Diff Checker
Local processing · never uploadedCompare two texts line by line and see exactly what was added, removed or kept.
Paste the original and changed text to compare them line by line.
What is Diff Checker?
Diff Checker is a fast, free online tool that compares two blocks of text line by line and highlights exactly what changed — added lines in green, removed lines in red, and unchanged lines in neutral — all instantly in your browser.
A line diff treats each text as a sequence of lines and finds the smallest set of insertions and deletions that turns the original into the changed version. Under the hood it computes the Longest Common Subsequence (LCS) of the two line arrays, then walks the result to decide, line by line, whether each line was kept, removed from the original, or added in the new version. This is the same family of algorithm that powers version-control diffs, so the output reads the way developers expect: a "−" prefix marks a line that exists only in the original, a "+" prefix marks a line that exists only in the changed text, and lines present in both are shown without a marker.
A line-based diff is useful in a surprising number of everyday situations. During code review you can drop two revisions of a file side by side and immediately see which lines a change touched, without scanning the whole file. When comparing configuration — an .env file, a YAML manifest, an nginx block, or a JSON document — a diff makes it obvious which keys or values drift between environments, which is often the root cause of "it works on my machine" bugs. Writers and translators use it for proofreading: paste an old draft and a new one to confirm that only the intended sentences moved. It is equally handy for comparing log excerpts, SQL schemas, or any two snippets where you need to know precisely what differs.
Reading the result is simple: scan for the colored lines. Green "+" lines were introduced in the changed text, red "−" lines were dropped from the original, and the summary at the top reports the totals so you can gauge the size of a change at a glance. Everything runs entirely on your device — your text is never uploaded, logged or stored — so it is safe to compare private source code, internal configuration or confidential documents. Because the comparison is local it is also instant, with no round-trip to a server.
FAQ
How does the diff decide what changed?
It splits both inputs into lines and computes their Longest Common Subsequence, then walks that table to label each line as unchanged, removed from the original, or added in the changed text. This produces the minimal, version-control-style set of edits.
Is this a line diff or a character diff?
It is a line-based diff: each line is compared as a whole unit. If a single character changes on a line, that line shows as one removed line plus one added line rather than an inline character highlight.
Is my text uploaded anywhere?
No. The comparison runs entirely in your browser. Your original and changed text never leave your device, so it is safe to diff private code, config or documents.
What do the "+" and "−" markers mean?
A green line with a leading "+" exists only in the changed text (added). A red line with a leading "−" exists only in the original (removed). Lines with no marker are identical in both texts.
Is there a limit on how much text I can compare?
For very large inputs the tool caps the comparison at 3000 lines per side to keep it fast and avoid exhausting memory. For normal documents and files you will never hit this limit.