How do plagiarism checkers work?

A plagiarism checker uses advanced database software to scan for matches between your text and existing texts. There are many plagiarism checker tools on the market, and they vary widely in features and in quality.

When you upload your text, the checker scans it and provides you with a percentage figure. This figure shows how similar your text is to existing texts in the database.

High-quality checkers will also provide you with a report, and link each instance of potential plagiarism with a source. From here, it’s easy to fix potential instances of plagiarism and submit your work with confidence.

How plagiarism checkers identify similarities

Plagiarism checker software crawls web content and indexes it, scanning your text for similarities against its database of existing content on the Internet.

Exact matches are highlighted using keyword analysis. High-quality checkers have also developed algorithms to identify non-exact matches, for content that has been paraphrased or altered but could still be considered plagiarism.

Interactive Scribbr sample report

Database size affects results

Not every plagiarism checker has access to the same database. This can lead to major differences in results.

Free plagiarism checkers often have smaller databases. This means that there are large gaps in their ability to find matches, especially with less readily available online content. The highest-quality plagiarism checkers have larger databases, which enhance their ability to find matches and ensure you are not plagiarizing accidentally.

Scribbr’s Plagiarism Checker has access to Turnitin’s comprehensive database. In addition to web sources, this database also contains scientific publications, scholarly journals, and books.

Quality of plagiarism software

The quality of plagiarism checkers available on the market varies widely. Many free checkers only recognize exact matches. If you have paraphrased a bit too closely or forgotten to add a citation, these checkers are unlikely to flag it.

High-quality plagiarism checkers use a process called “fingerprinting” to find non-exact matches among paraphrased or altered texts. Here, the software scans sentence fragments, searching for structural similarities. Just like a real fingerprint, each fragment in your text should be completely unique, and should not match existing document fingerprints. If there are matches, these checkers are able to identify them.

Example text

A match is found when several words in a sentence match.

Free plagiarism check
  • A match is found when several words in a sentence match.
  • A match is recognized from some matching words in a sentence.
High-quality plagiarism check
  • A match is found when several words in a sentence match.
  • A match is recognized from some matching words in a sentence.

To help you choose the best tool, we conducted in-depth research to identify the top plagiarism checkers in 2021.

What plagiarism checkers can’t identify

Although plagiarism checkers are consistently evolving and improving, they can’t recognize everything quite yet.

Ideas and non-text plagiarism

Plagiarism checkers often struggle to identify plagiarized ideas if they have been rewritten, as well as other non-text plagiarism, such as plagiarism of images or artwork.

Plagiarism of translated texts, ideas, images, or non-text content is still considered plagiarism, and has the same consequences.

Internal databases at educational institutions

Educational institutions often use plagiarism checkers to ensure students aren’t plagiarizing. In addition, most educational institutions also have their own internal database with uploaded work from current and former students.

This database is often not shared with outside parties. This means that plagiarism from fellow students will probably only be recognized by your educational institution, and not by a commercial plagiarism checker.

However, sometimes educational institutions share access to their internal database with other institutions, in order to better flag plagiarism or other types of academic dishonesty among students. Self-plagiarism or resubmitting a friend’s work as your own still counts as plagiarism, and is subject to the same consequences.

If you’re concerned about accidental plagiarism from unpublished or private documents, such as a previously-submitted paper or a friend’s thesis, Scribbr’s Own Sources Checker can help. It works similarly to standard plagiarism checkers, but you can submit documents not available on the Internet that may or may not be in your educational institution’s database.

Scribbr Citation CheckerNew

The AI-powered Citation Checker helps you avoid common mistakes such as:

  • Missing commas and periods
  • Incorrect usage of “et al.”
  • Ampersands (&) in narrative citations
  • Missing reference entries

Learn more

Interpreting your plagiarism score

Once you have submitted your text to a tool like Scribbr’s, you will receive a report. It highlights potential instances of plagiarism in your text, links them to identified sources, and gives you an overall percentage score.
Interpreting your Scribbr plagiarism score

How high can your plagiarism score be?

Your work should not contain any plagiarism, as it can have serious consequences for your academic and professional aspirations.

However, it is important to note that plagiarism checkers do not detect plagiarism, but rather similarities. Not all similarities necessarily constitute plagiarism.

Our checker can sometimes flag the following:

  • Properly-cited quotes
  • In-text citations or your reference list
  • Commonly-used phrases that are not considered plagiarism

Because of this, you can generally follow these guidelines:

  • A score higher than 10% is troublesome.
  • A score between 5% and 10% is concerning.
  • A score around 5% is reasonable.

