Diff Checker Developer Tool
Compare two blocks of text line by line and see what was added, removed, or unchanged.
Eyeballing two versions of a config file, a paragraph, or a chunk of JSON to spot what actually changed is slow and error-prone once either side runs more than a few lines. This free diff checker is a developer tool that runs a real line-based diff between two blocks of text and highlights additions and removals, entirely in your browser.
Original
Changed
Diff
Runs entirely in your browser — no data is sent anywhere.
How this diff is computed
Each side is split into lines, and the tool finds the longest common subsequence (LCS) of lines shared between the two versions — the largest set of lines that appear in the same relative order in both, whether or not they're adjacent. Lines in the changed text that aren't part of that shared subsequence are additions; lines in the original that aren't part of it are removals; everything else is unchanged and printed plain. This is the same underlying approach tools like diff and Git use, just applied at the line level rather than optimising for the more elaborate hunk-and-context output those tools produce.
Why line-based, not character-based
A character-level diff can technically find a smaller edit distance, but it tends to produce noisy, hard-to-read output for anything longer than a single sentence — a one-word change in the middle of a paragraph can fragment the whole line into alternating tiny green and red spans. Line-based diffing matches how source code, config files, and most structured text are actually organised and reviewed, and it's what every mainstream version control diff view defaults to for the same readability reason.
When to ignore whitespace
Reformatting — re-indenting a block, a linter changing trailing spaces, an editor converting tabs to spaces — produces lines that are semantically identical but not byte-identical, which would otherwise show up as a spurious remove-and-add pair. The "ignore whitespace" option trims each line before comparing (without altering what's displayed), so a pure formatting pass doesn't drown out the changes that actually matter.
Related tools
If you only need to know whether two files differ at all, without caring where, a hash generator is faster — identical hashes mean identical content, full stop. For comparing two drafts of a README or PR description before diffing them, render each through the Markdown previewer first. And when the two sides are JSON payloads rather than free text, the JSON viewer's formatting makes a subsequent line diff far more meaningful, since consistent indentation avoids diffing away purely cosmetic key-ordering or spacing differences.