Text Diff Checker — compare two texts line by line

A text diff checker compares two versions of the same content and shows you precisely which lines were added, which were removed, and which stayed the same — without you having to read both versions side by side and spot the differences yourself. It's the same underlying idea that powers git diff, Google Docs' version history, and the "compare revisions" screen in every content management system, boiled down to a single paste-and-compare page.

Everything here runs locally in your browser, which matters if you're comparing a client contract, a password file, or unpublished copy — nothing you paste is uploaded anywhere. At Arb Digital we diff content constantly: comparing a draft against a client's edits, checking what changed between two versions of a landing page, or verifying a find-and-replace pass didn't touch more than intended.

What This Diff Checker Does

Paste your original text into the first box and the changed version into the second. The tool splits both into lines and runs a line-by-line comparison algorithm to work out the shortest possible set of additions and removals that turns the original into the changed version. It then renders both texts merged into a single view: lines only in the changed version are highlighted green with a leading +, lines only in the original are highlighted red with a leading -, and lines present in both appear in neutral color with no marker. A summary at the top counts exactly how many lines were added, removed, and left untouched.

This is a line-level diff, which means the smallest unit of comparison is a whole line, not an individual character or word. If you change one word in the middle of a fifty-word line, the entire line is reported as removed-then-added rather than a small in-line edit — that's a deliberate trade-off explained in the next section.

How to Use It

1. Paste the original into the first textarea — the "before" version.
2. Paste the changed version into the second textarea — the "after" version.
3. Read the merged view. Green lines were added in the new version, red lines were removed from the original, and plain lines are identical in both.
4. Check the summary counts at the top to get a quick sense of how large the change was before reading line by line.
5. Copy the diff using the copy button if you need to paste it into a code review comment, a support ticket, or an email to a client explaining what changed.
6. Iterate — edit either box and the comparison updates instantly, which is useful for testing a proposed edit before you commit to it.

How the Comparison Is Calculated

This tool computes a longest common subsequence (LCS) between the lines of your two texts — the longest ordered list of lines that appears, in the same relative order, in both versions. Any line that isn't part of that common subsequence must have been either removed from the original or added in the changed version, and the algorithm labels each accordingly. This is the same core technique described in the classic diff literature and used by version control systems; you can read a technical overview of the underlying string-comparison problem on MDN and in general computer-science references on the longest common subsequence problem. The practical effect is that this tool finds the smallest possible edit — it won't report a line as both removed and re-added if it could instead be reported as unchanged.

Line-Level vs Word-Level vs Character-Level Diffs

Diff tools operate at different granularities, and each has a real trade-off. A line-level diff, like this tool, is fast, easy to read, and matches how most version control systems think about text — it's ideal for code, configuration files, and structured content where a "line" is a meaningful unit. A word-level diff breaks text into words and compares those, which is more useful for prose: it can show that only one word changed in a sentence instead of flagging the whole sentence as replaced. A character-level diff goes further still, useful for catching a single typo fix or a currency symbol change inside otherwise identical text. The trade-off is readability: character-level diffs on long documents produce a lot of visual noise for a small, easy-to-miss change. Most professional diff tools, including this one, default to line-level because it strikes the best balance between precision and a result you can actually scan quickly.

Why Whitespace and Line Endings Create Phantom Diffs

One of the most common sources of confusing diff results has nothing to do with the actual content — it's invisible characters. Windows-style line endings (CRLF, a carriage return followed by a line feed) and Unix-style line endings (LF, just a line feed) look identical on screen but are different bytes, so a file saved on Windows and the same file saved on macOS or Linux can register as "every single line changed" even though nothing visible is different. Trailing spaces at the end of a line, or a tab character where you expect a space, cause the same phenomenon: two lines that read identically to your eyes fail an exact string comparison because a comparison algorithm checks bytes, not appearance. If you paste two versions of a document and get a wall of red-and-green with no visible difference, invisible whitespace or line-ending mismatches are almost always the cause — try re-copying the text from its original plain-text source rather than from a rich-text editor, which often introduces exactly this kind of hidden formatting.

Why Diffing Matters Beyond Code

Diffing is usually associated with software developers reviewing a pull request, but the same technique is just as useful outside code. Editors comparing a submitted draft against a published version, marketers checking what a copywriter actually changed in a page rewrite, legal teams comparing two versions of a contract clause, and support teams verifying that a bulk find-and-replace only touched what it was supposed to — all of these are the same underlying problem as a code diff, just applied to different kinds of text. Because this tool runs entirely client-side, it's safe to use on sensitive drafts, internal documents, or anything you wouldn't want to paste into a server-backed tool of unknown origin.

How Version Control Systems Build on This Idea

Tools like Git don't actually store a full copy of every version of every file you commit — under the hood, much of what makes version history efficient and readable is exactly this kind of line-based comparison. When you run git diff, you're seeing a rendered version of the same algorithm this tool runs: lines common to both snapshots are treated as unchanged context, and everything else is grouped into additions and removals. Pull request review tools like the ones on GitHub or GitLab layer syntax highlighting, inline comments, and "moved block" detection on top of that same core diff, but the foundational computation — finding the longest common subsequence of lines — is identical. Understanding what this tool is doing is a reasonable shortcut to understanding what your code review tool is doing when it renders a "+12 -4" summary on a pull request, because it's the same math with a different coat of paint.

Reading a Diff Like a Reviewer

Experienced code and content reviewers don't read a diff top to bottom like prose — they scan for shape first. A large block of red immediately above a large block of green in the same area of a document usually means a section was substantially rewritten rather than lightly edited, which deserves a slower, more careful read than a single-line change buried among mostly unchanged content. Isolated single-line additions scattered throughout a document often indicate small clarifications or corrections, while a red block with no corresponding green block nearby means content was deleted outright with nothing replacing it — worth double-checking that the deletion was intentional. Getting comfortable reading a diff's shape before reading its words is a skill that transfers directly from code review to editing any long document, and it's exactly what the summary counts at the top of this tool are designed to support at a glance.

Common Mistakes to Avoid

• Pasting from a rich-text source like Word or Google Docs without converting to plain text first — hidden formatting characters can create false differences.
• Assuming a "changed" line means completely different content — a one-character edit in a long line still shows the whole line as removed-then-added at the line level.
• Ignoring the summary counts and trying to eyeball a long diff — the counts at the top tell you the scale of the change before you read a single line.
• Comparing text with different line-ending styles without expecting every line to register as changed — check the whitespace section above.
• Forgetting that order matters — moving a paragraph to a different position in the document shows as a removal at the old position and an addition at the new one, not as a "moved" line.

Related Free Tools From Arb Digital

Pair this diff checker with the Find and Replace Text tool to make a bulk edit, then diff the before and after to confirm exactly what changed. Use the Character Counter to check length limits on the final version, the Word Frequency Counter to see how word usage shifted, and the Regex Tester to build the pattern behind a more complex replacement. Browse everything in our free online tools hub.

Frequently Asked Questions

This tool runs entirely in your browser — nothing you paste is uploaded or stored anywhere.