Text Diff Checker

How word-level diffing works

The tool splits both texts into words and finds the longest common subsequence between them — the longest sequence of words that appears in both, in the same order. Everything outside that sequence is an addition or a removal.

This means moved text usually appears as a removal in one place and an addition in another, rather than being recognised as a move. That is a known limitation of subsequence-based diffing and is worth knowing when you reorder paragraphs: the diff will look larger than the change felt.

Whitespace differences are largely absorbed, so reflowing a paragraph without changing its words produces little or no diff — which is usually what you want when comparing text pasted from different sources.

What it is useful for

• Checking what an editor or client actually changed in a returned draft.
• Verifying that a rewrite genuinely changed something rather than shuffling a few synonyms.
• Comparing a live page's copy against your intended version after a CMS or migration issue.
• Reviewing AI-assisted edits to see precisely what was altered before accepting them.
• Confirming a translation update only touched the sections you expected.
• Auditing terms, policies or product copy against a previous approved version.

Reading the output

Green highlighted text exists only in the revised version. Struck-through red text exists only in the original. Unmarked text is common to both. The counts above the output tell you how many separate change regions there are, not how many words changed — a single rewritten sentence is one addition and one removal, however long it is.

A useful habit is to scan for large unbroken green blocks, which indicate genuinely new content, versus scattered small changes, which usually indicate copy-editing. The two need different levels of review.

Diffing and SEO work

The most common SEO use is verifying a content update. When you refresh an old article, a diff confirms you actually improved it rather than making cosmetic edits — Google's freshness signals respond to substantive change, not to a modified date on an unchanged page.

It is also useful after a migration. If a CMS move mangled entity encoding or dropped formatting, diffing the old page copy against the new one surfaces exactly what was lost, which a visual comparison rarely does reliably.

For comparing two genuinely different pages rather than two versions of one, the Text Similarity Checker is the better fit — it gives an overall score instead of a word-by-word breakdown.

Related comparison tools

If you are checking whether two pages overlap enough to be treated as duplicates, the Duplicate Content Checker gives a plain verdict rather than a diff.

For repetition inside a single document — the same sentence or paragraph appearing twice, which happens easily in content assembled from several sources — use the Duplicate Sentence Finder or the Duplicate Paragraph Finder.

And to measure how much a draft grew or shrank overall, the Word Counter on each version is quicker than counting diff regions.

Limitations worth knowing

• Moved text shows as a removal plus an addition, not as a move.
• Very large texts produce dense output that is hard to scan — compare section by section instead.
• It compares plain text, so HTML tags and markdown syntax are treated as ordinary words.
• Formatting changes such as bold or italics are invisible to it unless the markup is included in the pasted text.
• It has no concept of meaning, so a change from “can” to “cannot” looks the same size as any other one-word edit.

Frequently asked questions

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