Compare two texts and spot every difference instantly

Why use an online text diff checker

A diff checker earns its keep the moment two versions of something drift apart without anyone noticing exactly how. Code reviewers use line diffs to confirm a pull request only touches what it claims to. Lawyers and contract managers redline two drafts to catch a changed figure or a quietly deleted clause. Editors compare a submitted article against a published one to verify nothing was altered after approval, and writers use it to see what actually changed between drafts instead of re-reading both from scratch.

The common thread is that manual comparison is slow and error-prone — the human eye is good at reading text, not at spotting a single swapped word buried in a paragraph. Automating that comparison turns a tedious, mistake-prone task into an instant, reliable one, and doing it entirely in the browser means sensitive drafts never have to leave your device to get that benefit.

How the diff algorithm works

The two texts are first split into tokens — whole lines, words, or individual characters depending on the selected mode — and any shared prefix and suffix are trimmed off before the expensive comparison runs, which keeps typical edits fast even on long documents. What remains in the middle is compared with a longest-common-subsequence algorithm, the same family of technique used by tools like diff and Git, to find the smallest set of additions and removals that explains the change. Ignore whitespace and Ignore case only affect how tokens are matched — the original text is always what gets displayed, copied and exported.

Everything happens locally: there are no network calls, no uploads and no server involved, so the comparison works offline and your text never leaves the tab. If you also need word or character counts on either version, the word counter covers that without any extra setup.

Frequently asked questions

What's the difference between line, word and character diff?

Line mode compares whole lines, so it's best for code, config files or logs where you care which lines changed. Word mode splits text on runs of whitespace and highlights only the words that changed, which suits prose, emails and contracts. Character mode compares every individual character and is the most precise option for short strings like usernames, URLs or IDs, where even a single letter matters.

Is my text uploaded anywhere when I compare it?

No. Both text boxes stay in your browser's memory and the diff is computed locally — there are no network calls of any kind. If you're on Pro and save a comparison to your history, that history is written only to your browser's local storage, never sent to a server.

Can I ignore whitespace or case differences?

Ignore whitespace makes whitespace interchangeable rather than deleting it: in line mode, each line is trimmed and internal runs of spaces or tabs are collapsed to one before comparing; in word and character mode, a token made up entirely of whitespace — a run of spaces/tabs in word mode, or one whitespace character in character mode — is treated as equal to any other whitespace token, so a tab lining up against three spaces won't be flagged as a change. Ignore case simply lowercases both sides before comparing. Either way, the highlighted result always shows your original text unchanged — only the matching logic ignores those differences.

Can I diff code as well as plain text?

Yes. Line mode with the monospace preview suits code, configs and logs, where you typically want to know which lines were added or removed. Word or character mode is better for regular prose, since it highlights exactly which words or letters changed inside a line instead of flagging the whole line. Batch compare (Pro), for diffing several file pairs at once, always runs in line mode and does not apply the ignore-whitespace or ignore-case toggles, matching a straightforward code-review workflow.

What's the largest text this can compare?

There's no hard size limit, but very large inputs are protected against freezing the browser: if a comparison would need more than two million cell computations, a simplified add/remove diff is shown instead of a precise one. The on-screen preview is also capped — 2,000 rendered lines in line mode, or 100,000 characters worth of diff output in word and character mode — though Copy and the Pro downloads always include the complete, untruncated comparison.

What does Toolizto Pro add to the diff checker?

The free tier compares unlimited text in all three diff modes with no size restrictions, and lets you copy the diff at any time. Pro ($9.99/month or $79.99/year, covering every Toolizto tool) adds downloading the diff as a .txt or .html report, batch comparing multiple file pairs at once with a combined summary, and a saved comparison history you can reload later — plus it removes ads.