Diff Checker

Paste the original text into the left box and the changed version into the right box, then click Compare. Lines that were removed show in red with a minus sign, added lines show in green with a plus, and unchanged lines stay plain.

Yes — the comparison runs entirely in your browser with JavaScript. Neither text is transmitted, stored, or logged anywhere. You can disconnect from the internet after loading the page and the tool keeps working. That's an important difference from most online diff sites, which process your text server-side.

A diff (from 'difference') is the standard way to show changes between two versions of a text: a minimal list of lines removed from the old version and lines added in the new one. The format comes from the Unix diff program (1974) and powers code review, version control, and document comparison everywhere.

Most diff tools, including this one, find the Longest Common Subsequence (LCS) — the largest set of lines that appear in both texts in the same order. Everything in the old text that isn't part of the LCS is 'removed'; everything in the new text that isn't is 'added'. This produces the minimal, most readable set of changes.

Copy the text out of each document (Ctrl+A, Ctrl+C) and paste into the two boxes. Formatting (bold, fonts) is not compared — only the text content. For tracked formatting changes, Word's built-in Review → Compare feature is the right tool; for pure text changes, a diff is faster and clearer.

Yes — paste the two versions and compare. It's line-based, the same granularity as git diff's default output. For whole projects with many files you'll want proper version control, but for a quick 'what changed between these two files' check, pasting is faster.

This tool compares at line level: if any character in a line differs, the old line shows as removed and the new one as added. Line-level diffs are the standard for code and structured text. Word-level (inline) diffs exist but get noisy for large texts — reading the red/green line pair side by side shows the changed word quickly.

Minus (−, red) marks lines that exist in the original but not in the changed version — they were deleted or modified. Plus (+, green) marks lines that exist only in the new version — added or the modified replacement. A modified line appears as a −/+ pair.

The tool handles thousands of lines comfortably. Very large pairs (where lines-in-A × lines-in-B exceeds ~4 million) are declined to keep your browser responsive — split the comparison into sections if you hit that.

A diff compares two specific texts you provide and shows literal line changes. A plagiarism checker searches one text against a huge corpus (the web, academic databases) for similar passages. If you already have both texts, a diff gives an exact, deterministic answer.

Yes — a line with an extra space or a tab instead of spaces is a different line, which is often exactly what you need to find in code and config files (indentation bugs, trailing spaces). If you want to ignore whitespace, normalise both texts first — paste them through a find-and-replace.

This tool shows differences; merging is manual — copy the lines you want from each side. For frequent three-way merging (yours, theirs, common ancestor) use a dedicated merge tool like the one in VS Code or git mergetool.