What algorithm does this diff tool use?

It uses the Longest Common Subsequence (LCS) algorithm, the same approach used by Git's diff engine.

What does Ignore whitespace do in the diff tool?

Lines that differ only in leading/trailing whitespace are treated as identical, ignoring pure formatting changes.

What is a unified diff patch and how do I apply it?

A unified diff is a standard text format showing removed lines (-) and added lines (+) with @@ hunk headers. Apply it with: patch -p1 < changes.diff, or git apply changes.diff.

Is there a size limit for inputs?

The LCS algorithm works well for files under 5,000 lines. Files above 4,000 lines fall back to a simpler comparison to maintain responsiveness.

Can I use this to compare plain text, not just code?

Yes. The diff tool works on any text content — configuration files, Markdown, JSON, CSV, or any plain-text content.

Is code sent to a server?

No. All diffing is done entirely in your browser. No code is ever sent to a server or stored.

Code Diff Checker — Compare Code Online

Paste two versions of any code or text and instantly see exactly what changed between them. This free code diff tool online uses the Longest Common Subsequence algorithm — the same approach as Git — to highlight added lines in green, removed lines in red, and generate exportable unified diff patches for any workflow. Before comparing, you may want to format code with the regular expression tester to clean field patterns, or convert tabular data from CSV to a diffable format.

Original (Version A)

Modified (Version B)

Ignore whitespace Case insensitive Updates live as you type

Paste code in both panels above to see the diff.

Paste code above to generate the unified diff patch.

How to Use the Code Diff Tool

Paste your original code into the left panel (Version A).
Paste the modified code into the right panel (Version B).
The diff updates live. Green lines with a + prefix are additions; red lines with a − prefix are removals; grey lines are unchanged.
Use Ignore whitespace to ignore leading and trailing spaces when comparing lines.
Use Case insensitive for case-agnostic comparisons.
Switch to Unified Patch to generate a standard --- a ... +++ b ... @@ ... @@ patch you can copy or download as a .diff file.

Key Features

LCS (Longest Common Subsequence) algorithm — the same method used by Git's diff engine.
Visual diff view: green added lines, red removed lines, grey unchanged lines with line numbers.
Stats bar: added, removed, and unchanged line counts update in real time.
Ignore whitespace option for comparing reformatted code without false positives.
Case insensitive mode for comparing text regardless of capitalization.
Unified patch generation: standard --- / +++ / @@ format compatible with git apply and patch.
Copy patch or download as a .diff file for use in CI workflows.
Live update: diff recalculates automatically as you type.

Use Cases

Compare two code versions line-by-line before merging

When two developers modify the same file independently, a quick line-by-line diff reveals every conflict before a formal merge. Paste both versions, review the visual output, and identify lines that need manual reconciliation before touching version control.

Review configuration file changes between environments

Config files like nginx.conf, docker-compose.yml, or .env often diverge between staging and production. Paste both versions into the diff checker to get a precise inventory of every line that differs — no guessing, no manual scanning.

Generate a unified diff patch for a code review

Switch to the Unified Patch tab to get a standard .diff file compatible with GitHub, GitLab, and the Unix patch command. Download the file and attach it to a ticket or email when you cannot share a branch directly.

Check if two SQL queries return the same logic

When optimizing or refactoring a SQL query, it is easy to accidentally drop a condition. Paste the original and the refactored version into the diff tool to confirm that no WHERE clause, JOIN condition, or ORDER BY has been removed or changed unexpectedly.

Compare JSON responses for API regression testing

API responses can change subtly between deployments. Paste the expected JSON and the actual response into the diff checker with "Ignore whitespace" enabled to immediately spot new fields, missing keys, or changed values that could indicate a regression.

FAQ's

This tool uses the Longest Common Subsequence (LCS) algorithm to compute the minimal edit distance between two line arrays. It identifies which lines are shared between both versions and classifies everything else as additions or removals. This is the same fundamental approach used by Git's diff engine.

When enabled, leading and trailing whitespace is stripped from each line before comparison. Lines that differ only in indentation are treated as identical. This is useful for comparing code that was reformatted without changing logic — for example, after running a code beautifier on one version.

A unified diff is a standard patch format. To apply a .diff file on Linux/macOS, run: patch -p1 < changes.diff. On Windows, use Git Bash or WSL. You can also run git apply changes.diff in a Git repository. The patch command modifies the original file to produce the modified version.

The LCS algorithm runs in O(n×m) time and memory, which can be slow for very large inputs. For files under 5,000 lines, performance is instant. For files above 4,000 lines, the tool automatically falls back to a simpler line-index comparison to maintain responsiveness. For very large files, use git diff or vimdiff locally.

Yes. The diff tool works on any text content — configuration files, Markdown documents, JSON responses, CSV data, or any plain-text content. It compares line-by-line regardless of content type. For raw text analysis, pair it with the word counter to track document length changes between versions.

No. All diffing happens entirely in your browser using JavaScript. Your code never leaves your device and is never logged or stored. This tool works offline once the page has loaded.

Git diff requires a Git repository and a local installation. This online diff tool works with any two text inputs — no Git repository, no command line, no installation needed. It is ideal for quick one-off comparisons where setting up a Git workflow would take longer than the comparison itself.

A reliable diff tool is one of the most practical utilities a developer can have at hand. Whether you are reconciling changes between environments, reviewing someone else's edits, tracing a regression to a specific line change, or generating a patch file for a legacy workflow, being able to compare two text blocks instantly — without opening a terminal, configuring a Git repository, or installing any software — saves real time. This free online diff checker gives you the same LCS-based accuracy that powers Git's diff engine, presented in a clean visual interface with green/red line highlighting, line numbers, and a stats bar. The unified patch output means it is also useful for formal patch-based workflows. Everything runs in your browser with zero data ever sent to any server.