You usually don't need to compress a file that's already compressed

A compressed file — a ZIP, RAR, 7Z, or other archive — is already as small as that particular compression method can make it. Running compression on it again produces almost no size reduction and wastes time and processing power. The file you end up with is slightly larger than the original compressed file because the compression software adds its own overhead (headers, metadata) without removing meaningful data.

The only practical reason to compress an already-compressed file is to bundle multiple archives together, or to meet a specific requirement like a file size limit for email or upload. In those cases, you're not really compressing for space — you're packaging for convenience.

Key Takeaways

  • Compressing an already-compressed file saves almost no space because the data inside is already squeezed as tightly as that method allows.
  • The new archive will be slightly larger than the original because compression adds its own file headers and structure.
  • You might compress a compressed file to bundle multiple archives together or to meet a file size limit, but not to save disk space.
  • If you need smaller files, extract the original archive and re-compress with a stronger method like 7Z instead of double-compressing.
  • Some file types inside an archive — images, videos, PDFs — are already compressed and won't shrink further no matter what method you use.

Why compression doesn't work twice

Compression works by finding patterns in data and replacing them with shorter codes. A text file full of repeated words compresses well because those words appear many times. Once you compress it, the patterns are gone — the data now looks like random characters to a compression algorithm. A second pass finds almost nothing to compress.

Think of it like wrapping a present. The first layer of wrapping paper makes the box smaller and neater. Wrapping that wrapped box again doesn't make it smaller — it just adds more paper on top. The compressed file is already the "wrapped box." Adding another layer of compression is the extra paper that makes it bigger, not smaller.

Different compression methods use different algorithms, so a file compressed with ZIP might compress slightly more with 7Z or RAR. But the gain is tiny — usually less than 1 percent — and not worth the time it takes.

When you might compress a compressed file anyway

Email services and file-sharing platforms sometimes limit how large a single file can be. If you have five ZIP archives that are each 24 MB, and your email allows only 25 MB per attachment, you can't send them individually. Compressing all five into one 120 MB archive doesn't help with email, but it does let you upload them together to a service like Google Drive or Dropbox, which may have different limits for folders versus individual files.

You might also compress an archive if you're creating a backup package and want everything in one container for organizational reasons. The space cost is minimal — usually just a few kilobytes of overhead — and the convenience of having one file instead of five might be worth it to you.

Some older systems or specific software require files to be in a particular format. If you have a RAR file but need to send it to someone who can only open ZIP files, re-compressing it as ZIP is sometimes easier than extracting and re-archiving the contents.

What's actually inside a compressed file matters

The real factor in whether a second compression helps is what type of files are inside the archive. Text files, spreadsheets, and source code compress very well because they contain repetitive patterns. Images, videos, and PDFs are already compressed by their own formats — JPEG, MP4, and PDF all use compression built into the file type itself. Compressing an archive full of images a second time produces almost zero size reduction.

If you're trying to make files smaller and you're stuck with an archive full of images or videos, extracting and re-compressing with a different method won't help. The bottleneck is the file type, not the compression method. Your only real option is to reduce the quality or resolution of the images themselves, which is a different task entirely.

How to actually compress an already-compressed file

If you've decided you need to do this, the process is straightforward. Right-click the compressed file (the ZIP, RAR, or 7Z file itself, not the folder inside it). Select "Send to" on Windows or "Compress" on Mac, or use your compression software's menu. Choose your compression format — 7Z will give you the smallest result, though ZIP is more widely compatible.

On Windows, you can also open your compression software directly (WinRAR, 7-Zip, or the built-in compression tool), then drag the compressed file into the window to add it to a new archive. On Mac, the built-in Archive Utility works the same way — just drag and drop the file onto the process icon.

The process takes a few seconds to a few minutes depending on file size. You'll end up with a new archive containing the old archive. The new file will have a name like "filename.zip.zip" or "filename.zip.7z" depending on what format you chose.

A better approach: re-compress from scratch

If you're trying to make files smaller and you have access to the original uncompressed files, start over. Extract the compressed file, then create a new archive using a stronger compression method. 7Z typically produces files 10 to 20 percent smaller than ZIP for the same content, and it's free and widely supported.

To do this: right-click the compressed file, select "Extract All" (Windows) or double-click it (Mac). This creates a folder with the original files inside. Then right-click that folder, select your compression software's option to create an archive, and choose 7Z as the format. Delete the old compressed file and the extracted folder once you've confirmed the new archive opens correctly.

This approach actually reduces file size instead of adding overhead. It takes longer than double-compressing, but you get a real result instead of a slightly larger file.

Frequently Asked Questions

Will compressing a ZIP file make it smaller?

No. A ZIP file is already compressed, so compressing it again adds file overhead without removing data. The result will be slightly larger than the original ZIP, not smaller. If you need smaller files, extract the ZIP and re-compress with 7Z instead.

What if I compress a RAR file with ZIP compression?

You'll create a ZIP archive that contains the RAR file. The ZIP will be slightly larger than the RAR because of added headers. Different compression methods use different algorithms, so you might see a tiny reduction (usually under 1 percent), but it's not worth the time.

Can I compress a compressed file to make it password-protected?

No — if the original archive is already password-protected, compressing it again doesn't change that. If you want to add password protection to an unprotected archive, extract it first, then create a new archive with password protection enabled in your compression software's settings.

Why is my double-compressed file larger than the original?

Compression software adds file headers, directory information, and metadata to every archive it creates. When you compress an already-compressed file, you're adding a second layer of this overhead without removing any actual data. The overhead is usually just a few kilobytes, but it makes the file larger, not smaller.

Is there any reason to compress a file twice?

Only for convenience — bundling multiple archives into one file, or meeting a specific file size limit for upload or email. If your goal is to save disk space, compressing twice doesn't work. Extract and re-compress with a stronger method like 7Z instead.