Free · Fast · Privacy-first

Compare Two Text Files Online

Paste or upload two text files and see every difference highlighted the moment you click Compare.

⚡

Side-by-side and inline diff views

🔒

Highlights additions, deletions, and changes

✨

Supports plain text, code, and markup

⚡

Runs in browser, files stay private

Cost: Free forever
Sign-up: Not required
Processing: In your browser
Privacy: Files stay local

FreeNo signupWhite-label

Add this Diff Checker to your website

Drop the Diff Checker into any page — blog post, product docs, intranet, school portal — with a single line of HTML. Your visitors get the full tool, processed entirely in their browser. No backend, no uploads, no signup.

Files stay 100% in the visitor's browser
Responsive — adapts to any container width
Free forever, no API key needed

Embed code

<iframe
  src="https://www.fixtools.io/developer/diff-checker?embed=1"
  width="100%"
  height="780"
  frameborder="0"
  style="border:0;border-radius:16px;max-width:900px;"
  title="Diff Checker by FixTools"
  loading="lazy"
  allow="clipboard-write"
></iframe>

<iframe
  src="https://www.fixtools.io/developer/diff-checker?embed=1"
  width="100%"
  height="780"
  frameborder="0"
  style="border:0;border-radius:16px;max-width:900px;"
  title="Diff Checker by FixTools"
  loading="lazy"
  allow="clipboard-write"
></iframe>

Attribution-friendly: a small "Powered by FixTools" link appears in the embed footer.

Why Comparing Text Files Manually Falls Short

Text files accumulate changes over time through editing, versioning, and collaboration. Whether you are reviewing a configuration file that may have drifted from its baseline, checking whether a contract clause was altered between drafts, or verifying that a build script matches the version stored in your documentation, manual comparison is slow and unreliable. Human eyes miss subtle changes: an extra space, a transposed character, or a quietly removed line. A dedicated comparison tool removes that risk entirely by systematically scanning every character of both files and surfacing every difference, no matter how small or how deeply buried in a long document.

FixTools Diff Checker applies the Myers diff algorithm, an efficient method published by Eugene Myers that finds the shortest edit script required to transform one text sequence into another. It computes the longest common subsequence of lines between your two files, then marks everything outside that common sequence as either an addition or a deletion. The result mirrors the classic unified diff format: added lines tagged in green, removed lines in red, and unchanged context lines shown neutrally. Because all processing runs in your browser via JavaScript, there is no round-trip to a server, no latency from file upload, and no risk of confidential content being intercepted or retained.

Once you have the diff output, focus on red blocks first to confirm nothing was removed unintentionally, then check green blocks for unexpected additions. Use the side-by-side view for long files where reading context from both versions simultaneously helps you understand the intent behind each change. Switch to inline view when you need a compact single-column summary of all changes or are working on a narrow screen. Both views show character-level highlights within changed lines, making even a single-word modification easy to locate and evaluate.

Performance on very large text files depends on the longest common subsequence computation, which scales roughly with the product of the line counts of both inputs. For files in the tens of thousands of lines, this remains responsive on a modern laptop because the underlying algorithm runs in linear space with the optimisations described by Myers in the original paper. If you are comparing files that exceed a few hundred thousand lines, the browser may pause briefly while the diff is computed, but no data ever leaves the page. The entire pipeline is client-side JavaScript: the text you paste is parsed, tokenised by line, compared, and rendered without a single network request after the page itself has loaded. This privacy property is the main reason the tool is safe for confidential text such as unreleased contracts, internal memos, and proprietary configuration. Closing the tab clears all state immediately with no cached copy retained anywhere.

Myers diff algorithm paper GNU diff manual, unified format

How to use this tool

💡

Paste your original text in the left panel and your updated text in the right panel, then click Compare to see all differences highlighted.

