What Is a Text Diff Checker?
A text diff checker is a tool that compares two blocks of text and highlights exactly what changed — which lines were added, removed, or modified, and sometimes which words or characters within a changed line are different. The technique behind it (computing the Longest Common Subsequence between two sequences) is the same one Git, Mercurial, and every code-review tool use under the hood. The diff itself is a compact summary of the change: instead of having you re-read 500 lines to spot the 3 that moved, the tool shows you the 3 lines and asks no more of your eyes.
The audience extends well beyond developers. Writers compare draft revisions; editors review proposed changes; legal professionals confirm contract redlines line by line; QA engineers compare expected vs actual output; SEO analysts check whether a competitor's title tag changed; teachers compare student submissions for plagiarism. The Tooldit diff checker handles all these cases in a browser tab, computing the diff locally so confidential documents and unreleased copy never reach a server.
Compare Two Texts Line by Line
This text diff checker takes an original text and a changed version and shows you exactly what is different between them. Paste your before text on the left and your after text on the right, and the comparison updates instantly — added lines turn green, removed lines turn red, and lines that stayed the same are left plain. Instead of squinting at two windows side by side and hunting for the one word that moved, you get a clear, color coded map of every change.
Under the hood it aligns the two texts with a longest common subsequence algorithm, the same approach version control systems use. That means it finds the lines the two versions share and only flags the genuine insertions and deletions, even when a paragraph is added in the middle and everything below it shifts down.
The Controls You Get
Everything is on one screen, and every option updates the result the moment you change it:
- Original and Changed boxes — the left box is your starting point, the right box is the edited version. Type or paste into either one.
- Split view — shows the two versions next to each other, with removed and added lines paired across the columns so you can read the change horizontally.
- Unified view — stacks everything in one column with a plus for added lines and a minus for removed lines, like a git diff or a code review.
- Swap — flips the two sides instantly, which reverses every addition and deletion. Useful when you pasted the texts in the wrong order.
- Ignore case — treats uppercase and lowercase as identical, so a capitalization change alone will not be flagged.
- Ignore whitespace — collapses extra spaces and tabs and trims line ends, so reindentation and spacing tweaks are not counted as real differences.
- Live counts — a running tally of added, removed, and unchanged lines sits above the result so you can gauge the size of the change at a glance.
Why Compare Text Here
- Stays on your device — both texts are compared right in the browser tab, so nothing is uploaded and nothing is stored.
- Completely free — no metered character limits, no "pro" tier gating the split view or the toggles.
- No signup — open the page, paste, compare. No account or email required.
- No watermark or noise — the result is just your two texts and clean highlighting, nothing pasted in or branded onto your content.
- Instant feedback — results recalculate as you type or toggle an option, with no compare button to press.
When This Comes in Handy
- Reviewing contract edits — paste the old clause and the redlined version to see exactly which sentences a counterparty changed before you sign.
- Catching content changes — writers and editors compare two drafts of an article or landing page to confirm which lines a round of edits actually touched.
- Debugging config and code — drop two versions of a config file, log snippet, or function side by side to spot the single line that broke a build, with Ignore whitespace on to skip indentation noise.
- Verifying copied data — compare an exported list against the original to make sure rows were not dropped or duplicated during a copy or migration.
Text Diff vs Other Tools
Versus Git's git diff — git diff is the gold standard for source code and assumes versioned files. Tooldit is the right tool when the two texts aren't in git (a Notion doc before/after, a pasted email draft, a contract revision shared by email).
Versus Microsoft Word's Compare Documents — Word's comparison works well for .docx vs .docx but is fiddly to set up for arbitrary text. Tooldit accepts a paste from anywhere.
Versus dedicated apps (Beyond Compare, Meld, Kaleidoscope) — desktop diff apps are powerful but require install. For a single side-by-side comparison, the browser tool is faster.
Versus other online diff tools — many free diff sites upload your texts to a server before computing. For NDAs, legal contracts, or proprietary content, that's a leak risk. Tooldit computes the diff locally.
Troubleshooting & Common Issues
- Two identical-looking blocks show every line as different — usually a line-ending mismatch (Windows CRLF vs Unix LF). Open both files in a plain text editor, normalize line endings, then re-paste.
- Whitespace differences cluttering the result — toggle the "ignore whitespace" option. This treats trailing spaces, multiple spaces, and tab-vs-space as identical, which is usually what you want for prose comparison.
- Reordered lines flagged as changes — standard diff is order-sensitive; A→B and B→A look different. For order-insensitive comparison (e.g. unsorted lists), sort both inputs first with the duplicate remover's sort option.
- Output too large to skim — for big files, switch to "only show changes" mode (hides unchanged context) to focus on the differences.
- Diff shows whole block as removed + added — happens when many adjacent lines change at once. Word-level diff (if available) can show finer granularity within a paragraph; otherwise read both blocks side-by-side.
- Smart quotes vs straight quotes show as different — word processors often convert "..." to “...” automatically. Either enable ignore-punctuation mode (if available) or run both sides through a smart-quote normalizer first.
Frequently Asked Questions
+What kind of comparison does the text diff checker do?
It compares your two texts line by line. Each line that exists in the original but not the changed version is marked as removed (red), each new line in the changed version is marked as added (green), and lines that appear in both are shown as unchanged. The matching uses a longest-common-subsequence algorithm, so it lines up shared content even when lines are inserted or deleted in the middle.
+What do the Ignore case and Ignore whitespace options do?
Ignore case treats uppercase and lowercase as the same, so Hello and hello count as a match. Ignore whitespace collapses runs of spaces and tabs into a single space and trims the ends of each line, so changes that are only spacing or indentation are not flagged. The comparison uses the normalized text to decide what matches, but the result panel still shows your original lines exactly as you typed them.
+What is the difference between Split and Unified view?
Split view places the original on the left and the changed version on the right, with removed and added lines paired side by side so you can read across. Unified view stacks everything in one column with a plus sign for added lines and a minus sign for removed lines, similar to a code review or git diff. Use the buttons above the result to switch between them at any time.
+Does the Swap button change my comparison?
Yes. Swap flips the two sides, so what was the original becomes the changed text and vice versa. Because additions and deletions are defined relative to the original, swapping turns every added line into a removed line and every removed line into an added line. It is handy when you pasted the texts in the wrong order or want to see the change from the opposite direction.
+Are my texts uploaded anywhere when I compare them?
No. The whole comparison runs in your browser with JavaScript. The two texts you paste are never sent to a server, never stored, and never logged. Closing the tab clears everything, which makes the tool safe for drafts, contracts, source code, or any text you would rather not upload.
+What does "unified" vs "side-by-side" mean?
Unified diff stacks the old and new lines vertically with - for removals and + for additions, which is the format git diff uses. Side-by-side puts old and new in two columns, easier to scan for prose. Pick whichever matches the task — unified for code, side-by-side for writing.
+Can I diff JSON or structured data?
Text diff works on the literal characters, so two semantically-equivalent JSONs with different formatting will look very different. Pretty-print both first with the JSON formatter using the same indentation, then diff. For semantic JSON diff (key-by-key), use jq or jsondiff.
+Is there a maximum text size?
No hard cap, but the diff algorithm's memory use grows with the product of the two input lengths. Comparing two 10,000-line files works fine; comparing two 1,000,000-line files will stall the browser. For huge files, split into chunks or use git diff on the command line.
Related Text Tools
Cleaning up text before or after a comparison? Try the remove duplicate lines tool to strip repeated rows, or the word counter to measure each version's length. All Tooldit text tools run entirely in your browser.