find. Statistics always have a source, even famous Census totals.
Citing the source of a fact but not the source of the interpretation built on it. The Munich Pact is common knowledge; Parker’s counterfactual is not.
Assuming that changing the words removes the need to cite. A paraphrase is a citation with different wording, not a way around one.
Leaving a lecture idea uncited because "the professor already knows it." Cite it in the form your style uses for lectures or personal communications.
Citing the encyclopedia you used to confirm a piece of common knowledge. If it is common knowledge, no citation is needed; if you needed the encyclopedia to learn it, UNC says it probably is not common knowledge to you.
Reusing a paragraph from your own earlier paper without a citation. APA §8.3 and the MLA Style Center both treat that as self-plagiarism.
Putting a citation after every sentence of a paragraph that paraphrases one source. Cite at the first mention and again when the source changes; see citing the same source multiple times.
Writing "studies show" or "experts agree" with no citation. Name the study or cut the claim.
Copying a dictionary definition and calling it common knowledge. The wording is owned even when the concept is not. Use the APA dictionary specimen if you need a model.

Before you leave a sentence uncited

I did not learn this fact from a particular source while researching this paper.
My intended reader would accept it without asking who says so.
It could be confirmed in any general reference, not only in one author's work.
It is a fact, not an interpretation, estimate, statistic, or theory.
None of the wording is borrowed.
It did not come from a lecture, seminar, or conversation.
If any line above is false, I have added the citation in my style's form.

Frequently asked questions

What counts as common knowledge in academic writing?: Information that most readers in your audience already know, that can be verified in any general reference, that is not disputed, and that no single author discovered or phrased. Harvard’s examples are water as H2O and Andrew Jackson as the seventh U.S. president. MLA 9 §4.13 adds basic biographical facts and the dates of major historical events. Statistics, specific findings, arguments, and exact wording do not qualify, however familiar they feel.
Do I need to cite something my professor said in a lecture?: Yes, if you use their idea, framework, or example. In APA 7 an unrecorded lecture is a personal communication cited in the text only, such as (T. Nguyen, personal communication, February 24, 2020); posted slides use the official Mack and Spake Canvas form. Prefer the published research the lecture was based on when it exists. MLA 9 and Chicago 18 both allow a formal works-cited or note entry; Chicago’s current Quick Guide specimen is Eric Oliver’s 23 February 2022 virtual lecture.
If I find a fact in five different sources, is it common knowledge?: Probably, if the five sources are independent, reputable, and state the fact without citing anyone. That heuristic is the Purdue OWL classroom shortcut, not an APA, MLA, or Chicago rule. It fails when the five sources all copy one Census release, and it never covers borrowed wording or a borrowed interpretation, which need citations regardless. Purdue’s own fallback is still to cite when you are unsure.
Do I have to cite a paraphrase?: Yes. APA 7 §8.1, MLA 9 chapter 4, and Chicago all require a citation for paraphrased ideas; rewording changes the sentence, not the ownership. UNC’s handout answers the same question in one line: yes, you do. Whether the citation needs a page number depends on the style: APA makes it optional for paraphrases, MLA expects it when the source has pages, and Chicago notes give it.
Can I cite too much?: You can clutter a paragraph, but overcitation is a style problem while undercitation is plagiarism. APA §8.1 says not to repeat the same citation in every sentence when the source and topic have not changed: cite the first relevant sentence and keep going as long as the source stays clear. Do not remove citations from facts that need them to tidy the page.
Do dates and definitions need citations?: Widely known dates and standard definitions do not, as long as you are writing for readers who would accept them. A date that comes from one archival study, or a definition you quote word for word from a dictionary or a theorist, does. The official APA dictionary specimen is the 2018 APA Dictionary of Psychology entry for internet addiction. Quoted wording always needs quotation marks and a citation with a locator.
Do I need to cite a Census Bureau or BLS number?: Yes. MIT’s handbook lists U.S. Census Bureau and Bureau of Labor Statistics figures as examples of what is not common knowledge. The July 1, 2024 resident-population estimate of 340.1 million, published by Kristie Wilder on 19 December 2024, is a counted number with a vintage, not a proverb. Cite the release you actually opened.
Do I need to cite my own earlier paper?: Yes. Reusing your own submitted or published text or data without citation is self-plagiarism in APA 7 (§8.3). The MLA Style Center says the same: it is possible to plagiarize yourself, and many honor codes forbid reuse even with a self-citation. Cite the earlier work as you would any source, and check whether your course allows reuse at all.
When is a famous quotation common knowledge?: Almost never. Harvard’s rare exception is a line that has entered public memory, such as Kennedy’s "Ask not what your country can do for you—ask what you can do for your country," used as a passing comparison. Quote more of that speech, or analyze it, and you cite the speech. If you are not sure the line is that famous, cite it.
What if I am still not sure whether something needs a citation?: Cite it. Harvard’s worst-case extra citation is an instructor saying you did not need it; a missing one can cost the assignment. CMOS’s editors say to be generous without being silly. UNC’s moral is "When in doubt, give a citation." Keep the source in your notes so the citation is easy to add.

Format the citation once you have decided to cite

Build the in-text citation for the fact and the matching reference entry in APA, MLA, or Chicago, then run the finished list through the checker to catch a missing page, year, or italic. Use the official Niepel, Jack, Yu, Mack, and Nguyen forms on this page as the pattern, not an invented book.

In-text citation generator →Citation checker →