Resolving flagged similarities

As you work through the report, review each match individually to determine whether or not plagiarism has occurred.

You can ignore the match if you are confident that you have:

  • Correctly quoted or paraphrased
  • Cited the source you used in footnotes or in-text citations

You can also ignore any phrases that are so common they can’t be attributed to any specific source. But if you see that a complete, correct citation is missing or that you have paraphrased too closely, make the appropriate adjustments to avoid committing plagiarism and resolve the issue.

Some plagiarism scanners offer additional help, such as a citation tool that helps you generate the correct citations. For example, Scribbr offers a free Citation Generator for APA and MLA styles, to make sure your citations are perfect on the first try. You can also use Scribbr’s APA Citation Checker service for extra peace of mind.

Quoting correctly

In order to quote correctly, be sure to correctly cite the original source, and that the text within quotation marks is identical to the original.

Original text
“Arts and culture undoubtedly flourished in the ’20s as a shared American pop culture emerged thanks to the advent of radio broadcasting, widely circulated magazines and movies” (Thulin, 2021).
Quoted incorrectly
In the 1920s, arts and culture undoubtedly flourished in the U.S. due to the advent of radio broadcasting, widely circulated magazines and movies.
Quoted correctly
In the 1920s, “arts and culture undoubtedly flourished” in the U.S. due to “the advent of radio broadcasting, widely circulated magazines and movies” (Thulin, 2021).

Paraphrasing correctly

When paraphrasing, you must entirely rewrite the passage you are referencing without changing the meaning of the original text, citing the original source.

Original text
“Plastics harm wildlife in myriad ways, many of which scientists are just beginning to grasp. When birds, fish and other larger animals eat plastics, the material can get tangled up inside their bodies and cause damage; plastics can also make animals feel falsely full, so they stop eating” (Dzombak, 2021).
Paraphrased incorrectly
Plastics hurt animals in a lot of ways, many of which scientists are just starting to understand. When birds, fish and other bigger animals swallow plastics, the substance can get tangled up inside their stomachs and cause harm; plastics can also make animals feel artificially full, so they cease eating.
Paraphrased well
Scientists are still learning the extent to which plastics harm animals. According to Dzombak (2021), ingesting plastic can lead to internal damage if it gets tangled when swallowed, and can also lead animals to feel falsely full. Both prevent them from getting the nutrients they need.

Frequently asked questions

How is plagiarism detected?

Plagiarism can be detected by your professor or readers if the tone, formatting, or style of your text is different in different parts of your paper, or if they’re familiar with the plagiarized source.

Many universities also use plagiarism detection software like Turnitin’s, which compares your text to a large database of other sources, flagging any similarities that come up.

It can be easier than you think to commit plagiarism by accident. Consider using a plagiarism checker prior to submitting your paper to ensure you haven’t missed any citations.

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 recognizes 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).

What is an acceptable percentage of plagiarism?

Your work should not contain any plagiarism. This means that even a score of 1% is too high.

However, contrary to popular belief, plagiarism checkers work not by detecting plagiarism, but similarities. Not all similarities found by the Scribbr Plagiarism Checker constitute plagiarism. Our check sometimes flags the following:

  • Properly cited quotes,
  • In-text citations or your reference list, and
  • Commonly used phrases that are obviously not considered plagiarism.

Because of this, you can generally follow these guidelines:

  1. A score higher than 10% is troublesome.
  2. A score between 5% and 10% it is not bad per se.
  3. A score around 5% is reasonable.

Even if your score is 1%, you will need to review each similarity and decide whether it’s necessary to revise your work.

What should I do with a found similarity?

Should I check my paper for plagiarism before submitting it to my instructor?

If you’ve correctly cited all the sources you used, then you do not need to use a plagiarism checker before submitting your paper to your instructor. However, it is very easy to commit plagiarism accidentally, even if you’ve been very careful. To ensure that you didn’t forget to cite anything, then you should use a plagiarism checker yourself.

plagiarism checker works by using advanced database software to scan for matches between your text and existing texts.

To help you decide which checker to use, we conducted in-depth research comparing popular plagiarism checkers to find out which one is best.

Best plagiarism checker comparison

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Tegan George

Tegan is an American based in Amsterdam, with master's degrees in political science and education administration. While she is definitely a political scientist at heart, her experience working at universities led to a passion for making social science topics more approachable and exciting to students. A well-designed natural experiment is her favorite type of research, but she also loves qualitative methods of all varieties.