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. Well-known dates, geographic facts, textbook definitions, and proverbs qualify. Statistics, specific findings, arguments, and exact wording do not, 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 (R. Adebayo, personal communication, March 3, 2026); posted slides are cited as a classroom source with a reference entry. MLA 9 and Chicago 18 both allow a formal works-cited or note entry for a live 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 a classroom rule of thumb, not a style-manual rule. It fails when the five sources all copy one origin, and it never covers borrowed wording or a borrowed interpretation, which need citations regardless.

Do I have to cite a paraphrase?

Yes. APA 7, MLA 9, and Chicago 18 all require a citation for paraphrased ideas; rewording changes the sentence, not the ownership. 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.

You can clutter a paragraph, but overcitation is a style problem while undercitation is plagiarism. If you paraphrase one source across several sentences, cite it at the first mention and again only when the source changes or a new paragraph begins. 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. Quoted wording always needs quotation marks and a citation with a locator.

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), and MLA and Chicago take the same view. Cite the earlier work as you would any source, and check whether your course allows reuse at all.

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

Cite it. An unnecessary citation costs a few characters; a missing one can cost the assignment. If you want a second opinion, ask the instructor before the deadline, and keep the source in your notes so the citation is easy to add.

What Needs a Citation in APA, MLA, and Chicago? [2026]

Quick Answer: cite what you learned, not what everyone knows

A fact is common knowledge when an educated reader in your audience would accept it without asking "says who?" and could confirm it in any general reference. Everything else came from somewhere, and that somewhere gets a citation. The three styles agree on the principle and differ only in how the citation looks: APA 7 cites author and year, MLA 9 cites author and page, and Chicago 18 puts the source in a numbered note (or, in author-date, author and year with a page).

Templates for citing one fact taken from a book

APA 7 reference:

Author, A. A. (Year). Title of book. Publisher.

APA 7 in-text:

(Author, Year) or, pointing to the passage, (Author, Year, p. 12)

MLA 9 works cited:

Author Last, Author First. Title of Book. Publisher, Year.

MLA 9 in-text:

(Author 12)

Chicago 18 note:

1. Author First Last, Title of Book (Publisher, Year), 12.

Chicago 18 bibliography:

Author Last, Author First. Title of Book. Publisher, Year.

Chicago 18 author-date in-text:

(Author Year, 12)

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. The APA Style page on appropriate level of citation puts it in a sentence: writers do not need to cite common knowledge, which it describes as information that is generally known and that readers can find in many general references. The MLA Handbook, 9th edition, lists the same exemptions in chapter 4: common knowledge, passing mentions of a work, allusions, and epigraphs. Chicago does not carve out a separate rule; it simply does not require a note for a fact that is not borrowed from anyone.

Widely known. Most educated readers, or most readers in your field, already accept it: the Earth orbits the Sun; Shakespeare wrote Hamlet; 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.
Not contested. Nobody serious disputes it, and you are not using it to settle an argument.
Not owned. No single author discovered it, measured it, or phrased it the way you are phrasing it.

All four have to be true. 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 in a specific year). When a test fails, cite.

The "five sources" heuristic

Writing centers teach a practical shortcut: if you can find the same fact stated without a citation in five or more independent, reputable general sources, treat it as common knowledge. The number is not from any style manual. 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.

The heuristic has two known failure modes. First, five websites copying one 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.

What always needs a citation

Statistics and measurements. Percentages, counts, rates, survey results, experimental values. Someone produced the number; the reader needs to know who and when.
Specific research findings. "A 2023 study found that…" is a citation waiting to happen, however famous the study is.
Interpretations, arguments, and theories. The fact that the Berlin Wall fell in 1989 is common knowledge. The claim that it fell because of economic exhaustion is somebody's argument.
Exact wording. Any phrase you reproduce verbatim, even a short one, needs quotation marks and a citation with a locator. See paraphrasing vs. quoting.
Paraphrases and summaries. New words do not transfer ownership. APA, MLA, and Chicago all require a citation for a paraphrase; whether it needs a page number is covered in 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.
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.
Your own previously submitted or published work. Reusing it without citation is self-plagiarism in APA (§8.3), and MLA and Chicago treat it the same way.

Decision table: cite or not?

