Diff Checker — Free Online Diff Tool
Paste or drop two snippets of text, code or documents and see every change highlighted instantly. Word, line and character-level diff with a side-by-side view — and unlike most online diff checkers, nothing ever leaves your browser.
No comparison yet. Paste or upload text in both fields below — results appear here automatically, and nothing is calculated until you do.
Compare your two texts
Original Text (A)0 words
Modified Text (B)0 words
What a Diff Checker Does
A diff checker takes two inputs — call them A and B — and computes the smallest set of insertions and deletions that turn A into B. The result is rendered as a visual diff: additions in green, deletions in red, often with character-level highlights on changed words. Under the hood SnapTextCompare uses a Myers-style algorithm, the same family of diff used by git.
Diff checkers are the fastest way to answer "what exactly changed?" — whether that's a contract clause, a config file, a paragraph of marketing copy, or a function body. Manual reading misses things; a good diff doesn't.
Diff Modes
Word diff. Highlights changed words inside a line. Best for prose, configs and short scripts.
Line diff. Shows whole-line additions and deletions, unified-diff style. Best for source code and log files.
Character diff. Zooms into the changed characters within a word — perfect for catching a single-letter typo or a swapped digit in a contract number.
How the Diff Algorithm Actually Works
At the heart of every modern diff checker sits the longest common subsequence (LCS) problem: given two sequences, find the longest sequence of tokens that appear in both, in order. Everything not in the LCS is either a deletion (in A but not B) or an insertion (in B but not A).
Eugene Myers's 1986 paper "An O(ND) Difference Algorithm and Its Variations" gave a practical algorithm with runtime proportional to N × D, where N is the input length and D is the number of edits between A and B. The key insight: similar inputs are computed almost instantly, even when they're large — because D stays small. Two near-identical 50,000-line files often diff in milliseconds.
SnapTextCompare implements a Myers-style diff in pure JavaScript, with an additional post-processing step that promotes "changed" pairs (a deletion immediately followed by a similar insertion) into intra-line character highlights. That's why a single typo inside a long paragraph appears as a precise red/green character flash instead of a full-line strike-through.
SnapTextCompare vs Other Diff Checkers
| Feature | SnapTextCompare | Diffchecker | Diffnow |
|---|---|---|---|
| 100% in-browser | ✅ | ❌ uploads to server | ❌ uploads to server |
| Free, no signup | ✅ | Limited (paid Pro) | ✅ |
| No ads | ✅ | ❌ | ❌ |
| PDF + Word upload | ✅ | Paid | ✅ |
| Export TXT / HTML / JSON | ✅ | Paid | Limited |
Diff Checker Use Cases
Code review. Compare two versions of a function without opening a PR.
Contract redlines. Verify only negotiated terms moved between drafts.
Copy editing. Show authors exactly what was changed in their draft.
Config debugging. Find the one line that broke production.
Prompt iteration. Compare two AI prompts (or two outputs) word by word.
API response diffing. Paste two JSON responses and instantly see which field changed between calls.
Email approval. Diff a marketing draft against the legal-approved version before scheduling the send.
Power-User Shortcuts
Paste & go. The tool diffs continuously as you type — no "Compare" button to click.
Ignore whitespace when comparing reformatted code (after a Prettier run, for instance) — only logic changes remain visible.
Ignore case when comparing email lists, SKUs, or anything where capitalisation drifts but content is identical.
Swap sides if you accidentally pasted the new version into the "original" side — one click reverses the highlighting.
Export JSON when you want to feed the diff into a CI pipeline, a Slack bot, or any downstream automation.
Related comparison tools
How SnapTextCompare compares source code
Diff engine
Google's diff-match-patch (Apache 2.0), running in your browser. Character mode diffs raw characters; word and line modes encode tokens or lines first and diff those, then decode the result.
Similarity & importance
Similarity = 1 − Levenshtein / max(len). Importance weighs each change by category — money and dates score 90+, punctuation scores 3. Deterministic on any device.
Privacy
Files are read with the HTML5 File API and processed in your browser. No network request is ever made with your content — verifiable in the Network tab of your browser's DevTools.
Code is compared in the same word/line diff engine, then syntax-highlighted with highlight.js (100+ languages auto-detected). Whitespace-insensitive mode and Levenshtein similarity work identically to prose — useful for reviewing patches without a full IDE.
Diff Checker FAQ
Common questions about online diff checkers.