Compare two text blocks line-by-line with an LCS-based diff algorithm.
To compare texts: paste two versions of text into the panels — the tool highlights added, removed, and changed lines side by side, useful for code review and document comparison.
FreeToolHub Diff Checker is a free browser-based tool that compares two texts and highlights differences line by line, no signup, no upload.
About this tool
Compare two blocks of text or code side by side and highlight every difference line by line. Free, no signup, files never leave your browser.
What Is the Diff Checker?
The Diff Checker is a free browser-based tool that compares two blocks of text or code side by side and highlights every difference line by line. Unlike online diff tools that upload your text to a remote server for processing, this tool performs the comparison entirely in your browser using a JavaScript implementation of the Myers diff algorithm — your code, documents, and configuration files never leave your device. It handles additions, deletions, and modifications with color-coded highlighting (green for additions, red for deletions), and supports both line-by-line and word-level diff modes. The tool works with any text format including source code, markdown, JSON, YAML, and plain prose.
Who Should Use This Tool?
Software developers use this tool to review code changes before committing — pasting the original and modified versions to verify that only intended changes were made. DevOps engineers compare configuration files across environments to identify drift between staging and production settings. Technical writers use it to track changes in documentation drafts when version control is not available. Legal professionals compare contract revisions to spot modified clauses. Students use it to check their work against reference implementations. QA testers compare expected versus actual output logs to identify discrepancies. Anyone who needs to understand exactly what changed between two versions of a text file benefits from this instant visual comparison.
How to Use the Diff Checker
Paste your original text into the left panel and the modified text into the right panel. The tool highlights differences instantly as you type or paste — no submit button required. Added lines appear highlighted in green, removed lines in red, and unchanged context lines are shown in neutral gray. Use the line-level diff mode for a traditional comparison showing entire changed lines, or switch to word-level diff to see exactly which words within a line were modified. The synchronization scroll feature keeps both panels aligned as you navigate through long documents. For code comparison, the tool preserves indentation and syntax structure. All processing happens in your browser — close the tab and your data is erased.
Frequently asked
How does the diff comparison work?
It uses the LCS (Longest Common Subsequence) algorithm to identify the longest sequence of lines common to both texts. Lines present in one text but not the other are marked as additions (+) or deletions (-). Unchanged lines are shown as context. The result is displayed side-by-side or inline with color highlighting: green for additions, red for deletions.
Can I compare code files or large documents?
Yes. Paste two blocks of text or code (up to 50,000 characters each). The diff checker handles all text formats including source code, configuration files, JSON, XML, and plain text documents. It ignores no content by default—every character difference is shown. Use the inline view for a unified diff or the side-by-side view for a visual comparison.
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