The short answer
No amount of plagiarism is allowed. What a checker reports is a similarity percentage: the share of your text that matches other sources. That number includes quotations you cited correctly, so it is not a count of plagiarism. No similarity percentage is accepted everywhere.
Three things follow from that:
- The question has two answers. How much plagiarism is allowed? None. How much similarity is allowed? It depends on who you are submitting to and on what the matches are.
- A high number can be fine. A paper with many cited quotations will match a lot of text.
- A low number can still be a problem. One copied paragraph with no citation is plagiarism, even in a long paper.
Similarity is not plagiarism
A plagiarism checker finds matching text. It cannot tell whether a match is credited. Plagiarism is a judgment about credit, and only a person can make it. So the two words describe different things.
| Similarity | Plagiarism | |
|---|---|---|
| What it is | The share of your text that matches text in other sources. | Using someone else’s words or ideas without giving credit. |
| Who decides | Software. It counts matching words. | A person, such as your instructor or editor, under a written policy. |
| A quotation with quotation marks and a citation | Counted. It is matching text. | Not plagiarism. |
| An idea you took and put fully in your own words, with no citation | Often not counted. The words do not match. | Plagiarism. |
| Can it be zero? | Rarely, in a paper that uses sources. | Yes, and it should be. |
This is why the same percentage can describe an honest paper and a dishonest one. For what counts as plagiarism, see what is plagiarism? and the types of plagiarism. For how the matching itself is done, see how plagiarism checkers work.
Why there is no universal number
No body sets one percentage for everyone. Each school, journal and employer writes its own rules. Some give a guideline for when a report gets a closer look. Some decide not to use a number at all.
One example of the second kind is the London School of Economics. Its policy on the use of Turnitin (version 1.1, May 2024) says that no similarity score threshold should be used within its departments to decide when to investigate a piece of work. The policy gives its reasons:
- It describes the tool as a text-matching tool, not a plagiarism detection tool. The tool shows how much text matches. It cannot say what the matches mean.
- It says a high score is sometimes the result of lots of tiny matches, none of which point to misconduct, and that a lower score may still be a case to look into.
- It states that “there is no ideal overall similarity score (%), nor any % below which an assessment is likely to be plagiarism-free and above which it is not.”
- It adds that the idea of a threshold can lead students to focus on getting under a number instead of on the quality and originality of their work.
That is one university’s policy, written for its own staff and students. Yours may be different. A number from another school, another country or a forum post does not apply to you.
There are also practical reasons a single number cannot work:
- Assignments differ. A literature review quotes and names many sources. A personal reflection uses few. The same score means different things in each.
- Tools differ. Each checker searches different sources, so one paper gets different scores.
- Settings differ. Some reports count quotations and the reference list. Others leave them out.
When a high score is harmless
A score is high when a lot of your text matches other text. Much of that matching can be honest. These are the usual causes:
- Quotations. Words inside quotation marks, with a citation, still match the source word for word.
- The reference list. A correctly formatted reference looks the same in every paper that cites that source.
- Common phrases. “The results of this study suggest that” appears in thousands of papers.
- Technical terms and names. The name of a law, a method, a book or an organisation cannot be worded another way.
- Set text. An essay question, a template heading or a cover sheet that the whole class uses.
None of these needs fixing. If a match is a quotation, check that it has quotation marks and a citation. If it is a reference, check it is complete. Then leave it. Do not reword honest, cited work to move a number.
One caution. A paper made mostly of quotations is not plagiarism if every one is cited, but your instructor may still mark it down, because little of the thinking is yours. That is a question about the quality of the work, not about honesty.
When a low score is still a problem
A low score means little of your text matched what the checker searched. It does not mean every source was credited.
- One uncited copied paragraph. In a 3,000-word paper, a copied paragraph of 150 words is 5% of the text. The score is low. It is still plagiarism.
- Ideas without matching words. If you took an argument from a source and wrote it in your own words, nothing may match. It still needs a citation. See is paraphrasing plagiarism?
- Sources the checker cannot search. A printed book, a paper behind a login or a classmate’s essay may not be among the sources a tool checks.
- Matches in one place. A small share spread across the paper is different from the same share sitting in one section. The LSE policy linked above gives this example: 10% may be an insignificant amount of small matches, or a cause for concern if it is all in the conclusion of a short essay.
So a low number is a good sign, not a clearance. For what these cases look like on the page, see our plagiarism examples.
How to read your own report
Start with the matches, not the percentage. For each highlighted passage, ask three questions.
- What does it match? Open the source. A report that links every matching passage to its source lets you compare the two side by side.
- Is it credited in your paper? Exact words need quotation marks and a citation. A reworded idea needs a citation.
- If not, what does it need? Add the quotation marks, add the citation, or write the point properly in your own words and cite the source.
When every match is either credited or common wording, the paper is in good shape, whatever the number says. When one match is not credited, the paper has a problem, whatever the number says.
The Plagiarism Checker Plus report links every matching passage to its source, so you can answer these questions match by match. If you need to build a reference, the citation generator can help. Two other guides go further:
- How to check for plagiarism has the full method and a table of what to do with each kind of match.
- How to interpret a plagiarism report covers how a teacher reads the same report.
How to find the rule that applies to you
The rule you need is local. Look for it in this order:
- The assignment brief and the syllabus. If your instructor uses a guideline, it is often written here.
- Your school’s academic integrity policy. It defines plagiarism for your institution and says how reports are used.
- The journal’s author guidelines, if you are submitting for publication. Editors describe their own screening there.
- Your instructor, supervisor or editor. If nothing is written down, ask how they read a similarity report. It is a normal question.
If you are given a number, treat it as the point where someone looks more closely, not as a pass mark. Being under it does not make uncited text acceptable. Being over it with cited quotations is something you can explain.
The percentage on an AI report is a separate matter. For that, see what percentage of AI is acceptable?