Free Plagiarism Checker Online
Paste text or upload a file and instantly see similarity percentage, highlighted matches, and matching source URLs. Free local checks need no login, and your text never leaves your browser.
Last updated: 2026-08-11 · Privacy
What is a plagiarism checker?
A plagiarism checker compares your writing against other published content to find exact or near-exact matches. It highlights the copied sections and shows you the source so you can fix them before submitting or publishing.
CopyCheck Online fingerprints your text in the browser (shingle hashing), compares it against an embedded corpus, then optionally searches the live web via Tavily and UAPI for overlapping pages.
By the numbers
- 1Free tier: 20 local checks + 3 web checks per month, no login required.
- 2Local check runs 100% in your browser — text is never uploaded to a server.
- 3Web deep-check compares against live web pages (search + full-page crawl).
- 4Supports Chinese & English, mixed-language text.
- 5AI rewrite (DeepSeek) turns flagged paragraphs into original text.
CopyCheck vs traditional checker
| CopyCheck Online | Traditional tools | |
|---|---|---|
| No signup for free check | Yes | Usually required |
| Text stays in browser | Yes (local) | Uploaded to server |
| Web source URLs shown | Yes | Often hidden |
| AI rewrite included | Yes (DeepSeek) | Separate paid tool |
| Free monthly quota | 20 local + 3 web | Often 0 or tiny |
How to check plagiarism in 3 steps
- 1Paste your text
Paste up to 10,000 characters, or upload TXT / MD / PDF. Both Chinese and English are supported.
- 2Click Free Check
The browser compares your text against the embedded corpus instantly — no account, no upload.
- 3Read the report
See the similarity %, highlighted matches, and source URLs. Click AI Rewrite to fix overlaps.
Example report
Artificial intelligence is rapidly reshaping how we create and produce content. We believe algorithmic transparency and data privacy deserve serious attention.
Similarity 68% · 2 matches · Matched: "Artificial intelligence is rapidly reshaping" → source: news.example.com · Matched: "algorithmic transparency" → source: blog.example.org AI Rewrite: 1 suggested fix ready
FAQ
Is CopyCheck really free?
Yes. Free Check (local corpus + live web search) needs no login: 3 free searches per day for guests, 10 per day for logged-in users. Paid credits unlock deep full-page crawling.
Can it detect paraphrased text?
Yes — shingle fingerprinting catches near-duplicate and lightly rewritten passages, not just verbatim copies.
Is my text stored?
No. Local checks run in your browser and raw text is never uploaded. Only check summaries and source matches are kept for your history.
Does it work for Chinese text?
Yes. The engine tokenizes both Chinese and English, and mixed-language documents are supported.
Sources
- · Detection engine: shingle fingerprinting + FNV-1a hashing + Jaccard similarity (in-browser, lib/engine.ts)
- · Web search: Tavily Search API & UAPI (server-side, keys never exposed)
- · AI rewrite: DeepSeek chat completions (server-side)