Code Diff Tool: Compare Code Online Free

This free code diff tool compares two versions of code and highlights every line that was added, removed or left unchanged. Paste snippets or upload files, pick a side-by-side or inline view, and get change counts plus a similarity percentage in one click. It is free and needs no sign-in.

What Is a Code Diff and How Is It Measured?

A code diff is a line-by-line comparison of two versions of the same text that lists what was added, removed and kept. The name comes from the Unix diff command, and Git commits, pull requests and code reviews all present changes in this style.

The tool reads your Original Code and Modified Code in order and finds the longest run of lines the two versions share. Lines that exist only in the original are marked removed. Lines that exist only in the modified version are marked added. Everything else is unchanged. When you edit a line, it shows up as one removed line plus one added line, and the exact characters that changed are highlighted inside both.

Matching is exact. A line that differs by a single space, tab or trailing character counts as a different line, so invisible changes can show up in your results.

How to Use the Code Diff Tool

You can run a comparison in under a minute. Follow these steps:

1. Optional: click one of the sample buttons (JavaScript, Python, HTML or CSS) to load example code and see what a finished comparison looks like.
2. In the Original Code panel, stay on the Code tab and paste the older version into the box. You can also switch to the File tab, click to select a file, or drag a file onto the drop zone.
3. Do the same in the Modified Code panel with the newer version.
4. Under Diff View, choose Side by Side or Inline.
5. Click Compare, or press Ctrl+Enter from the keyboard.
6. Check the Added, Removed, Unchanged and Similarity figures, then scroll through the highlighted output. Click Swap to reverse the two sides, or Clear All to start over.

If either box is empty, you will see the message "Please enter code in both fields to compare." Fill both sides and run it again.

File upload accepts common code and text formats, including JavaScript, TypeScript, JSX, TSX, Python, PHP, HTML, CSS, JSON, XML, SQL, Ruby, Go, Rust, Java, C, C++, shell scripts, YAML, Markdown and plain text.

Worked Example: Finding a Pricing Bug

Say a checkout function returns the wrong total, and a teammate says they fixed it. Here is the original, five lines long:

• function total(price, qty) {
• const rate = 0.07;
• const subtotal = price * qty;
• return subtotal * rate;
• }

The modified version changes only the fourth line to return subtotal * (1 + rate);. After you click Compare, the counters read -1 Removed, +1 Added and 4 Unchanged. The old return line is shaded red, the new one green, and the character-level highlight marks the inserted (1 + and the closing parenthesis.

That tells you the fix touched exactly one line and nothing else moved. The old code returned only the tax amount, and the new code returns the price plus tax. If the counters had shown +12 and -9 for a one-line fix, you would know something else changed too.

How Do You Read the Diff Results?

The results panel gives you four numbers and a color-coded view, and each one answers a specific question.

The four counters

• Added (shown as +N): lines present only in the modified code.
• Removed (shown as -N): lines present only in the original code.
• Unchanged: lines that match in both versions.
• Similarity: a percentage with a progress bar showing how much the two versions overlap.

What good and poor results look like

For a targeted fix, you want a high similarity and small Added and Removed counts, like the example above. A feature addition usually shows many green lines and few red ones. A refactor shows both, with the unchanged count staying large if the logic was only reshaped.

A similarity far lower than you expected is a warning sign. The usual causes are pasting the wrong file, comparing different branches, or a formatter that rewrote indentation across the whole file. Run both versions through a formatter and compare again, and the real edits will stand out.

The colors

Red rows are removed lines, green rows are added lines, and white rows are unchanged. In side-by-side view, gray filler cells appear opposite a line that has no partner. Darker red and green patches inside a line show the specific characters that changed.

Should You Use Side by Side or Inline View?

Choose side by side when you want to see old and new code together, and inline when you want one continuous column. Both views show the same changes and the same counters.

If you are on a phone, the diff table scrolls sideways. Inline view is usually easier to read there.

Who Uses a Code Diff Checker, and When?

Anyone who needs to know exactly what changed between two pieces of text can use it. These are the situations where it saves the most time:

• Developers check their own edits before committing, or compare a snippet from a forum answer against their current code.
• Reviewers inspect a pasted patch when they do not have the repository open.
• SEO specialists compare page source before and after a release to confirm that the title, canonical tag, robots meta tag or JSON-LD schema changed as planned, and that nothing else did.
• Site owners compare redirect rules before and after an edit. If you built them with the .htaccess redirect generator, paste the old and new files to confirm only the intended rules changed.
• Admins compare configuration files in YAML, JSON or XML between staging and production. The same applies to header settings created with the CORS header generator.
• Students and teachers compare a working solution against a broken attempt to find the typo.

Best Practices and Troubleshooting Tips

A few habits make comparisons faster and the output more trustworthy.

• Format both versions first. Different indentation or line wrapping makes unchanged logic look edited. Running both through the code beautifier removes that noise.
• Put the older version on the left. Red then means removed and green means added, as you expect. If you reversed them, click Swap instead of re-pasting.
• Watch for whitespace-only changes. If a line looks identical in both panels but is marked changed, the difference is probably a trailing space, a tab instead of spaces, or a different line ending. Select the line in each panel and check.
• Decode encoded values before comparing. Base64 strings change completely after small edits, so decode them with a Base64 decoder and compare the readable text.
• Compare one file at a time. Mixing several files in one box produces a long, confusing diff.
• Check which tab is active. If you uploaded a file but the box on the Code tab is empty, make sure the file name appears in the drop zone before clicking Compare.
• Compare generated output too. After using the .gitignore generator, you can diff the result against your current file to see which patterns are new.

What Are the Limits of This Code Diff Tool?

This tool compares plain text line by line, and that has consequences you should know about.

• It does not understand language syntax. A reordered function appears as removed lines in one place and added lines in another, not as a move.
• It has no option to ignore whitespace, letter case or blank lines. Every character counts.
• It does not merge files or produce a patch. It shows differences for you to review.
• Very long edited lines may be highlighted as a full replacement instead of showing the exact characters that changed.
• Binary files such as images or compiled programs are not supported. Use text and code files only.
• It does not replace version control. For history across many commits, use Git. This page is for quick checks between two versions you already have.

A text diff tells you what changed, not whether the change is correct. Always run your tests after reviewing a diff.

Paste your original and modified code into the boxes above and click Compare to see the changes line by line. For more utilities that help with day-to-day coding, browse the web developer tools collection.