Direct answer
Direct Answer - Turnitin checks a submitted document against three main comparison sets: its internet archive (current and archived web pages), a repository of publications and periodicals, and its student paper repository of previously submitted work [1]. It then highlights any text in your document that matches those sources in a Similarity Report, expressed as a percentage of matched text rather than a plagiarism verdict [1]. Turnitin's own guidance stresses that a high similarity score is a signal to inspect matches, not proof that a student cheated [2].
What Sources And Databases Does Turnitin Compare My Paper Against?
Turnitin's comparison set is broader than most students assume. It draws on three source categories — internet sources, publications, and student papers — and each match in the report links back to the original text so the reader can judge it in context [2]. That means a phrase lifted from a decades-old journal article, a Wikipedia page, and a classmate's essay from last term can all appear as separate, labelled matches in the same report [2].
The percentage you see is not a measure of intent or guilt. It is simply the proportion of your document that overlaps with something in the comparison set, which is why a paper with heavy, correctly cited quotation can legitimately score high [1]. Turnitin itself recommends reading each match individually rather than reacting to the headline number [2].
Because the comparison set is continuously updated, the same draft can produce a different score weeks apart. New web pages get crawled, new publications are indexed, and new student papers enter the repository, so a match that did not exist last semester may exist today [2]. This is why a score is always a snapshot of one submission at one moment, not a permanent property of your writing [1].
Does Turnitin Flag My Own Previously Submitted Work, And How Does The Student Paper Repository Work?
Yes — this is the part that surprises students most. The student paper repository stores submitted papers specifically so that future submissions can be compared against them, which means your own earlier draft can be matched against your later one [3]. If you submitted a draft to a class in a previous term and the assignment was set to add to the repository, that text is now part of the comparison set [3].
Whether your paper is added to the repository is a setting, not a fixed rule. Instructors and institutions decide per assignment whether a submission is stored in the standard repository or merely checked against it, and papers can be excluded from the repository entirely if that option is configured [3]. So two students submitting identical text to two differently configured assignments can get very different long-term outcomes [3].
Self-plagiarism is therefore a real risk, and it is invisible unless you know the repository setting. Reusing paragraphs from your own graded coursework can register as a match against a student paper — your own — and your instructor will see the match labelled as a student submission [3]. If you plan to build on prior work, cite it the same way you would cite anyone else's [3].
How Can I See Exactly What Turnitin Would Flag In My Draft Before I Submit It?
The practical answer is to look at a Similarity Report on your own draft before the graded submission. A similarity report shows each matched passage with its source attached, so you can revise the specific sentences that triggered a match instead of guessing [4]. Reviewers routinely exclude quotations, the bibliography, and very small matches when interpreting a score, which is useful context for judging how your own draft will be read [4].
Reading the report yourself also teaches you what your discipline treats as a genuine match. A methods sentence that mirrors a standard protocol reads very differently from three uncited paragraphs copied from a website, and only the highlighted report makes that distinction visible [4]. Fixing an accidental match before submission is far cheaper than explaining it afterwards [4].
The catch is that the institutional Similarity Report is normally generated only when a paper is submitted to the assignment — which is exactly the moment you wanted to avoid. A pre-submission check against the same comparison sets closes that gap: you see the internet, publication, and student paper matches on your draft while you can still edit it [1][2].
If you want that same clarity before your deadline rather than after it, turnitin0 gives you a pre-submission preview of the reports your instructor will actually see — the AI writing report and the similarity report, delivered together.
※ Turnitin0.com - Actual Turnitin AI Report Cover, Score, Flag And Similarity Summary
FAQ
Does Turnitin check against every website on the internet?
No. It compares against the pages it has crawled and archived, which is a very large but not exhaustive index [2]. A match only appears if the text exists in a source Turnitin has indexed at the time of submission [1].
Will Turnitin flag a quote I cited properly?
It may still appear as a match, because the report flags text overlap regardless of citation [1]. Instructors typically exclude quotations and bibliographies when they interpret the score, so a cited quote is normally treated as legitimate [4].
Can I be matched against my own previous essay?
Yes, if that essay was added to the student paper repository [3]. Repository inclusion is an assignment-level setting, so the same paper may be stored in one course and excluded in another [3].
Does a high similarity score mean I plagiarised?
No. The score is the proportion of text matching the comparison set, and it can be high for correctly cited work [2]. Turnitin states the report is a tool for interpretation, not a verdict [1].
How do I know what my score will be before submitting?
Run a pre-submission check against the same comparison sets so you can see the matches and revise them while the draft is still yours to edit [4].