Text Diff Checker
Compare two blocks of text line by line to highlight added, removed, and unchanged content.
Free browser-based tool. Files are processed locally in your browser.
Open the Text Diff CheckerInteractive Text Diff Checker workspace loads in your browser.
How to Use the Text Diff Checker
Compare two blocks of text, code, JSON, or Markdown line by line to highlight added, removed, and unchanged lines locally in your browser.
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Enter or paste text into Original Text and Modified Text
Paste your baseline version into the left Original Text panel and your revised version into the right Modified Text panel (or click Load Sample Diff to load a realistic release-notes comparison).
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Use Swap Texts if you need to reverse the comparison direction
Click Swap Texts in the top action bar at any time to exchange the Original and Modified panels before running a comparison.
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Click Compare Text to compute the line-by-line diff
Click Compare Text. The tool runs a linear-space Myers diff algorithm locally and generates a unified diff table with dual line-number gutters (Original line `#` and Modified line `#`) plus summary counters for Added (`+`), Removed (`−`), and Unchanged (`=`) lines.
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Filter results and copy or download the diff report
Use the filter tabs (`All Lines`, `Changes Only`, `Added (+)`, `Removed (−)`, or `Unchanged`) to focus on specific edits, then click Copy Diff Output or Download .txt (`alltoolsfree-diff-report.txt`).
Understanding Line-by-Line Text Comparison (Myers Diff)
When reviewing contract revisions, configuration files, source code snippets, or edited article drafts, spotting subtle changes by eye across hundreds of lines is nearly impossible. A single changed number, deleted clause, or modified JSON property can easily go unnoticed.
The AllToolsFree Text Diff Checker uses Eugene Myers’s $O(ND)$ Divide-and-Conquer (Hirschberg linear-space) shortest-edit-script algorithm to compute the minimal set of line additions and removals needed to transform your Original Text into your Modified Text—all inside your browser with zero server uploads.
Common Practical Use Cases
- Comparing two versions of an article draft, legal clause, or policy document to verify exactly which sentences or paragraphs were edited.
- Inspecting changes between two configuration files (`.env`, JSON, YAML, or SQL scripts) before deploying an update.
- Finding which items were added to or removed from a multi-line inventory, roster, or URL list.
- Reviewing multilingual or Khmer (`ភាសាខ្មែរ`) text revisions without character encoding corruption.
How Unified Line Numbers and Diff Filter Tabs Work
In the comparison table, each row displays two line-number columns followed by a status prefix (`+`, `−`, or space) and the line content: **Unchanged lines** show both their Original line number and Modified line number; **Removed (`−`) lines** (highlighted in red) show their Original line number because they existed only in the Original Text; and **Added (`+`) lines** (highlighted in green) show their Modified line number because they exist only in the Modified Text.
When you modify a single word inside a line, a line-based diff algorithm records the old version of the line as Removed (`−`) and the new version immediately below it as Added (`+`). Using the **Changes Only** filter tab hides all unchanged context lines so you can review only the modified rows in long documents.
Helpful Tips to Avoid Common Mistakes
- Switch to the Changes Only tab when comparing long 1,000+ line files so you can jump straight to the added and removed lines without scrolling through unchanged sections.
- Note that line comparison is sensitive to leading and trailing spaces on a line; if two texts differ only by accidental whitespace, clean both texts in the Duplicate Line & Whitespace Cleaner first.
- If you edit either input panel after running a comparison, an amber "Comparison Results Are Outdated" banner appears with a one-click Refresh Comparison button.
- When you click Copy Diff Output or Download .txt, the exported report respects your currently selected filter tab (e.g., exporting only Changes Only if that tab is active).
Text Diff Checker Frequently Asked Questions
Why does changing one word on a line show one Removed (−) line and one Added (+) line?
This tool performs a line-by-line comparison. Whenever any character on a line changes, the original line is marked as Removed (−) with its original line number, and the updated line is marked as Added (+) with its new line number.
Do Windows (CRLF) vs. Mac/Linux (LF) line endings cause false differences?
No. Before comparing, the tool automatically normalizes Windows (\r\n) and classic Mac (\r) line breaks to standard Unix (\n) line endings so files from different operating systems compare cleanly.
What are the maximum text size and line limits per panel?
Each input panel accepts up to 200 KB (204,800 bytes) of UTF-8 text and up to 5,000 lines. These limits ensure the Myers diff algorithm completes almost instantaneously without freezing your browser tab.
Does Copy Diff Output copy all lines or only my active filter tab?
Both Copy Diff Output and Download .txt export the lines matching your currently active filter tab (All Lines, Changes Only, Added, Removed, or Unchanged), along with a summary header.
Related Browser Utilities for Text Diff Checker
Explore complementary AllToolsFree utilities that pair directly with Text Diff Checker:
Duplicate Line & Whitespace Cleaner – Normalize Spaces Before Comparing
Trim trailing spaces, remove blank lines, or sort two lists A–Z before running a diff.
Word Counter – Analyze Word & Character Counts
Check exact word, character, sentence, and paragraph statistics for your revised draft.
Case Converter – Standardize Letter Casing
Convert text blocks to consistent lowercase or Title Case before comparing them.