Someone sends back a contract with "just a few small changes", a colleague edits your article without tracking changes, or two copies of a config file behave differently and you cannot see why. A diff checker does it in an instant: it lines the two texts up and marks exactly what was taken out and what was put in.
Paste the original on the left and the changed version on the right. The comparison runs by itself as you type, highlights removals in red and additions in green, and counts how many lines or words fall into each group. Both texts stay in your browser tab the whole time.
What a diff checker actually shows
A diff is the shortest list of deletions and insertions that turns one text into the other. The tool looks for the longest run of content the two versions have in common, keeps it as unchanged, and reports everything else as removed from the original or added in the new copy. This is the same idea behind the change views in Git, code review sites and word processors.
There is no such thing as a "modified" item in a pure diff. When you rewrite a sentence, the old line is reported as removed and the new one as added. In the side by side line view these pairs are placed on the same row so you can read the before and after together, which is usually what people mean by a changed line.
Situations where a text diff saves time
Reviewing edits to contracts and articles
When a document comes back without tracked changes, compare it with the version you sent. Switching Compare by to Words makes small edits such as a changed date, a removed "not" or a new clause stand out inside long paragraphs.
Debugging configuration and code
Two .env files, two JSON responses or two snippets that should match but do not. Line mode with line numbers shows exactly which setting differs, and Ignore whitespace stops indentation changes from hiding the real difference.
Checking copied or rewritten content
Teachers, editors and SEO teams compare a source paragraph with a submitted one to see how much was really reworded. The Unchanged count gives a quick sense of how much text was kept word for word.
Options and how the comparison is made
Compare by Lines treats every line as one unit, which suits code, lists and structured files. Compare by Words splits the text into words, spaces and punctuation using your browser's Unicode word rules, so Arabic, Chinese and other scripts are compared word by word as well. Ignore case treats "Invoice" and "invoice" as equal. Ignore whitespace treats runs of spaces and tabs as a single space and, in line mode, also ignores spaces at the start and end of each line.
The View switch changes only the display. Side by side shows the original and changed text in two columns; in line mode each row has line numbers and changed lines are paired across. Inline merges both versions into one text, striking through the removed parts and highlighting the added ones. Swap texts exchanges the two panes if you pasted them the wrong way round.
The comparison uses a small open-source diff library that runs inside the page on your device. It recalculates about a fifth of a second after you stop typing and never sends either text to a server.
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- Paste the earlier version into Original text.
- Paste the newer version into Changed text.
- Choose Compare by Lines for code and lists, or Words for prose.
- Switch on Ignore case or Ignore whitespace if those differences do not matter to you.
- Read the highlighted Differences panel, in Side by side or Inline view, and check the Removed, Added and Unchanged counts below it.
What it does not do
- It compares pasted text only. To compare Word or PDF files, copy their text into the two boxes first; formatting such as bold or font changes is not compared.
- A block of text that was moved elsewhere appears as a removal in one place and an addition in another, not as a move.
- Line mode marks the whole line when anything in it changes. Use Words mode to see the exact words that differ.
- Very long texts with thousands of scattered changes can take a moment to compute on slower phones.
- There is no merge or export: the tool shows the differences but does not combine the versions for you.