Which view should I use?

Split on a wide screen, since it shows both versions at once. Inline on a narrow one. Unified when you want output you can paste into a tool that consumes patches.

Why is one changed line shown as both removed and added?

A line diff has no concept of an edit — a modified line is a removal plus an insertion. Pairing them side by side in the split view makes the actual change easier to spot.

Text Diff

Compare two blocks of text line by line or word by word.

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About this tool

A line-by-line comparison with split, inline and unified views. Ignoring whitespace helps when indentation changed but content did not; ignoring case helps when the difference is purely stylistic. Neither option alters the text you pasted — they only affect how the two sides are matched up.

Limitations

Very large inputs are diffed line-level only; word-level comparison on megabytes of text is not worth the wait.

Questions

Which view should I use?: Split on a wide screen, since it shows both versions at once. Inline on a narrow one. Unified when you want output you can paste into a tool that consumes patches.
Why is one changed line shown as both removed and added?: A line diff has no concept of an edit — a modified line is a removal plus an insertion. Pairing them side by side in the split view makes the actual change easier to spot.

Privacy

This page does its work in your browser. What you paste is processed by JavaScript running on your own device and is never sent to a DevToolery server — there is no server that could receive it. The page itself is a static file delivered by a CDN. See exactly what is stored.