Turnitin's similarity report is one of the most misunderstood tools in academic assessment. Students panic at scores of 20% and feel falsely safe at 4%. The percentage alone tells you almost nothing useful. What matters is what is matched and why.
What the Percentage Actually Measures
Turnitin compares your submission against its database of student papers, published articles, and internet sources, and reports the percentage of text that matches. Matches include your references section, quoted material, common phrases, and — critically — correctly referenced paraphrases may still match if the paraphrase is too close to the original.
A score of 0% does not mean no plagiarism. It means no matches in the database — which could mean the plagiarised source was not in Turnitin's database, or the work was purchased from a "contract cheating" service that submits to the database after yours.
What Is a "Normal" Score?
Most UK universities consider 10–25% acceptable for dissertations and essays, provided the matches are from properly cited sources. The critical question is what the highlighted sections are — if they are your reference list, quoted passages, and methodological terminology, a 22% score is entirely fine. If 15% is a matched passage that is unquoted and uncited, that is a serious problem.
How to Reduce a High Score Legitimately
- Exclude the bibliography using Turnitin's filter settings
- Exclude quoted material if your institution's policy permits
- Paraphrase more genuinely — aim for sentence restructuring + vocabulary change, not just word substitution
- Check whether common phrases in your field are flagging — these can be excluded as boilerplate
The Difference Between Similarity and Plagiarism
Similarity is a technical measure. Plagiarism is an academic integrity violation — presenting someone else's work as your own. A high similarity score is not automatically plagiarism, and a low score does not guarantee no plagiarism. Examiners read the full Turnitin report, not just the percentage, and assess whether matched sections are properly attributed.
AI Detection in Turnitin
Turnitin now includes an AI writing detection feature that identifies passages likely generated by AI. It reports a percentage of text flagged as AI-generated, and this is separate from the similarity score. Universities vary in how they treat AI detection results — check your institution's AI use policy carefully before using any AI assistance in your writing.