About This Tool
Spotting the difference between two blocks of text is a headache when done manually. Debugging code, checking document versions, and comparing lists all require careful attention. Looking for changes line by line is slow and error-prone, especially when the text is long or the changes are subtle. A single missing comma, an extra space, or a renamed variable can break an entire program or alter the meaning of a contract. Our free diff checker highlights differences instantly. Paste your original and modified text, and the tool shows exactly what changed, line by line. Added lines appear in green, removed lines in red, and unchanged lines stay neutral. Switch between a side-by-side view for direct comparison or an inline view to see changes in context. The comparison uses the Longest Common Subsequence algorithm, the same approach used by Git and professional code review platforms. Your text never leaves your device and nothing is sent to any server, which makes this tool safe for proprietary code, legal documents, and confidential business data.
How Diff Algorithms Work
Diff tools use the Longest Common Subsequence (LCS) algorithm. It finds the longest sequence of lines that appear in both texts in the same order. Everything else is treated as an insertion (added) or deletion (removed).
The algorithm builds a matrix comparing every line in the original text against every line in the modified text. It then backtracks through this matrix to produce the minimal set of changes needed to transform one text into the other. This is the same core algorithm that powers Git, SVN, and most version control systems.
The result is a clear, color-coded output where green lines are additions, red lines are removals, and plain lines are unchanged. This visual approach makes it easy to spot changes at a glance, even in documents with hundreds of lines.
When to Use a Diff Tool
- Programming: Check what changed in a file before committing to Git. Catch accidental edits, debug regressions, and review pull requests by comparing code versions.
- Writing: Compare two drafts of an essay or article to see what your editor changed. Track revisions without relying on Word track-changes.
- Data: Find missing or added items between two CSV lists, database exports, or inventory files.
- Configuration: Debug why a server config file is not working by comparing it to a known-good backup. Spot the one line that was accidentally modified.
- Legal Documents: Compare contract versions to see exactly which clauses were added, removed, or reworded between drafts.
Side-by-Side vs Inline View
This tool offers two viewing modes to match different comparison needs:
Side-by-Side View (Default): Shows the original text on the left and the modified text on the right. Matching lines appear in both columns, removals show on the left with a red background, and additions show on the right with a green background. This mode works best for comparing two complete documents where you want to see both versions at once.
Inline View: Merges both texts into a single stream. Each line is prefixed with a symbol: a plus sign for added lines, a minus sign for removed lines, and a space for unchanged lines. Removed lines appear with strikethrough styling. This mode works best for reviewing sequential changes, similar to how Git displays diffs in the terminal.
Toggle between views using the Inline View checkbox above the diff output. Both views show the same data, just formatted differently for different reading preferences.
Tips for Accurate Comparisons
Get better diff results by preparing your text before comparing:
- Consistent line endings: Windows uses CRLF and Mac or Linux uses LF. Mismatched line endings can cause every line to show as changed. Paste into a plain text editor first to normalize.
- Trim trailing spaces: Extra whitespace at the end of lines causes false differences. Many code editors have a setting to strip trailing whitespace automatically.
- Watch for encoding: Text copied from PDFs or Word documents may contain special characters like smart quotes or em dashes that look identical but have different character codes.
- Sort lists first: When comparing unordered lists, sort both alphabetically before diffing. Otherwise every line may show as changed even if the same items exist in both lists.
- Break long lines: Very long lines that wrap differently can make diffs hard to read. Consider adding line breaks at logical points before comparing.
Common Diff Use Cases in Software Development
Code Review: Before submitting a pull request, paste the old and new versions of a function to verify your changes are correct. This catches accidental deletions or unintended modifications that automated tests might miss.
Debugging: When a feature breaks after a deployment, compare the current configuration or code with the last known working version. The diff reveals exactly what changed between the two states.
Migration: When moving from one framework or library to another, compare generated output files to ensure the migration preserved all content. Database schema comparisons also benefit from diffing the before and after states.
Documentation: Technical writers compare documentation versions to track what was updated, added, or deprecated. API changelog generation often starts with diffing specification files.
Merge Conflict Resolution: When two developers edit the same file, diff tools help identify overlapping changes so you can manually merge them without losing either person's work.