Text Diff Checker

Paste two blocks of text and instantly see every added, removed, and changed line and word highlighted side by side — all in your browser, with nothing ever uploaded.

What is the Text Diff Checker?

The Text Diff Checker is a free online tool that compares two blocks of text and highlights exactly what changed between them. Paste two blocks of text and instantly see every added, removed, and changed line and word — color-coded so the difference jumps out at a glance. It is built for the everyday "what changed here?" question that developers, writers, and editors hit dozens of times a day, and it answers it without you having to read both versions line by line yourself.

Unlike many comparison sites, this Text Diff Checker runs entirely in your browser. Your two inputs are never uploaded, never stored, and never sent over the network — which makes it safe to drop in proprietary source code, signed contracts, or unpublished drafts.

How to use it

Paste your original text into the left (or top) box — this is the "before" version.
Paste the changed text into the right (or bottom) box — the "after" version.
Read the highlighted result instantly. Removed lines are marked in red, added lines in green, and unchanged lines stay neutral. Inside lines that were edited, the exact words that changed are highlighted inline.
Toggle the view between side-by-side and inline if you prefer one continuous column.

There is no button to wait on, no account to create, and no upload progress bar — the diff renders the moment both boxes have text. Get your answer, copy what you need, and close the tab.

The method behind it

A text diff works by treating each block of text as a sequence of lines and finding the longest common subsequence (LCS) — the largest set of lines that appear, in order, in both versions. Anything outside that common backbone is classified as an insertion (present only in the new version) or a deletion (present only in the old version). This is the same family of algorithms (Myers diff / LCS) that powers git diff and most code review tools.

For word-level highlighting, the tool takes each pair of lines that differ and runs a second, finer diff over their individual tokens (words and punctuation). That second pass is what lets you see that only a single value inside a long line actually changed, rather than re-reading the entire line. Whitespace and indentation are preserved so that code and config compare faithfully.

Examples

Edited config value — Old line: timeout = 30. New line: timeout = 60. The tool keeps the line aligned and highlights only 30 → 60 inline, so you instantly see the single value that moved.
Reworded sentence — Old: "The meeting is on Monday at noon." New: "The meeting is on Tuesday at noon." Word-level diff marks Monday as removed and Tuesday as added, leaving the rest untouched.
Inserted code block — Pasting a function before and after adding a guard clause shows the new if (!input) return; line in green while every surrounding line stays neutral, confirming nothing else shifted.

Common use cases

Code review — Spot exactly what a teammate changed in a snippet, config, or migration before approving it.
Document version control — Compare two drafts of an article, policy, or contract to catch every edit a collaborator made.
Debugging output — Diff two log files or API responses to find the one differing line causing a regression.
Proofreading and editing — Verify which words an editor changed without re-reading the whole piece.
Data sanity checks — Compare two CSV or JSON exports to confirm a transformation changed only what you expected.

Why use this one

Most free diff tools show you whole changed lines and leave you to hunt for the difference inside them. This Text Diff Checker adds word-level inline highlighting so the exact characters that moved are obvious. It also keeps everything 100% in your browser — nothing is uploaded — which the heavier, ad-cluttered incumbents cannot promise. And there is no sign-up and no upload wait: the comparison is instant.

Need to keep working with text and code? Pair it with our JSON Formatter to clean up JSON before diffing it, our Regex Tester to build and check patterns, and our Base64 Encode / Decode tool for quick payload inspection — all part of the same developer toolkit, all private and client-side.

Frequently asked questions

Is my text uploaded to a server when I compare it?

No. The entire comparison runs locally in your browser using JavaScript. Your two text blocks never leave your device, are never sent over the network, and are not stored anywhere — making it safe for proprietary code and confidential documents.

What is the difference between line-level and word-level diff?

A line-level diff marks whole lines as added, removed, or unchanged. A word-level (inline) diff goes further: inside a line that changed, it highlights the exact words or characters that were added or deleted, so you can pinpoint a single edited value instead of re-reading the whole line.

Can I compare code, JSON, or config files?

Yes. The tool treats input as plain text and preserves whitespace and indentation, so it works well for source code, JSON, YAML, CSV, logs, and config files — as well as ordinary prose like essays and contracts.

Is there a limit on how much text I can compare?

There is no hard server limit because everything runs in your browser. Very large inputs (tens of thousands of lines) may slow your browser, but typical files, documents, and code snippets compare instantly.

Does it work on mobile?

Yes. The Text Diff Checker is responsive and touch-friendly, so you can paste and compare two text blocks on a phone or tablet just as easily as on desktop.