Diff Checker

Compare two blocks of text and highlight every addition and deletion side by side.

Text A — Original

Text B — Modified

Paste two versions of text above and click Compare

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What Is the Diff Checker?

The Diff Checker is a free online tool that compares two text blocks side by side and visually highlights every difference between them. Lines that were added appear in green, lines that were removed appear in red, and lines that are identical appear in grey. It is the fastest way to see exactly what changed between two versions of any text — contract, article, code, document, or data — without reading every word manually.

The word "diff" comes from the Unix command-line utility diff, first developed in 1974. It performs a systematic comparison of two text inputs and outputs a structured description of which lines were added, modified, or unchanged. The same logic powers modern version control systems like Git, document comparison tools in Word and Google Docs, and code review platforms like GitHub. This tool brings that power to your browser with no installation required.

How to Use the Diff Checker

Using the diff checker takes about five seconds once you have both versions of your text ready. There are no settings to configure for basic comparison.

Paste the original version of your text into the left panel labelled "Text A — Original."
Paste the modified version into the right panel labelled "Text B — Modified."
Click the Compare button.
Review the results section below the panels, which shows each line annotated with its change type.
A summary bar shows the total count of added lines, removed lines, and unchanged lines at a glance.

What the Colors Mean

Green lines are additions — content that exists in Text B (the modified version) but was not present in Text A (the original). These are new lines that were inserted during editing. Red lines are deletions — content that was present in Text A but has been removed or replaced in Text B. Grey lines are unchanged — identical content present in both versions.

Common Professional Use Cases

Content Editing and Document Review

When an editor receives a revised draft, comparing it to the previous version manually by reading both documents side by side is time-consuming and error-prone. A diff checker makes every single change visible in seconds — words added, sentences restructured, paragraphs deleted, headings renamed. This is especially valuable for long documents where tracking changes by eye over 10, 20, or 50 pages would take hours.

Legal Contract Comparison

Lawyers and paralegals use diff tools to compare contract versions during negotiation. Spotting a clause change in a 50-page master service agreement by reading it twice is difficult and legally risky. A diff comparison makes every added word, deleted phrase, and modified clause immediately visible. This use case is so common that many legal practice management tools include built-in diff functionality.

Code Comparison Without Git

Developers who share code snippets via email, Slack, Notion, or Pastebin frequently need to compare two versions of a function, configuration file, or script without having access to a version control history. Pasting both versions into a diff checker gives the same information as git diff without needing a Git repository.

SEO Content Updates and Rewrites

Before and after an article rewrite for SEO purposes, content professionals use diff checkers to produce an audit trail showing what changed. This is useful for client reporting (showing what was revised), content freshness documentation, and ensuring that key information was not accidentally removed during a rewrite.

Translation and Localization

When a source document is updated after translation has begun, translators use diff tools to identify exactly which source segments changed so they only need to re-translate the modified portions rather than reviewing the entire document from scratch. This is critical for translation memory efficiency and quality control.

Academic Integrity Checks

Teachers and academic supervisors comparing a student's submission against a previous draft, or against a source text, use diff tools to detect unchanged portions that may indicate paraphrasing issues or unauthorized copying.

How the Comparison Algorithm Works

This tool performs a line-by-line comparison. Each line in Text A is compared sequentially to the corresponding line in Text B. Lines that match exactly are marked as unchanged. Lines that differ are marked as removed from A and added in B. This approach makes structural and content changes visually clear and readable. It follows the same conceptual model as the standard Unix diff algorithm, optimized for clarity in a browser interface.

Privacy

Both text inputs are processed entirely within your browser using JavaScript. No text you enter is transmitted to any server, logged, or stored anywhere. This means you can safely compare confidential legal documents, proprietary source code, sensitive business contracts, medical records, or any other private content. Close the browser tab and everything is gone immediately.

What This Tool Cannot Do

This is a text-only, plain-text comparison tool. It does not compare formatted documents with different fonts, styles, or layout — only the text content matters. It cannot process PDF or Word files directly; you need to paste the text content. It performs line-level rather than word-level comparison, so a line with one changed word will appear as a full line removal plus a full line addition rather than a word-level highlight. For word-level diffs within lines, a more specialized tool would be needed.

Frequently Asked Questions

Does the diff checker compare word by word or line by line?

This tool compares line by line. Each line in the original text is compared to the corresponding line in the modified version. If a single word on a line changed, the entire line is marked as removed from Text A and added in Text B. Line-by-line comparison is the most readable format for most content comparison use cases. For word-level diffs, you would need a more specialized tool.

Can I use this to compare code files?

Yes. Paste any code — JavaScript, Python, TypeScript, SQL, HTML, CSS, JSON, YAML, Go, Rust, or any other text-based format — and the comparison works identically. The tool does not do syntax highlighting, but all structural and content differences are clearly highlighted. This is very useful for comparing configuration files, scripts, and code snippets shared outside of a version control system.

What do the green and red colors mean?

Green lines are additions — content in Text B (the modified version) that was not present in Text A. Red lines are deletions — content from Text A (the original) that is not present in Text B. Grey lines are unchanged — identical in both versions. A modified line typically appears as one red line (old content removed) followed by one green line (new content added).

Is there a limit on how much text I can compare?

No. The tool processes text of any length in your browser. There is no server-side limit. Very large documents (tens of thousands of lines) may take a moment to compare on slower devices, but there is no hard limit on input size.

Can I compare formatted Word documents or PDFs?

Not directly. This tool works with plain text only. To compare Word documents, paste the text content from each document separately. To compare PDFs, copy the text from each PDF and paste it into the respective panels. Formatting like bold, italic, fonts, tables, and images cannot be compared — only the text content.

Is my text kept private?

Yes, completely. Both text inputs are processed locally in your browser using JavaScript. No data is transmitted to any server, stored, or logged. You can safely compare confidential legal documents, proprietary source code, sensitive contracts, or any other private content.

What is the difference between this and Git diff?

Git diff compares file versions stored in a Git repository, tracking changes over time with full history and the ability to compare any two commits. This tool is a standalone, one-off comparison: you paste two text blocks and see the diff immediately, with no repository, commit history, or setup required. It is ideal for quick comparisons of content that exists outside of version control.

Can I save or export the diff result?

Not directly from within the tool — the output is displayed on screen. You can select and copy the visible diff text to your clipboard, or take a screenshot for documentation purposes. If you need persistent records of document comparisons, a version-controlled workflow (Git, Google Docs version history) is more appropriate for ongoing tracking.