Text Diff Checker: Compare Two Texts Line by Line

A text diff checker compares two blocks of text and shows you exactly what changed between them, line by line. Paste an old version on the left and a new version on the right, and the tool highlights every line that was added in green and every line that was removed in red, leaving unchanged lines in grey. It runs entirely in your browser, so whatever you paste stays on your machine.

How the comparison works

Under the hood, the tool uses a longest common subsequence algorithm, the same idea behind the diff tools built into version control systems. It finds the longest run of lines the two texts share, then treats everything else as an addition or a removal. This is smarter than a plain top-to-bottom comparison: if you insert a sentence near the top, a naive tool would mark every line after it as changed, while this one correctly shows just the one inserted line and keeps the rest aligned as unchanged.

What you can use it for

Writers and editors use a diff to see precisely what a round of edits changed, which is far faster than re-reading two drafts side by side. Content and SEO teams use it to compare an old page version against a rewrite, to check what a client or colleague altered, or to confirm that a bulk find-and-replace only touched what it should have. Developers use it to eyeball changes to config files, copy blocks or data before committing them. Anywhere you have a before and an after, a diff turns hunt-the-difference into a highlighted list.

Privacy and practical limits

Because the comparison happens in your browser with JavaScript, nothing is uploaded or stored, which makes it safe for drafts, internal copy or anything you would rather not paste into a server-side tool. The comparison is line based, so it flags a whole line as changed even when only a word within it moved, which is the normal behaviour for line diffs and keeps the output readable. For very large documents the browser does the work, so extremely long inputs can take a moment. When you are managing content changes across many pages at scale, keeping track of what changed and why is part of the disciplined process behind effective SEO content work.