Should you run your thesis through a free plagiarism checker before you submit it? It is a natural instinct: the university will check your work with Turnitin or iThenticate, and you want to know the result first. But a free plagiarism checker is not a preview of the university’s report. It searches different content, often far less of it, and the terms of use may allow it to keep the file you upload.
This article is part of our guide How to Write a Master’s Thesis by the Mastermind PhD team and relates to our Paraphrasing & Plagiarism Check service. Sources last checked: November 2026.
This guide explains the six main risks, what independent research found about text-matching tools, and safer options for checking a thesis.
What is a free plagiarism checker? A free plagiarism checker is an online service that compares text you paste or upload with sources it can reach, usually public web pages, and reports matching passages, without a subscription. Free versions often limit the number of words, hide the source list behind a paid upgrade, or do not search subscription journals and student paper repositories that institutional tools use.
Why a free plagiarism checker is not a preview of Turnitin
The number a free plagiarism checker gives you depends on what it searches. University tools compare your thesis with large collections of published scholarly content and, in Turnitin’s case, with student papers stored by institutions. Many free tools search mainly the open web. The University of North Carolina’s library guide makes the same point: many free tools use web searching rather than comparing your work with other student papers.
So a low score from a free tool does not predict a low score in Turnitin, and a high score does not prove plagiarism. Both reports measure text matching against their own collections, not academic integrity.
Six risks of using a free plagiarism checker on a thesis
| Risk | What can happen | How to reduce it |
|---|---|---|
| 1. Storage of your file | The service keeps your text in its own database | Read the terms of use and privacy policy before uploading |
| 2. Later self-matches | Your stored text appears online or in another database and matches your own thesis later | Do not upload unpublished chapters to unknown services |
| 3. Limited coverage | Journal articles behind paywalls and student papers are not searched | Treat results as partial |
| 4. False reassurance | A low score hides real problems such as close paraphrasing | Review your sources and paraphrases yourself |
| 5. Data protection | Personal or participant data in your thesis is sent to a third party | Remove confidential data, or do not upload at all |
| 6. Policy breach | Some universities restrict uploading assessed work to external services | Check your regulations and ask your supervisor |
The UNC guide adds a further warning: there are unverified reports that some free software stores and sells student papers, and you should check what a site does with the papers you upload. We cannot confirm such practices for any particular service, which is exactly why reading the terms matters.
📱 Check your similarity with a Turnitin report — Every file comes with a Turnitin report and an AI check; similar passages are reworked without changing the meaning.
What independent tests found about text-matching tools
Researchers have tested these tools systematically. Foltýnek and colleagues (2020) ran a collaborative test of 15 web-based text-matching systems, with researchers from seven countries and test material in eight languages. Their conclusion was sobering: some systems can help identify some plagiarized content, but they do not find all plagiarism and at times flag non-plagiarized material as problematic. Software, they stress, cannot determine plagiarism; it can only support a human judgment.
The same group’s later study of AI-text detectors (Weber-Wulff et al., 2023) tested 12 publicly available tools and two commercial systems, and concluded that the detectors were neither accurate nor reliable, with a bias toward classifying text as human-written. Free “AI checkers” offered alongside a free plagiarism checker deserve even more caution than similarity tools.
What a free plagiarism checker can and cannot tell you
| Question | Can a free tool answer it? |
|---|---|
| Did I copy a sentence from a public website without quoting it? | Often yes |
| Is my paraphrase of a journal article too close? | Often no, if the article is behind a paywall |
| Will my thesis match a classmate’s submitted paper? | No; free tools rarely search student repositories |
| What will my Turnitin percentage be? | No |
| Is my thesis plagiarized? | No tool can decide this; examiners judge the matches |
Used with these limits in mind, a free plagiarism checker can catch an obvious missed citation from a website. It cannot certify a chapter as safe.
