Text Diff Checker
Free text diff checker. Compare two blocks of text and see added, removed and unchanged lines highlighted side by side.
Spotting what changed between two versions of a document by reading them side by side is slow and unreliable — the human eye skips over small edits, and those are usually the ones that matter. A diff tool compares them mechanically and highlights every difference.
Paste two texts and see added, removed and unchanged lines marked clearly. Both texts stay in your browser, so contracts and confidential drafts are safe to compare here.
How to use the Text Diff Checker
Paste the original text
The earlier or reference version goes on the left.
Paste the modified text
The newer version goes on the right.
Read the highlighted result
Added lines are marked in green, removed lines in red, and unchanged lines are left plain.
Check the summary
The count of added and removed lines tells you the scale of the change at a glance.
When a diff is the right tool
- Contract review. Comparing a returned draft against the version you sent catches the clause quietly altered in paragraph nineteen.
- Code review outside a version control system — comparing a config file against a known-good copy.
- Editorial changes. Seeing exactly what an editor or collaborator changed in a manuscript.
- Translation checking. Confirming which source strings changed and therefore need re-translation.
- Data comparison. Finding which rows differ between two exports of the same list.
- Plagiarism spot-checks on a passage you suspect was copied.
How diff algorithms work
Diff tools solve the longest common subsequence problem: find the longest sequence of lines appearing in both texts in the same order, and everything outside it is an addition or a removal.
The algorithm most tools use is Myers’ 1986 diff, which finds a minimal edit script efficiently. It works line by line, which is why moving a paragraph shows as a deletion in one place and an addition in another rather than as a move.
Line diff versus word diff
A line diff — what this tool does — treats each line as a unit. Change one word and the whole line shows as removed and re-added. This is ideal for code and structured text, where line boundaries are meaningful.
A word diff highlights the specific words that changed within a line, which reads better for prose. For long paragraphs, breaking your text into shorter lines before comparing makes a line diff considerably more readable.
Getting cleaner results
- Normalise line endings first. Windows uses CRLF and Unix uses LF; a file that has moved between systems can show every line as changed.
- Watch for invisible whitespace. Trailing spaces and tab-versus-space differences register as changes even though nothing looks different.
- Compare like with like. Diffing formatted output against source markup produces noise rather than insight.
- Reformat both sides consistently before comparing code — running the same formatter over each removes stylistic differences and leaves only real changes.
Frequently asked questions
Is my text uploaded anywhere?
No. The comparison runs entirely in your browser, which makes it safe for contracts, legal drafts and unpublished manuscripts.
Does it compare word by word or line by line?
Line by line. Changing a single word marks the whole line as changed. Splitting long paragraphs into shorter lines before comparing gives a more readable result for prose.
Can I compare files rather than pasted text?
Open each file, copy its contents and paste them in. Plain text, code, CSV and JSON all work; binary formats such as .docx and .pdf need converting to text first.
Why does it show every line as changed?
Almost always mismatched line endings — CRLF on one side and LF on the other — or a difference in indentation between tabs and spaces. Normalise both texts and compare again.
Can I compare Word documents?
Not directly, since .docx is a compressed binary format. Copy the text out of each document and paste it in, or use Word’s own Compare Documents feature to preserve formatting changes.
Is there a size limit?
No fixed limit, though very large texts of tens of thousands of lines will take a moment, since the comparison runs on your own device.
What do the colours mean?
Green marks lines present only in the second text, red marks lines present only in the first, and unmarked lines are identical in both.
Does it detect moved text?
No. A moved block appears as a deletion in the old position and an addition in the new one, which is standard behaviour for line-based diff algorithms.
100% private. Files are processed on your device and never uploaded.
Instant results. No upload queue. Processing time depends on your device.
Free forever. No signup, no watermark, no paid tier.
Works everywhere. Use a modern browser on your phone or computer.
Related tools
Text Diff Checker is one of all the free tools on ToolNest. No account or subscription needed — anywhere in the world.