How to Compare Two Blocks of Text and See Exactly What Changed

Here's the actual process for comparing two texts with Diff checker and reading the result correctly.

Step 1 - paste the original text

Paste or type the starting version into the Original field.

Step 2 - paste the changed text

Paste the newer version into the Changed field. The moment both fields hold real text, the comparison runs automatically - there's no button to click, and it re-runs a short moment after you stop editing either field.

Step 3 - choose Lines or Words mode

Lines mode (the default) compares the two texts line by line - the right choice for a whole document, a config file, or code. Words mode compares word by word instead, showing exactly which words changed inside a line that's mostly the same. This guide goes deeper on when to use which.

Reading the result

Added text appears underlined, removed text appears struck through, right inline in reading order - no separate red/green columns to reconcile in your head. A summary line above the result states the actual count: "N added, M removed," or an explicit "No differences" if the two texts are genuinely identical. That "No differences" message is a real, positive result, not a blank output that could just as easily mean the tool didn't do anything - seeing it is itself useful confirmation that two files or two drafts really are byte-for-byte the same, not just "close enough" at a glance.

Why the diff updates automatically instead of waiting for a click

Both textareas are watched for typing and pasting, and the comparison re-runs a short moment after either one goes quiet - fast enough to feel live, without recomputing on every single keystroke while you're still typing. There's genuinely no "run" step to remember, which matters here specifically: forgetting to click a button and reading a stale, outdated diff would be worse than no diff at all.

Copying the result

Click Copy result and you get a plain-text version suited to the mode you're in: in Lines mode, each line is prefixed with + (added), - (removed), or two spaces (unchanged) - the same convention `git diff` uses. In Words mode, the copy uses git's own --word-diff=plain format: {+added text+} and [-removed text-] inline, so the copied result stays readable as plain text even outside this tool.

If the two texts are almost entirely different

Two genuinely unrelated texts don't produce a useful line-by-line or word-by-word alignment - there's nothing meaningfully shared to align around. In that case the tool falls back to showing everything from the original as removed and everything from the changed version as added, with an explicit note that the texts were too different to compare in detail. That's an honest answer to "these don't share enough to diff meaningfully," not a bug - a detailed alignment between two unrelated documents would be misleading busywork, not useful information.

If either text is long

Each field works best under 200,000 characters. Longer text is still accepted, but comparison pauses and the tool says so instead of silently doing something different - shorten either field to compare it.

If one field is left empty

Nothing is compared, and a small inline note asks for text in both fields instead. This is deliberate - diffing real content against nothing would trivially show "everything added" or "everything removed," which is technically accurate but teaches you nothing about the actual comparison you wanted, so the tool doesn't pretend that's a useful result.

Nothing about your text ever leaves your browser

Both texts are compared entirely locally - there's no upload step, no server round trip, and nothing about either field is ever sent anywhere, whether the texts are two lines of config or the maximum 200,000 characters each. That matters more here than in most tools on this site, since a diff often involves pasting genuinely sensitive content: a contract draft, an internal config with real values in it, a password policy file.

Comparing more than one change at a time

The result covers the entire two texts in one pass - there's no need to compare section by section manually. If a config file has five separate changes scattered through it, all five show up together in one result, each in its own place, so you get the full picture in a single read rather than piecing it together from several smaller comparisons, and the summary count at the top tells you immediately whether you're looking at one small edit or a genuinely large rewrite before you even start reading the detail.

Switching modes without losing your text

Clicking between Lines and Words re-renders the same two texts you already pasted - neither field clears, so you can compare the same pair both ways in a few seconds to decide which view actually answers your question, without retyping anything.

For why lines and words suit genuinely different kinds of content, see the comparison guide. Or try it with your own two texts.