7Z file format: what it is and how to use it

The 7z archive packs files with LZMA2 compression that regularly beats ZIP by a wide margin, and it is open, documented and encryptable. Learn when to use it and what to choose instead.

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What is a 7Z file?

A .7z file is an archive created by 7-Zip, the open format Igor Pavlov introduced in 1999. Its default codec is LZMA/LZMA2, a range-coding algorithm that typically squeezes 20–40% more out of data than ZIP's DEFLATE, but the format is a container, not a single codec: it also accepts BZip2, PPMd and strong AES-256 encryption, and it can compress many files as one solid block so shared context shrinks far better.

When should you use 7z?

  • Maximum compression: backups and distributions where every megabyte counts.
  • Strong encryption: AES-256 can protect the file list itself, not just contents.
  • Solid archives of similar files: logs, sources or documents compress dramatically better in one block.

When is another format a better fit?

For recipients on stock Windows or macOS, ZIP opens with zero installs. On Unix pipelines tar + zstd is faster at comparable ratio. And if you need random access to many files, disable solid mode, or pick a non-solid format.

The short version: 7z gives the best size-to-effort ratio for archives, accepting that it needs a (free) tool on the receiving end.

How to open a 7Z file

On Windows

Double-click the file, or open it with 7-Zip.

On macOS

Keka opens 7Z files on macOS.

In your browser

Open 7Z files right in a browser tab or with the dexrr viewer. No install, no upload.

Also opens in

  • 7-Zip
  • WinRAR
  • PeaZip
  • Keka

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7Z technical details

Full name
7-Zip archive
Extensions
.7z
MIME type
application/x-7z-compressed
Magic bytes
37 7A BC AF 27 1C
Compression
Lossless
Developer
Igor Pavlov (7-Zip)
Specification
www.7-zip.org

7Z questions, answered

How do I open a .7z file?

7-Zip, WinRAR, PeaZip, Keka and most archive tools open it; Windows Explorer cannot. On dexrr you can inspect and extract 7z archives right in the browser.

Can I convert 7Z files online?

Yes, dexrr repacks 7z archives locally in your browser, so a recipient who cannot open 7z gets a ZIP instead, without anything being uploaded.

Is 7z better than ZIP?

On size, clearly, LZMA2 typically wins by 20–40%. On compatibility, ZIP still wins because every OS opens it natively. Pick 7z for storage, ZIP for handing files to strangers.

What does 'solid archive' mean in 7z?

Solid mode compresses the whole file set as one stream, so repeated patterns across files shrink together. Great for many similar files, but a single damaged region can affect more than one file.

Is the 7Z format outdated?

No, it is actively developed, the format is fully documented, and its compression still sits near the top of practical benchmarks.