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Side-by-Side Text Comparison Online

Side-by-side comparison is the clearest way to review differences between two versions of a text or document.

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Both versions displayed simultaneously

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Changed lines aligned across both panels

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Colour-coded additions and deletions

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Toggle between side-by-side and inline views

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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.

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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 Side-by-Side Is the Clearest Way to Review Changes

Side-by-side comparison is the preferred layout for reviewing document changes when context matters as much as identifying what changed. Unlike inline diff view, which interleaves deleted and added content in a single column requiring the reader to mentally filter alternating colours, side-by-side places both the original and the updated version in parallel panels with changed lines aligned horizontally. This layout lets reviewers read the original version on the left and the new version on the right and compare corresponding lines directly without context switching. For long documents with many small changes distributed throughout, side-by-side view significantly reduces the cognitive load of review by keeping both versions readable in their natural, uninterrupted flow rather than fragmenting each into alternating colour blocks.

FixTools implements the Myers diff algorithm to compute the minimum edit script between the two texts. The algorithm identifies the longest common subsequence of lines between the two inputs, which become the aligned anchor lines visible in both panels of the side-by-side view. Changed lines in the original appear highlighted in the left panel, and their corresponding replacements appear highlighted in the right panel at the same vertical position. Within changed lines, the specific words or characters that differ are highlighted at a finer granularity on top of the line-level highlighting. Both panels scroll in synchrony, keeping corresponding sections of both documents aligned as you navigate through long texts.

Side-by-side view is particularly valuable for legal and contract review, where understanding the original clause alongside its proposed replacement is essential for assessing the legal implications of the change. It is also the standard layout for code review in platforms like GitHub and GitLab, which use it as their primary diff display mode because it is the most readable format for multi-line changes surrounded by significant context. For very short diffs or when you need to copy the changes into a written summary, switching to inline view provides a more compact, sequential representation of the same information.

The performance characteristic of side-by-side rendering is worth understanding. The Myers diff itself is fast even for very large inputs, but rendering two aligned panels with character-level highlighting on every changed line is more layout work for the browser than a single-column inline view. For documents of a few thousand lines, the difference is invisible. For very large inputs in the tens of thousands of lines or more, the side-by-side renderer may take noticeably longer to paint than the inline view, particularly on older devices. If you encounter this, switching to inline view for navigation and back to side-by-side for detail inspection is a common workflow. The privacy guarantee is identical across both view modes: nothing is transmitted to any server, all computation runs in the browser tab, and closing the tab clears everything immediately. The two-way diff model is appropriate for the vast majority of side-by-side comparison tasks.

Myers diff algorithm paper Unix diff utility manual (man7.org)

How to use this tool

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Paste your two texts and choose the side-by-side view in Diff Checker for a parallel comparison where both versions are visible at once.

How It Works

Step-by-step guide to side-by-side text comparison online:

1
Open Diff Checker
Click "Open Diff Checker" to launch the comparison tool in your browser. No account or sign-up is required.
2
Paste both texts
Enter your original or baseline text in the left panel and your updated or revised version in the right panel. Paste the complete content of each version for a full comparison.
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Select side-by-side view
Choose the side-by-side view option to display both text versions simultaneously in parallel columns with changed lines aligned horizontally across both panels.
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Compare and review
Click Compare then scroll through both panels simultaneously. Both panels scroll in sync, keeping corresponding sections aligned. Use the colour-coded highlights to identify every change with full surrounding context visible on each side.

Real-world examples

Common situations where this approach makes a real difference:

Contract redlining and legal review

A lawyer reviewing a revised contract needs to read each changed clause in the context of both the original wording and the proposed replacement simultaneously to assess the legal implications accurately. The side-by-side layout makes it possible to read the original clause on the left and the proposed revision on the right in perfect horizontal alignment, assessing the impact of each specific word change with full surrounding context from both versions visible at once without switching between windows or documents.

Reviewing a technical specification update

An engineering lead reviews a revised technical specification before a project kick-off meeting where implementation decisions will be made. The side-by-side view shows exactly which requirements changed between the draft and the updated version, with both the original requirement wording and the new version visible in parallel for each change. This makes it straightforward to assess whether the revised wording materially changes the implementation scope or is simply a clarification of the original intent.

Comparing academic paper drafts for submission

A researcher incorporates peer review feedback and needs to verify that all requested changes were applied correctly before resubmission to the journal. The side-by-side view shows the original submitted text on the left and the revised manuscript on the right, making it easy to confirm that each reviewer-requested change was addressed as specified and that no content approved by the reviewers was accidentally removed or modified during the revision process.

Reviewing localisation changes in a software UI

A localisation manager reviews updated UI string translations for a software product release. The side-by-side view shows the original translated strings on the left alongside the updated translations on the right, making it easy to assess whether each changed translation preserves the intended meaning, matches the UI space constraints, and maintains a consistent tone with the surrounding unchanged strings without reading the two lists sequentially.

Pro tips

Get better results with these expert suggestions:

Use side-by-side for context-heavy changes

Side-by-side view delivers the most value when each changed line needs to be understood in the context of several surrounding unchanged lines. If changes are densely packed and most lines are highlighted, the inline view is faster to scan through from top to bottom. If changes are sparse and each individual change requires reading the surrounding context to understand its significance, side-by-side provides a significantly clearer picture of what was present before and what replaced it.

Widen your browser window for large documents

Side-by-side view divides the available screen width between two panels. For documents with long lines such as flowing prose paragraphs or wide code with deeply nested structure, maximise your browser window or use a wide monitor to give each panel sufficient horizontal space to display full lines without wrapping. Wrapped lines in side-by-side view are harder to align mentally across the two panels and slow down the review process noticeably on narrow screens.

