Compare two documents and see exactly what changed

Put two versions side by side — Word, Markdown, EPUB, HTML or plain text — and get a redline of every change, word by word. Neither file is uploaded.

Someone sends back “a few small edits” to a contract, a manuscript or a policy. Track changes was off, or the file has been through three people and two formats. This tool reads both versions, lines up their paragraphs, and marks each word that was added or taken out, so you can check the edits without reading the whole thing twice.

Three ways to read the changes

• Redline. The revised document, typeset, with deletions struck through and insertions underlined — the view a lawyer or editor expects. A paragraph rewritten from scratch shows as removed and added rather than as a confetti of tiny edits.
• Side by side. The two sources line by line, with the changed words picked out inside each changed line.
• Unified. A standard diff -u patch, for programmers and version control. Download it as a .diff file.

In every view, Next and Previous (or Alt+↓ and Alt+↑) step through the changes one at a time, and Changes only folds away everything that stayed the same.

Compare Word documents without uploading them

Most online comparison tools ask you to upload both files. For drafts of contracts, employment offers, grant applications or anything else under negotiation, that means handing both versions to a company you know nothing about. Here the .docx files are unpacked and read in your browser, with the same code as Recto's document converter, and nothing is sent anywhere.

How the comparison works

Both documents are turned into the same structure — headings, paragraphs, lists, tables — so that a Word file can be compared with a Markdown file or a web page. The blocks are aligned with Myers' diff algorithm, the method behind git diff, and each pair of matching blocks is then compared word by word. Edits are measured at the level of words and punctuation, so changing “colour” to “color” marks one word, not the sentence around it.

The engine was checked against a brute-force reference on six thousand random cases, where it found the smallest set of changes every time, and its unified diffs were applied with GNU patch to confirm they reproduce the revised text exactly.

Questions

How do I compare two Word documents?

Open the earlier .docx as the original and the later one as the revision, or drop both files onto the page at once. The text of each is read in your browser and the differences are shown as a redline: deletions struck through in red, insertions underlined in green.

Are my documents uploaded?

No. Both documents are read and compared by the page itself, on your computer. That matters when the documents are contracts, offers or anything confidential — most online comparison sites process them on their own servers.

Does it show formatting changes?

It compares the words, and notices when a block changes kind — a paragraph that became a heading, for example. Changes only to bold or italic on the same words are not shown, so the redline stays about what the document says.

Can I compare a Word file with a Markdown file?

Yes. Every format is converted to the same structure before comparing, so a .docx can be compared with a .md, a web page or plain text. That is useful for checking an exported copy against its source.

What is the unified view for?

It is the standard diff format used by programmers and version control. Download it as a .diff file and it can be applied with the patch command or read in any code review tool. It has been checked against GNU diff and patch.

How big can the documents be?

Book-length documents compare in well under a second on a typical laptop. Very large documents that have almost nothing in common are still handled, though past a point the tool reports large replaced sections rather than searching for the smallest possible set of edits.

Can I print or share the comparison?

Yes. Print it, save it as PDF from the print dialog, or download the redline as a single HTML file that opens in any browser and keeps the change marks.

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