AI Plagiarism Checker — free online duplicate content & paraphrase detector

Plagiarism is not one behaviour but several, and a similarity check helps most with the kinds that leave a textual trail.

• Direct (verbatim) plagiarism copies a passage word for word. It produces the highest similarity scores and the clearest highlighted matches.
• Mosaic or patchwork plagiarism stitches phrases from a source into your own sentences while swapping a few synonyms. Simple word-overlap tools miss it; semantic comparison still flags the borrowed structure.
• Paraphrasing plagiarism rewrites a source in new words but keeps its meaning and order without credit. This is the category sentence embeddings are built to catch.
• Self-plagiarism reuses your own earlier work, such as a past essay or a published article, without disclosure. Paste the old piece as the original text and your new draft as the suspect text to see the overlap.
• Accidental plagiarism comes from forgotten citations or sloppy note-taking rather than intent, but it is usually judged the same way.

Because this tool compares only the two documents you paste, it is ideal for checking a draft against a specific source you already have: a reading, an interview transcript, a competitor's article, or a previous version of your own writing.

A plagiarism or duplicate content checker asks whether your text matches another specific text. An AI content detector asks whether writing looks machine-generated, based on statistical patterns such as unusually even word choice. They are not interchangeable.

This tool is a similarity checker. It measures how closely the two documents you paste resemble each other and highlights copied or paraphrased sentences. It does not estimate whether either text was written by a person or a language model, and it does not search the web or any source database. It only knows the two texts you give it.

That distinction matters in practice. An AI model can produce writing that is completely original by a similarity measure yet still reads as machine-written, so a duplicate check will pass it. Conversely, a person can plagiarise heavily and trip a similarity check while an AI detector sees nothing unusual. If you need to confirm that a rewrite is genuinely different from its source, a similarity comparison like this one is the right tool. If you need to judge authorship, that is a separate task with separate limits and a far higher false-positive risk.

There is no universal cutoff where a match becomes misconduct; the threshold is set by your context, not by the tool. In academic settings, many instructors treat total similarity under 10-20% as normal, scrutinise 20-40% more closely, and investigate anything higher. For web publishing, editors usually want near-zero overlap with the source they are checking against.

Three things inflate a score without meaning you copied anything:

• Quotations and citations. Correctly attributed quotes still register as matching text.
• Common phrasing. Fixed expressions, technical terms, and standard definitions recur across writers by necessity.
• Reference lists and boilerplate. Bibliographies and templated sections match by design.

The reverse is also true: a low percentage does not guarantee originality if a handful of key sentences are lifted verbatim. That is why this tool separates copied highlights from paraphrased ones and reports sentence-level detail, so you can read each flagged passage in context instead of reacting to a single number. Judge the highlighted sentences, confirm each is attributed or reworded in your own words, and treat the overall percentage as a signal to look closer rather than a confession.

Swapping synonyms while keeping a source's sentence order is the single most common cause of accidental plagiarism, and it is exactly what a paraphrase checker based on sentence embeddings is designed to expose. To rewrite safely:

• Read, then look away. Close the source and write the idea from memory, so your sentence structure is your own rather than a shadow of the original.
• Change the frame, not just the vocabulary. Reorder the argument, combine or split sentences, and shift the point of view.
• Cite anyway. A paraphrase still borrows the idea, so it needs a citation even when no words match.
• Quote verbatim when the exact wording matters instead of lightly disguising it.

To pressure-test a rewrite, paste the original passage as the first text and your paraphrase as the second. If sentences light up as paraphrased and the similarity stays high, your version is still too close; rework the flagged lines and run it again. A rewrite that expresses the same point through a genuinely different structure will score low while preserving the meaning, which is the outcome you want before you cite the source and move on.

Astra's AI Plagiarism Checker is a free, ad-free, no-signup tool that compares two texts and detects copying, paraphrasing and systematic rewriting directly in your browser. It runs on top of TensorFlow.js and the Universal Sentence Encoder — nothing is uploaded.

How the score is built

• Sentence-level matches — each suspect sentence is embedded and compared to every original sentence; structural penalties handle large length and word-count differences.
• Topic similarity — embeddings on extracted key terms.
• Semantic similarity — embeddings on stop-word-stripped content.
• Coverage — share of original chunks with a strong match in the suspect text.
• Structure — paragraph counts and average chunk-length similarity.
• Classic metrics — Jaccard set overlap, cosine of term-frequency vectors, normalised Levenshtein distance.

Score bands

• 95–100% — direct copying, almost identical text.
• 90–95% — direct copying with light edits.
• 85–90% — plagiarism by minor rephrasing.
• 75–85% — heavier rephrasing.
• 60–75% — systematic rewriting / "spinning".
• 50–60% — partial reuse / similar topic.
• < 50% — generally original.