Compare two texts and instantly see what was added, removed or changed — side by side. Runs in your browser, nothing is uploaded. 🔍
Side-by-side comparison
Spot every change between two drafts, clauses, configs or code snippets in seconds.
Line-by-line, colour-coded: green = added, red = removed, so nothing slips through.
The comparison runs in your browser — your text is never sent anywhere.
A diff checker compares two pieces of text and shows exactly what changed between them — which lines were added, removed or kept. It's an everyday tool for developers reviewing code, legal assistants comparing contract drafts, writers tracking edits, and anyone who needs to find the difference between two versions of a document.
This tool uses a line-based comparison (longest-common-subsequence) and displays the result side by side with colour highlighting. Because it runs entirely in your browser, you can safely compare sensitive or confidential text with no upload and no sign-up.
A diff tool is not comparing the two texts character by character in order. It is solving a specific problem: find the longest sequence of lines that appears in both documents in the same relative order — the longest common subsequence. Everything in that sequence is unchanged; everything else is an insertion or a deletion.
Framing it that way explains the behaviour people find surprising. A block of text moved from the top of a document to the bottom usually shows as a deletion in one place and an addition in another, because a subsequence must preserve order and a move does not. Similarly, changing one word in a long paragraph can mark the whole line as changed, since line-level comparison has no smaller unit to work with.
Two details decide most confusing results. Whitespace and line endings are content unless the tool is told to ignore them, so a file edited on Windows and compared against one from macOS can appear entirely different when only the invisible line terminators changed. And comparing wrapped prose works far better if each sentence sits on its own line, which turns a paragraph-level diff into a sentence-level one.