Text Diff Checker

How to use the text diff checker

1. Paste the first version in the left box and the changed version in the right box. The differences appear below straight away.
2. Choose how to compare. Lines shows a classic line-by-line view with red and green rows, and highlights the exact words changed inside edited lines. Words shows the new text as one flowing passage with deletions struck through in red and additions shaded green.
3. Tick Ignore case or Ignore spacing to hide changes that do not matter, and Show only changes to fold long unchanged stretches away. Copy diff gives you a plain-text version with + and − marks.

Nothing is uploaded. The comparison runs inside your browser, which also makes it safe for drafts, contracts and code you would rather not paste into a website that stores input.

How the comparison works

The tool finds the longest common subsequence of the two texts: the largest set of items that appear in both, in the same order. Everything in that set is unchanged. Items only in the left text are shown as removed, and items only in the right text as added. It is the same idea used by version-control tools such as diff and Git.

unchanged = longest common subsequence removed = items in the original that are not in it added = items in the new text that are not in it similarity = 2 × unchanged ÷ (items in original + items in new)

In line mode, the items are whole lines. When a block of removed lines is followed by a block of added lines, the tool pairs them in order and then runs a second, word-level comparison on each pair, so a single changed word is marked inside the line rather than flagging the whole line. If a pair of lines has less than 30% in common, it is treated as a full rewrite and left unmarked inside.

Worked example

Press Try sample. The original has five lines and the new version has six. The result shows:

• Line 2 changed: Monday became Tuesday. Only that word is highlighted.
• Line 3 changed: $50 became $75.
• One new line was added: the sentence about tracking the parcel.
• Three lines are unchanged: the greeting, the thank-you line and the sign-off.

That is 3 unchanged lines, 2 lines removed and 3 lines added (the 2 rewritten lines each count once as removed and once as added, plus the new line). Similarity: 2 × 3 ÷ (5 + 6) = 54.5%, shown as 55%. Switch to Words mode to see the same two edits in running text, with only MondayTuesday and $50$75 marked.

Choosing the right options

Common uses

• Editing and proofreading: see exactly what an editor or a co-author changed between two versions of a document.
• Contracts and policies: spot a quietly changed number, date or clause between a draft and the version sent back to you.
• Code and configuration: check what differs between two environment files, SQL scripts or snippets.
• Plagiarism and rewrites: a high similarity percentage between two passages shows how much was copied unchanged.
• Data checks: compare two exports of a list to find the rows that were added or dropped.

Accuracy and limits

The result is a correct minimal difference, but a text can often be matched in more than one valid way, so another tool may mark a moved paragraph differently. Moved blocks appear as one removal and one addition rather than as a move. The comparison is limited to about six million item comparisons, which in Words mode means roughly 1,200 words between the first and the last change (spaces count as items), so very large files should be split or compared by line first. Spaces in line mode are compared as typed unless you tick Ignore spacing. Formatting such as bold or colour is not part of plain text and cannot be compared.

Frequently asked questions