What to cite comes down to one rule: cite anything that is not yours and not common knowledge — every direct quote, paraphrase, borrowed idea or theory, statistic, figure, table and contested claim must be attributed to its source. You do not need to cite common knowledge, widely accepted facts, or your own original analysis and findings. This guide explains exactly what to cite and what not to cite, gives you a quick-reference table, walks through the ‘common knowledge test’, shows worked examples, and points you to a free plagiarism checker so you can catch any citations you missed before you submit.
Why getting citation right matters
Citation is the mechanism that separates honest scholarship from plagiarism. When you cite, you tell your reader three things at once: that an idea came from somewhere, where they can verify it, and that you are not passing off someone else’s work as your own. Get it wrong in either direction and you create problems. Under-cite, and you risk an academic-misconduct case even when the borrowing was accidental. Over-cite — attributing things that are common knowledge or your own conclusions — and your writing reads as if you have no independent thought of your own, which weakens your argument and your marks.
Most students do not plagiarise deliberately. The far more common failure is a genuine uncertainty about where the line sits: when a fact is “well known enough” to skip the citation, when a paraphrase has drifted close enough to the original to need quotation marks, or whether a statistic everyone repeats still needs a source. This guide draws that line clearly, with a test you can apply in seconds and examples you can copy.
What you MUST cite
If material falls into any of the categories below, it needs an in-text citation and a matching entry in your reference list or bibliography. The format — author–date, footnote or numeric — depends on your style guide, but the obligation to cite does not change.
1. Direct quotations
Any time you reproduce someone’s exact words — even a single distinctive phrase — you must place them in quotation marks (or a block quote for longer passages) and cite the source with a page or paragraph number. Copying wording without quotation marks is one of the most easily detected forms of plagiarism, because matching strings are exactly what similarity software is built to find.
2. Paraphrases and summaries
Restating someone else’s point in your own words does not remove the obligation to cite — the idea is still theirs. A proper paraphrase changes the sentence structure and vocabulary substantially and carries a citation. If you only swap a few words for synonyms, you have produced “patchwriting”, which most institutions treat as plagiarism. If you are unsure whether your rewrite is far enough from the original, our guide on how to paraphrase walks through the technique, and a paraphrasing tool can help you restructure — but you must still add the citation.
3. Someone else’s ideas, theories, arguments or interpretations
Concepts, frameworks, models, classifications and original interpretations belong to whoever developed them. Maslow’s hierarchy of needs, Porter’s five forces, a specific reading of a poem proposed by a named critic — all need a citation even when you never quote a word. The fact that you have explained the idea in your own language does not make the idea yours.
4. Data, statistics and specific facts from a source
Numbers, survey results, percentages, dates and findings that you took from a particular study, report or dataset must be cited so the reader can check them. “Roughly two-thirds of UK undergraduates work part-time during term” needs the source that produced that figure. The more specific or surprising the number, the more important the citation.
5. Images, tables, figures, diagrams and media
Visual and aud-visual material is intellectual property too. Any photograph, chart, graph, map, illustration, screenshot or table you did not create yourself needs a citation — and often a copyright permission line as well. If you adapt or redraw a figure from a source, cite it as “adapted from”. Reusing a chart without attribution is plagiarism even though no words were copied.
6. Contested, surprising or field-specific facts
Some facts are not settled. Anything debatable, recently discovered, attributed to a particular school of thought, or surprising enough that a reader might doubt it should be cited so your claim is defensible. “The minimum wage reduces youth employment” is contested — cite the study you are relying on. “Water boils at 100°C at sea level” is not.
When you use another writer’s words, you must use quotation marks and cite the source. When you put the idea into your own words, you still must cite the source. The only thing that changes is the punctuation.
— Adapted from MLA and APA style guidance on quoting and paraphrasing
What does NOT need to be cited
Just as important as knowing what to cite is knowing what to leave uncited, so your work does not drown in needless references. Here are the 5 things that do not need to be cited.
- Common knowledge. Facts that an educated reader in your field already knows and that appear, uncited, across many sources — for example, that London is the capital of the UK, or that World War II ended in 1945.
- Widely known, uncontested facts. Established information no reasonable reader would dispute and that is not traceable to one original source, such as “the human heart pumps blood” or “Shakespeare wrote Hamlet”.
- Your own original findings, data and analysis. Results from your own experiment, your own survey, your own reading of a primary text, and the conclusions you draw — these are your contribution and should not be attributed to anyone else.
- Your own arguments, opinions and structure. The way you connect ideas, the thesis you build, your transitions and commentary are yours and need no citation (though the evidence you cite to support them does).
- Generally accepted, self-evident information. Common sayings, dictionary definitions of everyday words, and observations any reader could verify by simple reasoning or experience.
One caveat: “your own findings” still need a citation if you have already published them elsewhere — reusing your own previous work without acknowledgement is self-plagiarism, and many universities treat it seriously. When recycling earlier coursework or a published paper, cite yourself.
Cite vs. don’t cite: quick-reference table
Use this table as a fast lookup while you write. The right-hand column shows a concrete example of each case.
| Material | Cite? | Example |
|---|---|---|
| Direct quotation | Yes | “Knowledge is power” — must be in quotation marks with source and page. |
| Paraphrase of a source | Yes | Rewording a journal article’s argument in your own sentences. |
| Another author’s theory or model | Yes | Explaining Bloom’s taxonomy or Bowlby’s attachment theory. |
| Statistic from a study or report | Yes | “42% of respondents agreed” taken from a named survey. |
| Image, chart or table from a source | Yes | A graph copied or adapted from a published paper. |
| Contested or surprising claim | Yes | “Bilingualism delays dementia onset” — attribute to the research. |
| Common knowledge | No | “Paris is the capital of France.” |
| Widely accepted, uncontested fact | No | “Water is made of hydrogen and oxygen.” |
| Your own experiment results / data | No | The numbers your own lab work produced. |
| Your own analysis, argument, opinion | No | The conclusion you reach after weighing the evidence. |
| Your own previously submitted work | Yes* | Reusing your earlier essay — cite to avoid self-plagiarism. |
*Self-citation: only required when reusing material you have already submitted or published elsewhere.