The same test applied to typical sentences from a student paper.
Statement in your draft	Cite?	Why
Water boils at 100 °C at sea level.	No	Established, verifiable in any general reference, owned by no one.
The Berlin Wall fell in November 1989.	No	A widely known date.
East Berlin lost 8% of its residents in the five years after 1989.	Yes	A specific statistic that someone counted and published.
Historians disagree about whether the collapse was inevitable.	Usually no	A general statement about a debate; cite if you name a position or a historian.
Okafor argues that the collapse was economically inevitable.	Yes	An interpretation belongs to its author.
"The wall did not fall; it was pushed."	Yes, in quotation marks	Exact wording is always cited, with a page or other locator.
Photosynthesis converts light energy into chemical energy.	No, in most contexts	Textbook-level knowledge in any science course.
A 2024 trial found the drug cut symptoms by 22%.	Yes	A specific finding from a specific study.
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.
As Professor Adebayo explained in lecture, aquifers recharge slowly.	Yes	Cite the lecture or the person; the framing is theirs.
"Actions speak louder than words."	No	A proverb with no author.
A map redrawn from a government report.	Yes	Reproduced or adapted material is cited in the caption.
A definition copied from a dictionary.	Yes	Quoted wording, even for a common word.
The idea for your thesis came from a blog post.	Yes	Borrowed ideas count even when no words are borrowed.

Examples side by side

No citation needed

Paris is the capital of France.
DNA carries genetic information.
The United States Constitution was ratified in 1788.
Jane Austen wrote Pride and Prejudice.
Inflation reduces the purchasing power of money.
Your own observations, fieldwork, or experimental results, described in your methods.

Citation needed

Paris recorded 2.1 million residents in the 2022 census.
A 2021 study sequenced the genome of a particular bacterium.
A historian's claim about why ratification succeeded in New York.
A critic's reading of Elizabeth Bennet's irony.
An economist's estimate of inflation's effect on renters in a given year.
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. The APA appropriate-citation page ties the decision to the audience: what is common knowledge in one field may need a citation for readers outside it. A biologist writing for biologists does not cite the structure of DNA. The same sentence in a paper for a first-year composition course, or in a policy brief for non-scientists, may well need one, because the reader has no way to check it without help.

The same kind of statement, in three fields.
Field	Common knowledge for that audience	Still needs a citation
History	Dates of major events; names of rulers; the outcome of a well-known war	Casualty figures; an interpretation of causes; a quotation from a primary source
Sciences	Textbook mechanisms; standard definitions; physical constants	Any measured value from a study; a method adapted from a paper; a contested hypothesis
Literature	Plot, characters, and publication dates of canonical works	Any critical reading; biographical claims; a passage you quote, with its page or line
Social sciences	Broad definitions (GDP, socialization)	Survey results; effect sizes; 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. 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 (see APA personal communications). If the slides or a recording live on the course site, APA cites that file as a classroom source. MLA and Chicago both allow a formal entry for a live lecture. The APA lecture guide and personal communications guide cover the variants.

One idea from a lecture, three styles

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

Aquifers can take decades to recharge after heavy pumping (R. Adebayo, personal communication, March 3, 2026).

APA 7, slides posted on the course site:

Adebayo, R. (2026). Urban water systems [PowerPoint slides]. Canvas. https://lms.lakeside.edu/courses/envs210/week7

APA 7 in-text for the slides:

(Adebayo, 2026)

MLA 9 works cited:

Adebayo, Rita. "Urban Water Systems." ENVS 210: Environmental Systems, 3 Mar. 2026, Lakeside University. Lecture.

MLA 9 in-text:

(Adebayo)

Chicago 18 note:

1. Rita Adebayo, "Urban Water Systems" (lecture, ENVS 210, Lakeside University, March 3, 2026).

Chicago 18 bibliography:

Adebayo, Rita. "Urban Water Systems." Lecture, ENVS 210, Lakeside University, March 3, 2026.

How each style cites a fact

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 invented book and one invented journal article so that you can see the same fact and the same statistic move through all three systems.

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 optional for a paraphrase; add it when the book is long and the fact is on one page. Full rules are on the APA basic principles page and in the APA in-text citation guide.

APA 7, a fact from a book and a statistic from an article

Reference (book):

Okafor, L. M. (2024). Rivers under the city: A history of urban water supply. Meridian University Press.

Parenthetical:

Most European capitals covered their smaller rivers during the nineteenth century (Okafor, 2024).

