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Diff Checker

Compare two text blocks and highlight differences.

Comparing two blocks of text to find differences is a core task for developers reviewing code changes, editors proofreading revisions, writers comparing document versions, and support teams checking updated policies. Our free Diff Checker performs an optimized line-by-line comparison and highlights additions, removals, and unchanged lines side by side.

Paste the original text in the left box and the modified text in the right box, then click Compare. The result shows two synchronized panels: removed lines are highlighted in red with a minus sign on the left, added lines are highlighted in green with a plus sign on the right, and unchanged lines show both sides normally with no highlight.

The diff algorithm is based on the classic Myers diff algorithm, which produces a minimal set of edits (longest common subsequence) between the two texts. Line-level comparison is ideal for source code, config files, and prose documents where each line represents a meaningful unit of change.

All comparison happens in your browser — your text never leaves your device. You can confidently compare proprietary code, legal documents, contracts, or personal notes. No accounts, no uploads, no history stored.

Frequently Asked Questions

What algorithm does the diff use?
An implementation of the Eugene Myers O(ND) difference algorithm, which finds the shortest edit script (minimum number of line insertions and deletions) between two texts. This is the same class of algorithm used by Git diff, GitHub PRs, and most professional diff tools.
Can it compare character-by-character or word-by-word?
The current version compares line-by-line, which works best for code, lists, and structured text. Word-level or character-level intra-line highlighting for changed lines is on our roadmap. Vote for this feature via our contact form if you need it.
Can I compare files instead of pasting text?
Paste-only for now. File upload (comparing two .txt, .json, .csv, or code files directly) is coming in a future update.
Is there a size limit?
The tool works best with files under ~50,000 lines total. Extremely large comparisons may slow the browser due to DOM rendering of thousands of highlighted spans. For large code diffs, use git diff or a dedicated desktop tool like VS Code Compare.

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