When and why you might compress an already-compressed file

You can compress a file that's already compressed, but it usually won't get much smaller. A ZIP, RAR, or 7Z file is already squeezed down as far as the compression method can take it. Running compression on it again typically reduces the size by only 1 to 5 percent, if at all — sometimes the new file is actually larger because of the extra layer of formatting.

The real reasons to do this are practical rather than about saving space. You might compress an already-compressed file to password-protect it, to combine multiple compressed files into one, to convert it to a different format your recipient can open, or to meet a file-size limit on an upload or email system. You might also do it if you're archiving and want everything in a single consistent format.

The process itself is straightforward: you treat the compressed file like any other file and run your compression software on it. The steps depend on which program you're using and what you're trying to accomplish.

Key Takeaways

  • Compressing an already-compressed file rarely saves meaningful space because the file is already squeezed as much as the original compression method allows.
  • You compress a compressed file mainly to password-protect it, combine multiple archives, convert formats, or meet file-size restrictions.
  • The process is the same as compressing any other file: select it, right-click, and choose your compression option from the context menu.
  • Double-check that your recipient can open the format you're creating before you send it — nested compression can create compatibility problems.

Compressing a compressed file on Windows

On Windows, the simplest method uses the built-in compression. Right-click the ZIP, RAR, or 7Z file you want to compress. From the menu that appears, hover over Send to and select Compressed (zipped) folder. Windows creates a new ZIP file containing the original compressed file. The new file will have a name like "filename.zip.zip" or similar.

If you want to password-protect the compressed file, the built-in Windows tool won't do it — you'll need third-party software. Download and install 7-Zip (free, open-source) or WinRAR (paid, 40-day trial available). Open the program, navigate to the compressed file you want to protect, right-click it, and look for an option like Add to archive or Add to 7z. In the dialog that opens, find the password field, enter your password twice, and click OK. The program creates a new compressed file with password protection applied.

Compressing a compressed file on Mac

On Mac, right-click the compressed file (ZIP, RAR, or 7Z) and select Compress from the context menu. macOS creates a new ZIP file containing the original. Like Windows, the built-in tool doesn't offer password protection.

To add a password, download The Unarchiver (free) or BetterZip (paid). Open the program, drag your compressed file into it, and look for a password or encryption option in the menu. Set your password and save. The program wraps the original file in a new password-protected archive.

Converting a compressed file to a different format

If you need to convert a RAR file to ZIP (or vice versa) because your recipient can't open RAR, you have two options. The first is to decompress the original file completely, then compress it again in the new format — but this defeats the purpose if the file is large. The second is to use software that can repackage without decompressing.

7-Zip handles this well. Open 7-Zip, navigate to your RAR file, right-click it, and select Add to archive. In the dialog, change the Archive format dropdown from "7z" to "zip". Click OK. 7-Zip creates a new ZIP file containing the contents of the RAR without fully extracting it first. The process is faster than decompress-and-recompress, though the new file won't be noticeably smaller than the original.

Combining multiple compressed files into one

If you have several ZIP or RAR files and want to send them as a single archive, compress them together. On Windows, select all the compressed files you want to combine, right-click, and choose Send to > Compressed (zipped) folder. Windows creates one new ZIP containing all of them.

On Mac, select the files, right-click, and choose Compress. macOS creates a single ZIP with all the files inside. If you're using 7-Zip or WinRAR on either system, you can also open the program, select multiple files at once, and use Add to archive to combine them in one step.

What to watch out for with nested compression

When you compress a compressed file, you create what's called nested compression — one archive inside another. This can cause problems. Some older email systems reject files with multiple layers of compression. Some recipients may not realize they need to extract twice. Some antivirus software flags nested archives as suspicious, even though they're harmless.

Before you send a nested compressed file, test it with your recipient if possible. Ask them to extract it and confirm they can open what's inside. If they're using very old software or a restrictive corporate system, decompress the original file completely, then compress it once in a standard format like ZIP. It's slower but avoids compatibility headaches.

When to decompress instead of recompressing

If your only goal is to make the file smaller, don't compress it again — you'll waste time for almost no gain. If you need to edit the contents, decompress it first rather than compressing and decompressing repeatedly. If the file is already in a format your recipient can open, send it as-is instead of adding another layer.

The exception is password protection. If you need to secure a compressed file and the original compression method doesn't support passwords, recompressing with a password-capable tool is the right move. Otherwise, ask yourself whether the extra step actually solves a real problem. Often it doesn't.

Frequently Asked Questions

Will compressing a compressed file make it significantly smaller?

No. A compressed file is already squeezed as much as possible by its original compression method. Recompressing typically reduces the size by 1 to 5 percent at most, and sometimes increases it slightly due to the overhead of the new archive format. If file size is your goal, decompress the original instead and compress it once in a more efficient format like 7Z.

Can I password-protect a file that's already compressed?

Yes. Use 7-Zip or WinRAR to add the compressed file to a new password-protected archive. The built-in Windows and Mac compression tools don't support passwords, so you'll need third-party software. The process takes a few seconds and creates a new file with encryption applied.

What if my recipient can't open the nested compressed file?

Ask them to extract it twice — once to remove the outer layer, then again to remove the inner one. If they're still stuck, decompress the original file completely on your end, then compress it once in ZIP format, which is supported by nearly all systems. This adds time but eliminates compatibility problems.

Is it safe to compress a RAR file to ZIP format?

Yes. Use 7-Zip to convert it without fully decompressing. Open 7-Zip, right-click the RAR file, select Add to archive, change the format to ZIP, and click OK. The new ZIP file will contain the same contents as the original RAR, just in a different wrapper.

Should I compress multiple files or compress them separately?

Compress them together into one archive if you want your recipient to download and manage a single file. Compress them separately if they need to use the files independently or if some are much larger than others. One archive is simpler to send; separate archives give more flexibility.