About the Text Diff Tool
The Text Diff Tool compares two versions of any text using a Longest Common Subsequence (LCS) algorithm and highlights every difference inline. Additions appear green, deletions red, and in-line character changes are tracked so single-letter substitutions and punctuation tweaks do not get hidden under whole-line markers. Side-by-side, inline, and merge views support different review styles.
It is built for writers reconciling editor feedback against an original draft, lawyers redlining contract revisions, developers reviewing untracked code changes, students comparing essay drafts, translators auditing a TM-segmented version against the source, and anyone who has ever asked “what changed between these two PDFs?” The merge mode lets you accept or reject each individual change to build a final combined version without retyping.
All comparison runs in JavaScript on your device. Whatever you paste — unpublished manuscripts, confidential contracts, NDA-bound legal redlines, internal-only documentation — never leaves your browser. The page makes no network call after first load. This is the only acceptable posture for a diff tool intended to handle real working documents.
LCS is computationally O(n × m) in the worst case — comparing two 50,000-character documents may briefly freeze the tab on slower hardware. For very long texts, prefer the line-level mode first to locate the differing regions, then run character-level only on the relevant segments. The tool is built for natural-language and short structured text; for binary diffs or whole-repository code review, use Git’s built-in diff or a purpose-built code-review platform.
Readability comparison and word frequency changes between versions.
How to Use the Text Diff Tool
Paste your original text on the left and the revised version on the right. Click “Compare” to see every difference highlighted. Green marks additions, red marks deletions, and the character-level mode shows exactly which characters changed within modified lines. Use the inline mode for a single-stream view.
Use Cases for Text Comparison
Text diff tools are essential for writers comparing draft revisions, editors tracking changes between versions, developers reviewing code modifications, lawyers comparing contract versions, and students checking essay revisions. Any time two versions of a document exist, a diff tool reveals precisely what changed.
Line-Level vs Character-Level Diff
Line-level diff identifies which lines were added, removed, or modified between versions. Character-level diff goes deeper, highlighting the specific words or characters that changed within modified lines. This is the same approach Git uses to show code changes, presented in a user-friendly visual format.
Understanding Similarity Percentage
The similarity score measures how much the two texts have in common relative to their total content. A score of 100% means the texts are identical, while 0% means they share no common lines. This metric helps you quickly gauge the scope of changes between versions without reading every line.
Diff Algorithms: Myers, Patience, and Histogram
Not all diff algorithms produce the same output. The choice of algorithm affects how changes are grouped, where boundaries are drawn, and ultimately how readable the result is for a human reviewer.
The Myers algorithm, published by Eugene Myers in 1986, finds the
shortest edit script between two sequences — that is, the minimum number of
insertions and deletions needed to transform one text into the other. It is the default
engine in most diff tools and produces mathematically optimal results, but it can
sometimes generate counterintuitive groupings when code is restructured or lines are
repeated. The Patience diff algorithm, used as an optional mode in Git
(git diff --patience), takes a different approach: it first anchors the
comparison on lines that appear exactly once in both versions, then recursively diffs
the regions between those anchors. This tends to produce diffs that align with the
logical structure of code rather than just minimizing edit count, making it noticeably
easier to read when functions are moved or reordered. The Histogram
algorithm is a further refinement of Patience that handles repeated lines
more gracefully by tracking how frequently each line appears and weighting unique lines
more heavily. Git offers it via git diff --histogram and many developers
find it produces the cleanest output for real-world code changes.
Code Review and Version Control Applications
Text diff is the foundational technology behind every pull request, merge request, and code review workflow in modern software development. Git does not store full file snapshots for every commit — it stores the diff between versions, which is far more space-efficient and makes it trivial to reconstruct the history of any individual line across thousands of commits.
Platforms like GitHub and GitLab render these diffs with color coding that follows a universal convention: lines removed from the original appear in red with a minus sign, lines added in the new version appear in green with a plus sign, and unchanged context lines appear in gray. Understanding how to read a diff fluently makes code review significantly faster and more accurate. Instead of reading every line of a file, a skilled reviewer can focus only on what changed, mentally model the intent of the change, and spot potential bugs introduced by the edit. The same skill applies to reviewing AI-generated text edits, legal document redlines, and content revisions in a CMS — any workflow where two versions of the same document need to be compared precisely.
Looking for other text utilities? The Word & Character Counter helps you measure document length before and after revisions, and the Case Converter lets you transform text casing in a single click. Browse all Writing & Content tools for more tools that streamline your writing and editing workflow.
Frequently Asked Questions
What is a text diff?
A text diff (short for difference) is a side-by-side or inline comparison of two versions of the same document that highlights exactly what changed. Green marks additions, red marks deletions, and modifications are shown as combined deletion and insertion. It is the same concept Git uses to track code changes.
What is the difference between line-level and character-level diff?
Line-level diff marks entire lines as added, deleted, or modified. Character-level diff goes deeper and highlights exactly which characters within a modified line changed, which is essential for catching single-word substitutions, typo fixes, and punctuation changes that line-level views hide.
When should a diff tool be used?
Writers comparing draft revisions, editors tracking changes between versions, developers reviewing code, lawyers comparing contract redlines, and students checking essay revisions all benefit from diff tools. Any time two versions of a document exist, a diff reveals precisely what changed.
Does the tool run in the browser?
Yes. The comparison runs entirely in the browser with JavaScript. No text leaves the device, which makes it safe for confidential documents, unpublished manuscripts, and legal drafts under NDA.
What is merge mode?
Merge mode lets the user accept or reject each individual change between the two versions to assemble a final combined document. It is the fastest way to reconcile feedback from multiple reviewers without retyping the full text.
