Are plagiarism checkers accurate?

The accuracy depends on the plagiarism checker you use. Per our in-depth research, Scribbr is the most accurate plagiarism checker. Many free plagiarism checkers fail to detect all plagiarism or falsely flag text as plagiarism.

The accuracy is determined by two factors: the algorithm (which recognises the plagiarism) and the size of the database (with which your document is compared). Plagiarism checkers work by using advanced database software to scan for matches between your text and existing texts.

Size of the database

Many free plagiarism checkers only check your paper against websites – not against books, journals, or papers previously submitted by other students. Therefore, these plagiarism checkers are not very accurate, as they miss a lot of plagiarism.


Most plagiarism checkers are only able to detect ‘direct plagiarism‘, or instances where the sentences are exactly the same as in the original source. However, a good plagiarism checker is also able to detect ‘patchwork plagiarism‘ (sentences where some words are changed or synonyms are used).