Switch to inline for generating written change summaries

When you need to write a written summary of what changed, whether for a release note, an audit log entry, or a change management document, switch from side-by-side to inline view after completing your initial review. The single-column format of inline view makes it easier to copy specific changed sections into a document without inadvertently including content from both panels, and reads more naturally as a sequential list of changes when pasted into a text document.

Print in landscape orientation for side-by-side PDFs

When saving a side-by-side diff as a PDF for record keeping, formal review, or sharing with stakeholders, use landscape page orientation in the print dialog rather than portrait. Landscape orientation gives each panel more horizontal space and prevents lines from wrapping in the printed output, producing a more readable PDF that preserves the side-by-side layout with both versions clearly legible on each printed page.

FAQ

Frequently asked questions

Side-by-side comparison displays two versions of a text simultaneously in parallel columns. Changes are highlighted in both versions at the same time so you can see exactly what was added, removed, or modified while reading context from both sides without switching between documents. Changed lines are aligned horizontally across both panels so that the original version and its replacement appear at the same vertical position. It is the standard layout for code review, contract redlining, and document comparison in professional tools.

Side-by-side is better for longer documents where understanding the surrounding context around each change is important, for reviewing changes in documents where the original wording matters such as contracts and specifications, and for code review where seeing both the before and after version of each changed block in parallel is more informative than seeing them interleaved. Inline view is better for short snippets, for generating written change summaries to copy into other documents, and when most lines have changed and dense interleaving makes the parallel layout less useful.

Yes. The side-by-side view synchronises scrolling between both panels so corresponding lines stay vertically aligned as you scroll through the document. Changed lines in the original on the left always appear at the same vertical position as their corresponding replacement lines in the right panel. This synchronised scrolling is essential for maintaining context alignment throughout long documents and is a key advantage of the side-by-side format over manual comparison of two separate windows. The split-pane scroll lock tracks both vertical and horizontal scrolling, which keeps wide content like wrapped prose or indented code aligned even on narrow screens. For maximum legibility during long review sessions, the panels render in a fixed-width monospace font where each character occupies the same horizontal space, so corresponding columns line up cleanly across both sides without the proportional drift you get with variable-width fonts. This choice matters most for code, tabular text, and structured data where vertical column alignment carries meaning that a proportional font would visually disrupt.

Yes. Side-by-side comparison is a standard method for contract redlining and legal document review, allowing lawyers and reviewers to assess changes between document versions with full context from both the original and revised versions visible simultaneously. All processing runs entirely in your browser, so confidential contract text is never transmitted to any external server. This makes the tool appropriate for comparing sensitive legal, financial, and business documents.

Yes. Within changed lines, the diff highlights the specific words or characters that changed at a finer granularity than the whole line. This word-level highlighting makes it easy to spot a single changed word or value within a long line of otherwise unchanged text, which is important for catching subtle wording changes in contracts, specifications, and compliance documents where a single word can materially change the meaning or obligation.

You can use your browser's print function with Ctrl+P on Windows or Cmd+P on Mac to print or save the diff view as a PDF for record-keeping, sharing, or formal review purposes. For side-by-side diffs specifically, use landscape page orientation in the print dialog to give each panel sufficient horizontal space for readable line widths and to prevent lines from wrapping in the printed output.

When one document has more lines than the other, the diff algorithm aligns common lines across both panels and inserts blank space in the shorter panel to maintain the vertical alignment of corresponding content. Lines that exist only in one document, such as added or deleted paragraphs or blocks, appear highlighted in their panel against a blank area in the corresponding position of the other panel, making it immediately clear that the content is present in one version but absent in the other.

Side-by-side view requires sufficient screen width to display two panels simultaneously with readable content in each. On narrow mobile phone screens, the layout typically switches to a stacked or single-column view automatically to maintain readability on the smaller screen. For the full side-by-side experience with both panels visible simultaneously, use a tablet in landscape orientation, a laptop, or a desktop browser with the window at full width.

The side-by-side renderer does more layout work than the inline view because it paints two aligned panels with character-level highlighting on every changed line. For very large inputs, this can be noticeably slow on older hardware. Three remedies usually help. First, switch to inline view for navigation and reserve side-by-side for inspecting specific blocks in detail. Second, close other browser tabs to free memory, since the rendering also consumes the page heap. Third, split the comparison into logical sections rather than pasting the entire file at once, which reduces both the diff computation time and the rendering load. Restarting the browser between very large comparisons clears any accumulated state.

Run git show commit:path/to/file in your terminal to extract any version of any file from any commit, then paste the extracted versions into the left and right panels of the side-by-side view. This gives you the same parallel-column layout used by the GitHub pull request diff viewer, but for any pair of versions across commits, branches, or even separate repositories. The output is portable: screenshot it for a Slack thread, save it as a PDF in landscape orientation for an audit log, or share the URL with a reviewer who needs to assess a specific change without repository access. The colour convention matches the major Git platforms so the visual translation is direct.

In side-by-side view, the left panel shows the original version and the right panel shows the revised version. Red highlighting in the left panel marks content that was removed in the revision. Green highlighting in the right panel marks content that was added in the revision. Modified lines appear as a red row on the left aligned horizontally with a green row on the right at the same vertical position, so you can read the old wording and the new wording in direct parallel. Within each highlighted line, a brighter shade marks the specific characters that differ between the two sides, which is essential for spotting a single changed word inside a long line.