Free Text Tool · Diff Checker · Text Compare · Side by Side · Word · Line · Character
Difference Checker
Compare two texts and highlight every difference. See additions in green, deletions in red and unchanged text in grey. Switch between word, line and character-level comparison. Ignore whitespace and case differences. Export colour-coded HTML.
How to Use the Difference Checker
Paste your original text in the left panel and the modified version in the right panel. Select your comparison granularity (word, line or character) and click Compare. Furthermore, additions appear in green, deletions in red and unchanged text in grey. Additionally, toggle options to ignore whitespace or case differences, and swap texts with one click.
- Paste original textEnter the original version in the left textarea.
- Paste modified textEnter the changed version in the right textarea.
- Choose granularityWord mode for prose. Line mode for code. Character mode for precise typo detection.
- Click CompareView the colour-coded diff with statistics showing additions, deletions and similarity.
- Copy or exportCopy the diff text or download as an HTML file with preserved highlighting.
What Is a Difference Checker?
A difference checker (also called a diff tool or text comparator) compares two versions of text and highlights every change. It identifies additions, deletions and modifications between an original and a modified version. Furthermore, diff tools are essential in software development, content editing, legal review and academic proofreading.
The concept originates from the Unix diff utility, created by Douglas McIlroy at Bell Labs in the early 1970s. The original diff program used an algorithm based on the Longest Common Subsequence (LCS) problem. Furthermore, this algorithmic foundation remains the basis for modern diff tools including git diff, GitHub pull request views and this tool.
Comparison Granularity Explained
| Granularity | Comparison unit | Best for | Detail level |
|---|---|---|---|
| Line | Each line of text | Source code, configuration files, CSV data | Low (shows whole lines) |
| Word | Each word (space-separated) | Prose, articles, documentation, contracts | Medium (shows changed words) |
| Character | Each individual character | Legal documents, typo detection, passwords | High (shows every character) |
How Diff Algorithms Work
This tool uses the Longest Common Subsequence (LCS) algorithm. LCS finds the longest sequence of elements that appears in both texts in the same order. Furthermore, everything not in the LCS is either an addition or a deletion. The algorithm produces an optimal diff that minimises the number of reported changes.
The LCS algorithm has O(mn) time complexity where m and n are the lengths of the two texts. For very large texts (10,000+ lines), optimised variants like the Myers diff algorithm reduce computation time. Furthermore, this tool uses the standard LCS approach, which handles texts up to several thousand lines comfortably in a browser.
Difference Checker for Code Review
Developers use diff tools daily for code review. Every pull request on GitHub, GitLab and Bitbucket shows a diff of the proposed changes. Furthermore, line-level comparison is the standard for code because each line of code is typically a single statement. Changes to a single line highlight the entire line, and the reviewer examines the full context.
The ignore whitespace option is especially useful for code diffs. Indentation changes, trailing spaces and blank line additions are formatting-only changes that do not affect program behaviour. Furthermore, filtering them reduces visual noise and lets reviewers focus on logical changes rather than cosmetic ones.
Difference Checker for Content Editing
Writers and editors use diff tools to track changes between document drafts. Word-level comparison shows exactly which words were added, removed or replaced. Furthermore, this is more precise than Microsoft Word's Track Changes feature, which sometimes marks entire paragraphs as changed when only a few words differ.
Content teams use diff comparisons to verify that approved changes were applied correctly. After an editor returns a marked-up draft, the writer can diff the original against the revised version. Furthermore, this catches accidental deletions, unintended additions and changes that went beyond the approved scope.
Difference Checker for Legal Documents
Contract review requires character-level precision. A single changed word, number or punctuation mark can alter the legal meaning of a clause. Furthermore, character-level diff catches changes that word-level comparison might display as a single word swap. The difference between "shall" and "may" is a single word change that fundamentally alters a contractual obligation.
Lawyers use diff tools to compare contract versions during negotiation rounds. Each party may propose changes to specific clauses. Furthermore, comparing the sent version against the returned version ensures that no unexpected changes were introduced beyond the agreed modifications. This process is called "redlining" in legal practice.
Unified vs Side-by-Side View
Unified view shows both versions interleaved in a single column. Additions are prefixed with + and deletions with -. Furthermore, this format is used by git diff and Unix patch files. Furthermore, unified view is compact and works well on narrow screens and in terminals.
Side-by-side view shows the original on the left and modified on the right in parallel columns. Additionally, lines are aligned so you can see each change in context. Furthermore, side-by-side is easier to read for document comparison but requires more horizontal space. GitHub uses side-by-side as the default view for pull request reviews.
Understanding Diff Statistics
The statistics bar shows five metrics. Added count is the number of units present in the modified text but absent from the original. Removed count is the number of units present in the original but absent from the modified. Furthermore, unchanged count is the number of units identical in both texts.
Similarity percentage measures how much of the combined text is unchanged. Consequently, a similarity of 95% means the texts are nearly identical. In contrast, a similarity below 50% indicates a major rewrite. Furthermore, the total count shows the combined number of diff operations. Therefore, use these statistics to quantify the magnitude of changes between versions.
Best Practices for Text Comparison
Choose the right granularity for your task. Use word mode for articles and documentation. Use line mode for code and configuration files. Furthermore, use character mode only when precision matters more than readability, such as legal contracts and password verification.
Enable ignore whitespace for code comparisons to filter formatting noise. Moreover, enable ignore case when capitalisation differences are not meaningful. Furthermore, always review the statistics before reading the full diff. If the similarity is above 95%, you can focus on just the highlighted changes. If similarity is below 50%, it may be faster to read the modified text fresh rather than tracking individual changes.
When comparing configuration files, pay attention to line ordering. Two configuration files may contain identical settings in a different order. Furthermore, a line-level diff will report every line as changed even though the content is semantically identical. In these cases, sorting both files before comparing produces a cleaner, more meaningful diff result.