Parenthetical with a page:

London had buried more than a dozen tributaries by 1870 (Okafor, 2024, p. 112).

Narrative:

Okafor (2024) traced the pattern to a sanitation panic rather than to planning.

Reference (article):

Hallberg, T., & Nkemelu, C. (2023). Household water sources and trust in municipal supply: Evidence from a regional survey. Journal of Urban Environmental Studies, 18(2), 145–162.

Statistic in text:

In the survey region, 38% of households relied on private wells (Hallberg & Nkemelu, 2023).

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 fact and statistic

Works cited (book):

Okafor, Lena M. Rivers under the City: A History of Urban Water Supply. Meridian UP, 2024.

Parenthetical:

London had buried more than a dozen tributaries by 1870 (Okafor 112).

Author named in the sentence:

Okafor traces the pattern to a sanitation panic rather than to planning (114–15).

Works cited (article):

Hallberg, Tove, and Chidi Nkemelu. "Household Water Sources and Trust in Municipal Supply: Evidence from a Regional Survey." Journal of Urban Environmental Studies, vol. 18, no. 2, 2023, pp. 145–62.

Statistic in text:

In the survey region, 38% of households relied on private wells (Hallberg and Nkemelu 151).

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 page numbers (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, the same fact and statistic

Sentence:

London had buried more than a dozen tributaries by 1870.¹

First note:

1. Lena M. Okafor, Rivers under the City: A History of Urban Water Supply (Meridian University Press, 2024), 112.

Shortened note:

2. Okafor, Rivers under the City, 114–15.

Note for the statistic:

3. Tove Hallberg and Chidi Nkemelu, "Household Water Sources and Trust in Municipal Supply: Evidence from a Regional Survey," Journal of Urban Environmental Studies 18, no. 2 (2023): 151.

Bibliography (book):

Okafor, Lena M. Rivers under the City: A History of Urban Water Supply. Meridian University Press, 2024.

Bibliography (article):

Hallberg, Tove, and Chidi Nkemelu. "Household Water Sources and Trust in Municipal Supply: Evidence from a Regional Survey." Journal of Urban Environmental Studies 18, no. 2 (2023): 145–62.

Author-date reference list:

Okafor, Lena M. 2024. Rivers under the City: A History of Urban Water Supply. Meridian University Press.

Author-date in-text:

(Okafor 2024, 112)

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	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	(Okafor, 2024)	(Okafor)	Note without a page; (Okafor 2024)
Two authors	(Hallberg & Nkemelu, 2023)	(Hallberg and Nkemelu 151)	(Hallberg and Nkemelu 2023, 151)

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's appropriate-citation page warns against both undercitation and overcitation, but the two are not symmetrical: overcitation is a style problem, undercitation is plagiarism. If you cannot decide whether a fact is common knowledge for your reader, cite it. If a paragraph is starting to look like a wall of parentheses, the fix is usually to consolidate the citations, not to drop them.

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. Interpretations and estimates 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 ones.
Citing the source of a fact but not the source of the interpretation built on it.
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, it probably is not.
Reusing a paragraph from your own earlier paper without a citation. Every style treats 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.

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. Well-known dates, geographic facts, textbook definitions, and proverbs qualify. Statistics, specific findings, arguments, and exact wording do not, 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 (R. Adebayo, personal communication, March 3, 2026); posted slides are cited as a classroom source with a reference entry. MLA 9 and Chicago 18 both allow a formal works-cited or note entry for a live 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 a classroom rule of thumb, not a style-manual rule. It fails when the five sources all copy one origin, and it never covers borrowed wording or a borrowed interpretation, which need citations regardless.
Do I have to cite a paraphrase?: Yes. APA 7, MLA 9, and Chicago 18 all require a citation for paraphrased ideas; rewording changes the sentence, not the ownership. 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. If you paraphrase one source across several sentences, cite it at the first mention and again only when the source changes or a new paragraph begins. 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. Quoted wording always needs quotation marks and a citation with a locator.
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), and MLA and Chicago take the same view. Cite the earlier work as you would any source, and check whether your course allows reuse at all.
What if I am still not sure whether something needs a citation?: Cite it. An unnecessary citation costs a few characters; a missing one can cost the assignment. If you want a second opinion, ask the instructor before the deadline, and 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.

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