What Counts as Plagiarism? Seven Versions of One Passage, Judged

Plagiarism is using someone else's words or ideas without clear credit, whether you mean to or not. Copying text, rewording a passage without citing it, and patching sentences together with a few words swapped all count. So does handing in your own old work again without permission. Quoting in quotation marks with a citation, or restating an idea fully in your own words and citing it, does not.

Rules differ between schools, journals and employers, so your own policy has the last word. Below, we take one real passage, show seven ways it could end up in an essay, mark which cross the line, and measure how much of the original each one still carries. Then come quick answers on similarity scores, permission, public-domain sources, old essays and AI.

The rule in one sentence

Harvard's Guide to Using Sources says that in academic writing it is plagiarism to take "any idea or any language from someone else" without adequately crediting the source. Three things follow from that.

• Ideas count, not only words. If the argument, finding or number came from a source, it needs a citation even when every word is yours.
• Accidents count. Harvard says the rule applies whether you plagiarize intentionally or by accident, and the University of Oxford warns that even inadvertent plagiarism can bring a penalty. Both name careless note-taking as one way it happens.
• The kind of source doesn't matter. Harvard's list includes published authors, other students and websites without clear authorship, and Oxford's definition covers all published and unpublished material, including material generated wholly or partly by AI, unless that use was authorized.

One passage, seven versions

Here is the source: three sentences from the US Environmental Protection Agency page Benefits of Trees and Vegetation (last updated May 15, 2026). The third comes two sentences later in the original, which the [...] marks.

Trees and vegetation (e.g., bushes, shrubs, tall grasses) lower surface and air temperatures by providing shade and cooling through evapotranspiration. Evapotranspiration is a process in which trees and vegetation absorb water through their roots and evaporate it through their leaves. [...] One review of 308 studies found that, on average, urban forests were 3.0° F (1.6°C) cooler than urban non-green areas.

Here are the ways it could end up in a student's essay on city heat.

Version A, copied. The three sentences pasted in as they are (without the [...]), with no quotation marks and no citation.

Version B, copied and cited. The same pasted text with "(U.S. EPA, 2026)" added at the end, but still no quotation marks.

Version C, synonyms swapped, no citation:

Trees and plants (such as bushes, shrubs and tall grasses) reduce surface and air temperatures by giving shade and cooling via evapotranspiration. Evapotranspiration is a process where trees and plants take in water through their roots and release it through their leaves. A review of 308 studies showed that, on average, city forests were 3.0° F (1.6°C) cooler than city areas without greenery.

Version D, synonyms swapped and cited. Version C with "(U.S. EPA, 2026)" at the end.

Version E, the student's own words, cited:

Shade is only part of the reason trees cool a street. Plants also pull water up from the soil and release it through their leaves, and that evaporation draws heat out of the surrounding air. Across 308 studies, urban forests averaged about 1.6°C (3°F) cooler than built-up areas with no greenery (U.S. EPA, 2026).

Version F, a short quotation, cited: the EPA reports that urban forests were "3.0° F (1.6°C) cooler than urban non-green areas" on average across 308 studies (U.S. EPA, 2026).

Version G, own words, no citation. Version E with "(U.S. EPA, 2026)" deleted.

B and D are the rows that catch people out. A citation doesn't make copied words your own, and a thesaurus doesn't turn someone else's sentences into a paraphrase.

How much of the source is still there? We measured it

To see the difference in numbers, we split each version and the source into words (numbers, unit letters and each half of a hyphenated word counted separately, citations left in) and counted two things: the share of the version's three-word sequences that also appear in the source, and the longest unbroken run of identical words. This is a rough check we ran ourselves on November 9, 2026, not the method of any commercial checker.

B, D and G aren't listed because they share A's, C's and E's wording; only the citation differs. An overlap count can't see whether you gave credit.

What stands out:

• The swapped version kept a third of its phrases and the whole skeleton. It has the same three sentences in the same order, the same bracketed list of plants and the same run of numbers. Oxford names exactly this pattern: altering a few words, changing their order, or closely following the structure of the original argument.
• The real paraphrase shares almost nothing but the facts. Its longest match is four words ("it through their leaves"). It opens with its own point, explains evapotranspiration without naming it, and rebuilds the source sentences around its own point, which is what makes it the student's writing rather than the EPA's.
• The acceptable quotation scored higher than the swapped version. Almost half of its phrases match, because it is a quotation. That is why a percentage on its own decides nothing.