How It Works

Step-by-step guide to compare two text files online:

1
Open the Diff Checker
Click "Open Diff Checker" to launch the tool in your browser. No installation or sign-in is required. The tool opens immediately and is ready to accept text in both panels.
2
Paste your original text
Paste or type your original (baseline) text into the left panel. This is the version you are treating as the reference point, such as the approved draft, the committed version, or the document before editing began.
3
Paste your updated text
Paste or type the newer version of the text into the right panel. This is the version you want to check against the original, such as the revised draft, the deployed file, or the document after editing was completed.
4
Click Compare
Click the Compare button to run the diff. Results appear in seconds. Additions are highlighted in green, deletions in red, and unchanged lines remain unformatted. Within changed lines, the exact words or characters that differ are highlighted at a finer level.
5
Review the differences
Scroll through the results to review every change in order. Switch between side-by-side and inline views as needed. Use side-by-side to read context from both versions together, and inline to get a compact list of all changes in a single scrollable column.

Real-world examples

Common situations where this approach makes a real difference:

Server configuration audit

A sysadmin copies the live nginx.conf from a production server and the version stored in the team wiki. Pasting both into the Diff Checker immediately reveals three extra location blocks added to production that were never documented. The audit takes under two minutes instead of a slow manual line-by-line review, and the precise list of differences goes directly into the incident report without any additional work to summarise what changed.

Contract revision review

A legal assistant receives two versions of a vendor agreement and needs to confirm which clauses changed before a signing deadline. Rather than printing both and annotating by hand, they paste both documents into the Diff Checker and get a colour-coded summary of every altered sentence in seconds. No confidential text is sent to any external server, and the diff output can be saved as a PDF for the audit file.

Build script verification

A DevOps engineer wants to confirm that a deploy script on a staging server exactly matches the canonical version stored in the repository. Copying both into the Diff Checker immediately shows a commented-out debug line left in staging that would have caused a silent failure in production. The fix takes thirty seconds and the incident is prevented before it occurs.

Academic draft comparison

A researcher is finalising a paper and needs to confirm that every reviewer-requested revision from one draft was correctly carried into the final submission version. The Diff Checker highlights each rewritten paragraph and changed citation, making it straightforward to verify that no requested change was accidentally reverted and no approved content was unintentionally removed during the revision process.

Pro tips

Get better results with these expert suggestions:

Normalise line endings before comparing

Files edited on Windows use CRLF line endings while Unix and macOS files use LF. If your diff shows every single line as changed even though the content looks identical, the cause is almost certainly mixed line endings. Open both files in a text editor that can convert line endings, normalise both to the same format, then paste the corrected versions to get a clean diff showing only real content differences.

Use inline view for quick summaries

Side-by-side view is ideal for reading context and understanding the intent behind each change, but inline view condenses both versions into a single column and is faster to scan when you need a quick count of total changes, want to copy the diff into a summary document, or are working on a screen too narrow for two columns.

Compare sorted versions for unordered lists

If your files contain unordered lists such as dependency lists, permission sets, or feature flags, sort both files alphabetically before diffing. This prevents the diff from flagging reordered items as changes when only the order shifted, and makes genuinely new or removed items stand out clearly against the sorted background.

Paste incrementally for large files

For very large files on older devices, paste one logical section at a time rather than the entire file at once. Comparing a few hundred lines at a time keeps the browser responsive, produces shorter and more manageable diffs for each section, and makes it easier to investigate the changes within each part before moving to the next.

FAQ

Frequently asked questions

Paste your two texts into the left and right panels of the FixTools Diff Checker and click Compare. All differences are highlighted immediately with no software installation or sign-up required. The tool works for any plain-text content including documents, code, configuration files, and markup. Added content appears in green, removed content in red, and changed lines show both the removed and added version for easy side-by-side reading.

Yes. You can paste text directly or upload plain text files (.txt) into either panel. The diff runs in your browser on the uploaded file contents without sending anything to a server. For other file types such as Word documents or PDFs, copy the text content from the file and paste it into the panels. The comparison works on the text itself regardless of how you enter it.

