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Turnitin: what the similarity score really is

Almost everything people believe about turnitin is wrong in the same way, and the company corrects it in the first line of its own guide for students.

“Turnitin does not check for plagiarism. What we actually do is compare your submission against our database.”

Turnitin’s own guide for students, read 21 August 2026. Everything that follows on this floor is a consequence of that sentence — including why a correctly quoted, properly referenced essay still comes back with matches.

The score is not a plagiarism percentage. Turnitin explains the arithmetic: “The similarity score is the percentage of matched text your submission contains, or overall similarity. We calculate this by dividing the total words in a submission by the amount of words matched to outside sources.” Matched, not copied.

What the colours mean, in Turnitin’s own words

Blue — no matching text
Green — one word to 24% matching text
Yellow — 25–49% matching text
Orange — 50–74% matching text
Red — 75–100% matching text

A colour is not a verdict. Turnitin states that “there is no fixed number to receive as a score”, and that the acceptable amount differs by school, instructor and assignment.

It is worth knowing what this floor is not. A plagiarism checker you choose is a private check on your own writing before you hand it in. This is the opposite: a report generated by your institution, that you often cannot see, feeding a percentage to somebody who will decide something about you. Understanding how does turnitin work is therefore not a technical curiosity — it is the difference between defending yourself with evidence and arguing with a number.

That is why a scrupulously honest essay comes back with matches. The company says so outright: “If you’ve used quotes and references correctly, that will still be highlighted as a match.” Quoting properly does not lower the number. It raises it.

Where to start

Five ways in.

“My score is high and I did not copy anything.”
Start at reading your score
“My score is 100% and I wrote every word.”
There is a documented reason — when the number lies
“What number am I supposed to be under?”
There is no such number — reading your score
“Does re-using my own work count?”
That is the grey areas
“I am the instructor looking at this report.”
Read for instructors

Reading your score

What the percentage is, what the colours mean, and why the question people ask about it has no answer. All of it quoted from Turnitin’s own student guide.

What the percentage measuresTotal words divided by words matched to outside sources — matched, which is not the same as copied.Being built
The colour bandsBlue, green, yellow, orange, red — and why none of them is a judgement.Being built
There is no target numberTurnitin: “There is no fixed number to receive as a score… Every school, instructor, or assignment could have a different amount.”Being built
Quotes and references still matchDoing citation properly raises the score, and the company says so plainly.Being built
Why zero is not the goalTurnitin: “Zero similarity may indicate that you aren’t including any source-based evidence.”Being built

When the number lies

The documented cases where the score is badly misleading, taken from Turnitin’s own list of scenarios. Two of them can put an innocent student at 100%.

Your own earlier draftSubmitting several drafts to a private repository can leave the final one matching itself at 100%.Being built
Somebody submitted your work firstTurnitin’s own scenario: the person who copied scores 25%, and the author who wrote it scores 100%.Being built
How collusion is actually foundA final check run against every submission after the deadline, so timing does not decide who is suspected.Being built
A rounded zeroA long document can show 0% while the report still contains matches, because the figure was rounded.Being built
High score, no plagiarismTurnitin: a high score “does not always suggest that a piece of writing has been plagiarized”, and a low one does not clear it.Being built

The grey areas

The questions that come up once the arithmetic is understood, and where the answer depends on your institution rather than on the software.

Self plagiarismRe-using your own earlier work, why it matches, and why the rules on it differ so widely.Being built
Common phrases and method sectionsThe parts of academic writing that are supposed to look the same as everybody else’s.Being built
BibliographiesWhy a reference list matches almost entirely, and whether it should be excluded.Being built
Group workShared sections, shared data, and agreeing in advance who submits what.Being built

For instructors

Reading a report as evidence rather than as an answer, and the policies that survive being challenged.

What the report is forTurnitin describes it as “a tool to determine if the matches are acceptable” — a judgement, not an output.Being built
Setting expectations firstTelling students what is acceptable before the deadline, which prevents most of these conversations.Being built
The conversation about a high scoreStarting from the matches themselves rather than from the percentage.Being built
Similarity and AI are separate reportsTwo different measurements, constantly confused, with different error rates.Being built

What this tower will not do

It will not tell you what a safe percentage is. Turnitin says there is no such number and that every institution differs; any page giving you a figure is inventing it.

It will not treat the similarity score as a plagiarism finding. The company that sells it opens its own student guide by saying it is not one, and that sentence is the reason this floor exists.

And it will not help anybody defeat it. The floors here explain what the number measures so that honest work is not mistaken for dishonest work — which is a different project from teaching people to hide.

One structural note, since it decides how the rest of this tower is arranged. The similarity report and the AI writing report are two separate measurements with two separate error rates, produced by the same company inside the same interface. They are confused constantly, including by people acting on them, and the turnitin percentage somebody quotes in an email is frequently not the one they think it is. Each has its own floor here for that reason. What holds instead is simple: every quotation on this floor comes from Turnitin’s own guide for students.

Where this page got its facts

  1. Turnitin’s guide for students on understanding the similarity score — what it compares, how the percentage is calculated, the colour bands and the scenarios where the number misleads — guides.turnitin.com, read 21 August 2026.

Written by Alberto Gulotta

Founder and editor of AI Tools Primer, writing from Palermo, Italy. Thirty-five years of taking computers apart, starting with a Commodore 64 — the long version is on the about page.

Something wrong on this page? Write to aitoolsprimer@gmail.com and it gets fixed.

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