Zellio.io

Online Diff Checker

Compare two text blocks and highlight every added or removed line.

Runs locallyWorks offlineShare link carries settings, never your data
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Frequently asked questions

Is my text uploaded to compare it?

No. Everything runs locally in your browser using standard web APIs — your text, files and inputs are never uploaded to a server, so the tool works even offline once the page has loaded.

Can it compare code and config files?

Yes — it works on any plain text, including source code, JSON, YAML, logs and prose. It compares line by line and marks each difference, with added lines highlighted in green, removed lines in red, and modified lines shown side-by-side. This makes it ideal for code reviews before committing to version control, reviewing configuration changes in production environments, or auditing changes to deployment scripts. The line-by-line granularity catches all structural changes while remaining readable for both code and human-readable formats.

Does it do word-level differences?

It highlights changes at the line level, which is the clearest view for most reviews. Lines that differ are marked as added or removed, making it easy to spot which entire lines were added, deleted, or modified between the two versions. While word-level diffs can sometimes obscure the overall structure of changes, line-level comparison gives you the context you need to understand the scope of modifications. For text and code reviews, line-level comparison is typically the best balance between detail and readability, especially when tracking changes across multiple sections of a file.

Pro tips

  • Put the older version on the left. The colours only mean what you expect if the sides are in chronological order.
  • Suspect whitespace first when a diff shows a change you cannot see — trailing spaces and CRLF endings are the usual culprits.
  • Diff a section at a time for long documents. A thousand-line comparison is technically correct and practically unreadable.
  • Remember that moved text reads as a deletion plus an insertion, so a reorganised document looks far more changed than it is.
  • Compare exports rather than screenshots when checking a contract or policy, since only the text can actually be diffed.

About Online Diff Checker

Comparison is line by line, so an edited line reads as one removal plus one addition rather than a character-level patch. That suits config reviews and copy revisions, where which lines changed matters more than which characters did.

Compare two blocks of text side by side and highlight every line that was added, removed or changed. It is the fast way to review edits to copy, configuration files, code snippets, contracts or any two versions of a document without installing a desktop diff tool.

Paste the old version on one side and the new on the other for a colour-coded breakdown of exactly what moved. A reordered block shows as a deletion plus an insertion rather than a move, which is worth knowing before reading a long diff.

Granularity decides what a comparison is good for. A line diff treats each line as a unit, which suits code and configuration where lines are meaningful, and reads poorly on prose — correct one word in a paragraph and the whole paragraph shows as removed and re-added. Knowing that in advance stops a large block of red and green from looking like a bigger change than it is.

The most valuable diffs are the ones that surface a change nobody mentioned. Comparing a contract against the version you agreed, a configuration against the one that was working, or a policy against last quarter's is how you find the edit that was not in the summary. When two texts look identical but compare as different, the cause is almost always invisible: trailing spaces, tabs against spaces, or Windows line endings against Unix ones.

Common use cases

  • Reviewing a contract or terms document against the version you previously agreed to.
  • Comparing a configuration file against a known-good copy while troubleshooting a service that stopped working.
  • Writers and editors checking exactly what a collaborator changed in a draft.
  • Developers comparing two code snippets or log excerpts without committing anything to version control.
  • Anyone verifying that a document they received matches the copy they sent.
How it comparesGit and desktop tools like Beyond Compare or Meld offer word-level diffs, three-way merges and directory comparison, and all of them assume the content is already in files you control. Pasting two blocks is faster when the text arrived in an email, and the contract never lands in someone else's storage.