"Is 20% plagiarism?" What similarity scores mean

Similarity checkers don't decide whether you plagiarized. Turnitin's own student guide to the similarity score opens its explanation with "Turnitin does not check for plagiarism." It compares your text with a database of web pages, periodicals and journals, and papers other students have submitted, and highlights what matches.

From the same guide, as of November 2026:

• Correct quotes and references still show up as matches. A cited quotation like Version F raises your score even though it is fine.
• There is no fixed acceptable number. Every school, instructor or assignment may accept a different amount of matching text. Check the syllabus or ask.
• A high score doesn't always mean plagiarism, and a low score doesn't always mean there was none. The guide gives examples, such as a paper scoring 100% because earlier drafts of the same paper were submitted to the school's repository.
• The colors are just ranges. For classic assignments in Feedback Studio, blue means no matching text, green one word to 24%, yellow 25-49%, orange 50-74% and red 75-100%; Turnitin Similarity and SimCheck swap blue and green. The guide doesn't say that any color means plagiarism.

So if you see 20% or 25%, open the report and look at each highlighted passage. Ask whether it is a cited quotation, your reference list, a common phrase, an earlier draft of your own, or copied wording you forgot to credit. Only the last one is a problem, whatever the total says.

Quick answers: "Is it plagiarism if…"

…I cite the source? Not if the copied words are in quotation marks, or if you have genuinely rewritten the passage. A citation alone doesn't fix copied or lightly edited wording (Versions B and D).

…I change a few words or reword it? Without credit, yes. With credit, it still is if the result follows the original too closely.

…I have the author's permission? Yes, it can still be. Oxford's definition covers using someone's work "with or without consent of the original author", and it separately bans submitting work someone else wrote for you, even with their consent.

…the source is public domain or has no named author? Still yes. Plagiarism is about credit, copyright is about permission to copy, and they are separate rules. An article on Southern New Hampshire University's website, quoting the director of information literacy at its library, says plagiarism is usually a matter of ethics and academic integrity rather than law, though it can lead to legal trouble if it infringes copyright. Our EPA example shows the gap: US copyright law (17 U.S.C. § 105) doesn't protect works of the US Government, so the passage is very likely free to copy under US law, yet Version A is still plagiarism.

…I reuse my own earlier essay? Often yes. Oxford calls it auto-plagiarism and bars resubmitting work you have already handed in, in part or in full, unless your course rules allow it. If you build on your earlier work, ask first and cite it.

…I use AI? It depends on the rules you are under. Oxford's definition covers material generated wholly or partly by AI, unless that use was authorized beforehand. The University of Kansas Medical Center library's plagiarism guide says using generative AI isn't plagiarism in itself, but passing off its output as your own without acknowledging the tool could be. Read your course or workplace policy, and if AI help is allowed, say how you used it. AI detector scores are estimates and can be wrong, as we covered in how accurate AI detectors are.

…it's common knowledge? Widely known facts, such as that trees give shade, don't need a citation. The 308 studies and the 1.6°C figure do: specific numbers, findings and arguments come from somewhere. When in doubt, cite.

…an idea came from a classmate? It can be. Oxford asks you to acknowledge help from fellow students and any guidance that substantially changed your content or approach; ordinary proofreading and your tutor's advice don't count. Group work has its own limits, so check them.

A five-minute check before you submit

1. Every copied phrase is in quotation marks, with the citation right next to it.
2. Every paraphrase passes the closed-book test. Put the source away, reread your sentence and ask whether its structure is yours, then check that the citation is still there.
3. Every specific number, finding or argument has a source. If you can't remember where it came from, find it again or cut it.
4. The edges are clear. One citation mid-paragraph can make the sentences after it look like yours, as an example in Harvard's guide shows. Mark where the source stops and you start.
5. Nothing is reused from your earlier work without permission and credit.
6. AI use matches the policy and is disclosed where required.
7. Get a second read. For a quick look at patched-together or boilerplate wording, our free plagiarism checker gives an originality score, a risk level and suggestions for the weak parts. Know what it isn't: it reads only the text you paste and does not compare it with the web or paper databases, so it can't tell you whether a sentence matches a source. For matching against sources, use the tool your school approves.

The bottom line

What counts as plagiarism comes down to one test: could a reader tell which words and ideas are yours and which came from somewhere else? Quote what you copy, truly rewrite what you paraphrase, cite both, and treat a similarity percentage as a list of passages to check, not a verdict.