Markdown Diff Viewer

See exactly what changed between two Markdown versions

You have an old README and a new one, a draft and an edited draft, or the text an AI assistant gave you yesterday and the version it gave you today. Reading both side by side to spot the differences is slow and error-prone. The Markdown Diff Viewer lines the two versions up and marks every line that was added or removed, so you can review an edit in seconds instead of minutes.

The viewer is built for Markdown, but because it compares plain lines of text it works just as well for any line-based file: changelogs, YAML config, plain notes, CSV rows or code.

How the comparison works

Both inputs are split into lines. The tool then computes the longest common subsequence of lines, which is the largest set of lines that appear in both versions in the same order. Those lines are treated as unchanged. Every other line in the original is marked as deleted, and every other line in the updated version is marked as added. This is the same basic idea that classic diff programs use, applied to whole lines.

Lines must match exactly to count as unchanged. A single extra space, a changed letter or a trailing tab makes a line different, and it appears as one deletion plus one addition. Where a single line was edited in place, the old version normally appears first with a minus sign and the new version follows with a plus sign.

The whole comparison runs in your browser. Neither version is sent to this site's server.

Reading the result

The Diff Result card shows every line of the comparison in order, in three styles:

Above the result, three status cards summarise the outcome. Diff status reads "Not compared yet", "Changes found" or "No changes". Added lines and Deleted lines give the counts. The footer shows the length of the raw diff and the total number of compared lines.

The diff is a full view rather than a trimmed patch: unchanged lines are kept so you can see each change in its surrounding context. There are no @@ hunk headers.

Step-by-step

1. Put the older text into Original Markdown (the "Before version"). You can type, paste with your keyboard, or press Paste Original in the header.
2. Put the newer text into Updated Markdown (the "After version"), or press Paste Updated.
3. Check the character and line counts under each editor to confirm you pasted the full documents.
4. Press Diff.
5. Scroll through the Diff Result. Red minus lines were removed, green plus lines were added.
6. Press Copy Diff to copy the plain-text diff, or Download to save it as markdown-diff.txt.

Each editor has its own copy button, Copy Original and Copy Updated. Clear empties both editors and the result. Typing in either editor after a comparison resets the result, so press Diff again whenever you change something.

The plain-text diff format

Copy Diff and Download both use the same simple format: one line per compared line, starting with +, - or a space, followed by the line content. For example:

# Project
-Version 1.2 is out.
+Version 1.3 is out.
## Install

This reads naturally in chat, email or a code review comment. It is not a full unified patch, so tools like git apply or patch will not accept it as-is.

Who finds it useful

• Technical writers reviewing an editor's changes to a documentation page before publishing.
• Developers comparing two copies of a README or changelog without opening a terminal or creating a commit.
• Content teams checking what a translator, client or reviewer changed in a Markdown article.
• People working with AI writing tools, who often get a slightly different answer on each run and want to see what actually moved.
• Students and researchers tracking edits between drafts of notes written in Markdown.

Getting a clean, meaningful diff

• Normalise formatting first. If one version was run through a formatter and the other was not, nearly every line may differ. Format both the same way, then compare.
• Watch for trailing spaces. They are invisible but make lines unequal. If a line looks identical yet shows as changed, trailing whitespace is the usual cause.
• Remember that line endings are normalised. Windows and old Mac line breaks are converted to plain line feeds, so a file saved on Windows compares cleanly with the same file saved on Linux.
• Hard-wrapped paragraphs exaggerate changes. If one version wraps at 80 characters and the other uses one line per paragraph, rewrap one of them so the lines line up.
• For long documents, compare a section at a time. The result is easier to read and the comparison finishes faster.

A quick review routine

For an edit review, start with the counters rather than the full list. If Added lines and Deleted lines are both small, read the marked rows one by one. If both are large and roughly equal, the change is probably formatting, such as rewrapped paragraphs or converted list markers, and normalising both versions first will give you a much shorter, more useful diff. If only Added lines is large, someone has written new sections, so jump to the green blocks.

What not to use it for

• Do not expect word-level or character-level highlighting. A one-word edit shows the whole line as removed and re-added.
• Do not treat it as a merge tool. It shows differences but cannot combine two versions or resolve conflicts.
• Do not use the downloaded text as a patch file for version control.
• Do not expect rendered output. The viewer compares the Markdown source, not the formatted page.

Limitations

• Line-based comparison only, with exact matching including whitespace and letter case.
• The algorithm builds a table the size of the two line counts multiplied together. Very large files, tens of thousands of lines each, can make the browser slow or unresponsive. Split them into parts if that happens.
• Moved blocks are shown as a deletion in one place and an addition in another; moves are not detected as such.
• There is no side-by-side layout, ignore-whitespace option or context-trimming option in the current interface.

Related tools

If formatting noise is hiding the real changes, run both versions through the Markdown Beautifier first, then compare. To see how either version renders, open it in the Markdown Previewer.

Frequently Asked Questions