Safer options for checking your thesis
Before turning to any external service, try the options your university already provides:
- A draft submission point. Many programs open a Turnitin draft assignment so students can see a report before final submission. Ask your supervisor or program administrator whether one exists.
- Your supervisor’s check. At many institutions supervisors check theses with iThenticate. UNSW Sydney, for example, expects supervisors to check their candidates’ work and discuss the report with them.
- Your library or writing center. Librarians can explain which tool the university uses, how settings such as excluding quotes and references work, and how to read the report.
- A self-audit of your sources. Go chapter by chapter: every quotation in quotation marks with a page number, every paraphrase rewritten from your notes and cited, every summary checked against the original.
- Your reference manager. Make sure every source in your notes is in Zotero, EndNote or Mendeley, so that nothing you used is left uncited.
If you choose a paid service, ask direct questions: which database it uses, whether your file is stored or deleted, how long it is kept, and who can access it. A reputable provider answers these in writing. At Mastermind PhD, files checked in our paraphrasing and plagiarism service come with a Turnitin report and an AI check, and files are retained for 6 months.
How to read any similarity result sensibly
Whatever tool you use, the interpretation is the same:
- Look at matches, not the percentage. One long match from a single source is more serious than many short matches of common phrases.
- Separate quotes and references. Correctly quoted and cited text matches by design.
- Check methods sections calmly. Standard descriptions of instruments and statistical tests often match published wording.
- Fix the cause, not the number. Rewriting to fool a tool, for example by swapping characters or synonyms, can be detected and treated as misconduct.
📱 Ask on WhatsApp now — 5 years of experience across all disciplines and 14 academic services. Two-stage payment (80/20); revisions are free until supervisor approval, within 30% of the chapter.
Frequently asked questions
Is a free plagiarism checker accurate?
It is accurate only for what it searches. A free plagiarism checker can find copied web text, but it usually misses paywalled journals and student papers, so its score is partial and cannot predict Turnitin.
Will a free checker save my thesis?
Some services state that they store uploads, and others do not. Read the terms of use and privacy policy before uploading anything, and avoid uploading unpublished chapters to services you cannot verify.
Can a free checker cause my thesis to match itself in Turnitin?
It can if your text ends up stored in a place that the university’s tool searches later. This risk is the main reason to avoid uploading full chapters to unknown services.
Is it allowed to check my thesis with an external tool?
Policies differ. Some universities restrict uploading assessed work to third-party services, especially when it contains participant data. Check your regulations and ask your supervisor first.
What score should I aim for?
There is no universal safe percentage. Examiners read where the matches are and why. Our guide to acceptable Turnitin similarity percentages explains how universities interpret scores.
Sources
- Foltýnek, T., Dlabolová, D., Anohina-Naumeca, A., Razı, S., Kravjar, J., Kamzola, L., Guerrero-Dib, J., Çelik, Ö., & Weber-Wulff, D. (2020). Testing of support tools for plagiarism detection. International Journal of Educational Technology in Higher Education, 17, 46. https://doi.org/10.1186/s41239-020-00192-4
- Weber-Wulff, D., Anohina-Naumeca, A., Bjelobaba, S., et al. (2023). Testing of detection tools for AI-generated text. International Journal for Educational Integrity, 19, 26. https://doi.org/10.1007/s40979-023-00146-z
- University of North Carolina at Chapel Hill Libraries. Plagiarism checkers. https://guides.lib.unc.edu/plagiarism-citing/plagiarism-checkers (accessed November 2026)
- UNSW Sydney, Conduct and Integrity Office. iThenticate. https://www.unsw.edu.au/assurance-integrity/conduct-integrity/research-integrity/responsible-conduct-research/IThenticate (accessed November 2026)
- Turnitin. Understanding the similarity score. https://guides.turnitin.com/hc/en-us/articles/23435833938701 (accessed November 2026)
Sources last checked: November 2026. This article is general educational guidance and does not replace your university regulations or your supervisor’s instructions. How we prepare our content.