Text Diff Checker at a glance
- What it does
- Compare two blocks of text side by side and see exactly what was added, removed or changed.
- Where it runs
- Entirely in your browser — no data is uploaded
- Works offline
- Yes, once the page has loaded
- Cost
- Free, with no account and no usage limit
- Category
- Text Utilities
How to use the diff checker
- Paste the original into the left panel.
- Paste the revised version into the right panel.
- Compare, and read the highlighted output: additions and removals are marked distinctly.
How to read a diff
A diff describes the edit that turns the first text into the second. It is not a description of meaning, only of characters, so a paragraph moved from the top of a document to the bottom shows as a deletion and an unrelated addition, even though nothing was rewritten.
Most tools use a variant of the same algorithm - find the longest common subsequence, then treat everything else as inserted or deleted. That is why a diff sometimes aligns two unrelated lines that happen to share their opening words: it is optimising for the smallest edit, not for readability.
What people use it for
- Contracts and agreements. Comparing a returned draft against the version you sent, when the other side did not track changes. This is the use case where a missed clause costs real money.
- Configuration. A service works on staging and fails in production. Diff the two configs before doing anything else.
- API responses. The same request returning something different after a deployment.
- Editing. Seeing exactly what an editor or a collaborator changed in your copy.
- Data exports. Comparing two CSV extracts to find which rows changed.
- Plagiarism and duplication. Checking how much of one document appears in another.
Getting a cleaner comparison
Normalise before comparing. Differences in line endings between Windows and Unix make every line appear changed. So does a switch between tabs and spaces. Normalise one side first if the whole output lights up.
Break long lines. A minified file or a single enormous paragraph gives you one changed line and no useful detail. Run it through the JSON formatter or add line breaks at sentence boundaries first.
Compare like with like. Diffing a PDF's extracted text against the original Word document mostly shows extraction artefacts.
Watch for invisible characters. Non-breaking spaces, zero-width joiners and smart quotes pasted from a word processor differ from their plain equivalents while looking identical on screen. If a line is marked as changed and you cannot see why, this is almost always the reason.
Comparing sensitive documents
The comparison runs entirely in your browser. Nothing is uploaded, which is the whole point for the most common serious use of a diff tool: checking a contract, a settlement, an employment agreement or an unreleased set of results against an earlier draft. Pasting that material into a service that processes it server-side is a disclosure; pasting it here is not.
Frequently asked questions
It highlights differences at a granularity fine enough to show which parts of a line changed, rather than marking the whole line. That is more useful for prose, where most edits are a few words inside a long paragraph.
Open each file and paste its contents. For binary formats such as .docx or .pdf you will need to extract the text first, since the raw file bytes are compressed and will not diff meaningfully.
No. Both versions stay in your browser, which is what makes the tool safe for contracts and other confidential documents.
Usually invisible differences - line endings, trailing spaces, or smart quotes replacing straight ones. It can also be a paragraph reflow, which changes where every line break falls.
Nothing you enter here leaves your browser
Text Diff Checker does its work in JavaScript running on your own device. The page loads once, and after that there is no upload step and no server involved — which matters here because unpublished drafts, contracts and internal documents stay on your machine.
You can verify this rather than taking our word for it: load the page, disconnect from the internet, and the tool keeps working. Our privacy policy sets out what is and is not collected, and this guide explains why the distinction matters.