No. All diffing happens entirely client-side in your browser using JavaScript. Your text content is never sent to FixTools servers, which makes the tool safe to use with confidential contracts, internal configuration files, proprietary source code, or any other sensitive text. Closing the browser tab clears everything with no data retained.

Any plain text content works: source code in any programming language, configuration files, log files, markdown, HTML, CSS, JSON, XML, YAML, and more. The tool is language and format agnostic. If the content can be opened in a text editor and read as plain text, it can be compared here. Binary file formats like images, PDFs, and Word documents require extracting the text content first. File encoding can affect what counts as the same text: a file saved as UTF-8 and a file saved as UTF-16 contain different byte sequences for the same logical characters, and a file saved as Windows-1252 or another legacy single-byte encoding can decode incorrectly when the browser expects UTF-8. If accented characters or non-Latin scripts appear as garbled question marks or replacement glyphs in the panels, re-export both files from the source application as UTF-8 without BOM and paste again. Normalising both inputs to the same encoding and the same line-ending style is the single most effective preparation step for getting a clean diff on files that originate from different operating systems or different export tools.

Yes. The browser-based diff handles files limited only by your device memory, which on modern machines is typically several hundred megabytes. For very large files where the full diff is hard to navigate, consider splitting the content into logical sections and comparing each section separately. This produces shorter, more manageable diffs and makes it easier to review each part in detail.

This almost always means the two files use different line endings: CRLF on Windows versus LF on Unix and macOS. When line endings differ, every line is technically different, causing the diff to flag the entire file. Open each file in a text editor that can display and convert line endings, normalise both to the same format (LF is the most portable choice), then paste the normalised versions into the Diff Checker. A closely related cause is a byte-order mark (BOM) at the start of one file but not the other. A UTF-8 BOM is three bytes (0xEF 0xBB 0xBF) that some editors prepend silently on save, particularly Windows-era tools and certain export pipelines from legacy systems. The BOM is invisible in most viewers but counts as content during the diff, which makes the first line appear changed even when the visible text is identical. Strip the BOM in both files using an editor that exposes the option, or normalise both inputs through a tool that writes UTF-8 without BOM, and re-run the comparison for a clean result.

You can copy the diff output from the result panel and paste it into an email, document, or chat message. For a formatted report that preserves the colour coding, use your browser print function (Ctrl+P on Windows or Cmd+P on macOS) and save the result as a PDF. This creates a portable, shareable record of the differences that retains the visual highlighting for easy reading.

Yes. The Diff Checker handles full Unicode text, so you can compare documents in any language including scripts with right-to-left reading order such as Arabic and Hebrew, and character-based scripts such as Chinese and Japanese. Character-level differences within changed lines are highlighted correctly regardless of the script used. There is no need to configure the language or encoding before running the comparison.

When a file is large enough to push the browser tab close to its memory ceiling, the diff can stall or the page can become unresponsive. The fix is almost always to split the file into smaller logical sections and compare each in turn. Close other open tabs to free memory, then paste a few thousand lines at a time rather than the entire file. If you are on a low-memory device, consider exporting the file as text from the source application with no trailing binary content, since stray bytes can also cause the parser to slow down. Restarting the browser between large comparisons clears any accumulated heap state.

For day-to-day work inside a Git repository, the native git diff command is faster because it operates directly on the working tree. The browser tool covers the cases where Git is not available or convenient: comparing a file pulled from a backup to the current source, comparing text pasted from email to a committed version, or sharing a visual diff with a reviewer who does not have repository access. The standard workflow is to run git show or git checkout to extract the two file versions to compare, then paste both into the tool to get a colour-coded view that you can save as a PDF or screenshot and attach to a ticket.

The output follows the convention used by GitHub, GitLab, and the GNU diff utility. Green highlighting marks lines present in the right panel but absent in the left, which the tool treats as additions. Red highlighting marks lines present in the left panel but absent in the right, which are treated as deletions. A modified line appears as a deletion on one row immediately followed by an addition on the next row, with both rows aligned in the side-by-side view. Within each changed line, a second colour pass highlights the specific characters that differ from the matched line on the opposite side.