The common knowledge test
“Common knowledge” is the slipperiest category, because it is relative — what is common knowledge to a third-year chemistry student is not common knowledge to a first-year historian. Rather than guess, run a fact through this two-part test. A fact is common knowledge (and needs no citation) only when both answers are yes:
- Is it widely known? Could you find the same fact, stated without a citation, in multiple independent general sources — textbooks, reputable encyclopaedias, mainstream references — rather than tracing it back to one specific study or author?
- Is it uncontested? Would an informed reader in your field accept it without demanding evidence, because it is settled and not a matter of debate, interpretation or recent discovery?
If either answer is “no” — the fact is specialised, traceable to a single source, surprising, debatable or recent — then cite it. The audience matters: write for the educated general reader of your discipline, not for an expert who would find everything obvious. And remember the asymmetry of risk: an unnecessary citation costs you nothing but a few words; a missing one can cost you an academic-integrity case. That is why the safest rule, and the one every style guide endorses, is the one below.
When in doubt, cite. A citation you didn’t strictly need is a minor stylistic blemish; a citation you needed and omitted is potential plagiarism.
Worked examples
Theory is easiest to absorb through real cases. Here are four common situations and the correct call for each.
✘ “The Earth orbits the Sun (NASA, 2021).” — over-citing. This is settled common knowledge; the citation is unnecessary clutter.
✔ “In 2023, global average surface temperature was about 1.45°C above the pre-industrial baseline (WMO, 2024).” — a specific, recent, sourced figure. Cite it.
Original: “Active recall strengthens memory more effectively than passive review.”
✘ Patchwriting: “Active recall builds memory more effectively than passive reviewing.” — only synonyms swapped; this still counts as plagiarism even with a citation.
✔ Proper paraphrase: “Testing yourself on material — rather than simply re-reading it — produces stronger, longer-lasting retention (Roediger & Karpicke, 2006).” — restructured, reworded, and cited.
✔ Cite: “Smith (2019) found that remote workers reported 13% higher productivity.” (Smith’s finding — cite it.)
✔ No citation: “These results suggest that flexibility, rather than location itself, may be the real driver of the productivity gain.” (Your interpretation of the data — this is your contribution and needs no citation.)
✘ Pasting a results table from a published study under your own heading with no attribution. — plagiarism, even though you typed none of the numbers yourself.
✔ Reproducing it with a caption such as: “Table 2. Adapted from Jones (2020), p. 47.” — and, for direct reuse, a copyright permission note where required.
Check your work free
Paste up to 3,000 words into our free plagiarism checker to spot unmarked quotes and missed citations before you submit — no sign-up, no storing of your work.
How to format the citations you do need
Once you have decided something needs citing, the format follows your required style. The obligation is the same; only the punctuation and ordering differ:
- Author–date styles (APA, Harvard referencing) put the author surname and year in the text — e.g. (Brown, 2021) — with a full entry in the reference list.
- Numeric and footnote styles (Vancouver, Chicago notes, IEEE, OSCOLA) use a number or footnote marker pointing to a numbered list or note.
- Source-specific rules matter: citing a website differs from citing a book, a journal article or a dataset. Match the template for the source type.
Whatever the style, every in-text citation needs a matching reference-list entry, and vice versa. For the wider mechanics of attributing sources correctly, see our overview of referencing.
Common citation mistakes to avoid
Most academic-integrity slips come from a small set of recurring errors. Watch for these:
- Citing the paraphrase but forgetting the quote. If even a few exact words survive your rewrite, they need quotation marks, not just a citation.
- “Synonym-swapping” and calling it paraphrasing. Patchwriting is a recognised form of plagiarism — restructure the sentence, do not just trade words. See how to remove plagiarism for legitimate fixes.
- Citing a source you never read. Don’t cite a study you only saw quoted somewhere else; cite the secondary source you actually used, or read the original.
- Dropping citations from figures and tables. Visuals are the most commonly forgotten — every borrowed image or table needs attribution.
- Treating AI output as citation-free. Unverified text from an AI tool is neither your own original work nor a citable source. Check your institution’s policy, verify every claim against a real source, and run the result through an AI detector if your university screens for AI-generated content. Note that AI detection is a separate question from source plagiarism — our plagiarism overview explains the difference.
Catch missed citations before you submit
Even careful writers miss the odd quotation mark or forget where a half-remembered fact came from. The reliable safeguard is to run your finished draft through a similarity check, read each flagged passage, and confirm that anything matching a source is either properly quoted and cited or genuinely common knowledge. Our free plagiarism checker handles drafts up to 3,000 words as a basic web check — useful for a quick scan. For a full, Turnitin-level similarity report (and AI-writing detection) on a longer dissertation or thesis, the paid plagiarism report gives you a detailed breakdown you can act on.
To be clear about what a checker does: it helps you find matching text so you can add the citations you legitimately owe and improve weak paraphrases. It is a proofreading-and-integrity aid, not a way to “beat” detection — reducing your similarity score should always mean better attribution and cleaner writing, never disguising borrowed work. When you cite what you should and leave common knowledge alone, a low score takes care of itself. For the bigger picture on staying on the right side of the line, see our guide to the types of plagiarism and the best paraphrasing tools for rewording sources honestly.
