Plagiarism Checker — Compare Text for Duplicate Content (Free)
Compare two texts side by side, highlight matching passages, and compute a similarity score. Useful for citation review. Free.
About Plagiarism Checker
A plagiarism checker compares a target text against one or more reference texts (or pastes), highlights matching passages, and computes a similarity score — useful for verifying that quoted material is properly attributed, that a draft hasn't accidentally repeated source phrasing, and that two documents are genuinely independent. The ZTools Plagiarism Checker runs side-by-side comparison entirely in your browser, supports paste-vs-paste comparison (no web search), highlights matched n-grams (3-, 5-, 7-word sequences), computes the percentage of overlapping words, and exports an annotated diff for documentation.
Use cases
- Verifying a student's draft against a source paper. Teacher pastes the draft and the suspected source. Highlighted overlap shows exactly which passages match — settling "did they copy this?" disputes objectively.
- Self-check before submitting a research paper. Author pastes their draft and a reference they cited heavily. Confirms the paraphrasing is sufficient; surfaces accidental near-quotation that should be marked as a direct quote.
- Editorial review of guest posts. Editor compares a submitted article against the writer's previous published work. Catches re-purposed content that the writer should disclose as a re-edit, not original.
- Code-comment review. Compare two README files or documentation pastes — useful for confirming whether two open-source projects copied each other's docs without attribution.
How it works
- Paste your target text. The document being checked.
- Paste one or more reference texts. Sources to compare against. Tabs for adding multiple references; aggregate score includes all of them.
- Choose match sensitivity. Loose: 3-word matches (catches paraphrase tendencies). Standard: 5-word matches (catches most direct copies). Strict: 7+ word matches (only true verbatim copies).
- Read the highlighted output. Matching passages are highlighted in both texts, color-coded by reference. Non-matching text remains uncolored.
- Read the similarity score. Percentage of matched words / total words in target. Above 20% suggests substantial reuse; below 5% is normal incidental overlap.
Examples
Input: Paper A vs Paper B (well-cited paraphrase)
Output: 8% match — within normal range for paraphrased content with proper citation.
Input: Draft vs source (direct quote, no quotation marks)
Output: 24% match — likely undeclared quote; should be marked as direct citation.
Input: Two independent essays on the same topic
Output: 3% match — only common phrases ("on the other hand", "in conclusion"). Independent.
Frequently asked questions
Does this search the web for matches?
No — this tool compares pastes against pastes. For web-wide plagiarism scanning (Turnitin-style), you'd need a service that crawls and indexes the web. The ZTools tool is for known-source comparisons and self-checks.
What's a normal similarity score?
Below 5% for independent writing on the same topic — common phrases and structural cues. 5-15% for well-paraphrased content with quoted attribution. 15%+ suggests substantial reuse that needs explicit citation. >40% is essentially copy-paste.
How does the tool handle citations?
It doesn't parse citations — it shows you what overlaps. Whether the overlap is properly cited is a judgment call you make from the highlights. Tools like Turnitin try to auto-recognize quoted/cited passages; this one doesn't.
Why use this instead of a web plagiarism service?
Privacy (your text never leaves your browser), zero cost, instant results, and best when you already know the candidate source. Use a paid web service when you need broad-spectrum scanning.
Does it handle paraphrase detection?
Loose mode (3-word matches) catches some paraphrasing, but true paraphrase detection requires semantic similarity (LLM-based). The tool is best at verbatim and near-verbatim overlap.
Pro tips
- Always paraphrase AND cite — the tool can catch missing citations, but it won't fix the "I forgot to credit" omission.
- For best results, compare against the suspected single source rather than mixing many references — the score is clearer.
- Common phrases ("in conclusion", "on the other hand") create incidental overlap; ignore matches under 5 words.
- For institutional plagiarism review, use a service with web index access (Turnitin, iThenticate); use this tool for self-checks.
Reviewed by Ahsan Mahmood · Last updated 2026-05-05 · Part of ZTools.
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