What Is a 7Z File? The Format Behind 7-Zip, Explained

A 7Z file is an archive created by 7-Zip, the free and open-source compression tool by Igor Pavlov. The .7z container holds any number of files and folders, compressed with the LZMA or LZMA2 algorithm that gives the format its reputation for small sizes.

Where 7z comes from

7-Zip shipped in 1999 with one big idea: a completely open archive format with compression that beat the commercial tools of the day. The 7z format specification is public, which is why dozens of independent tools - including this site - can read and write it without any licensing deal.

Why 7z files are smaller

LZMA2 uses a large “sliding window” and range encoding, so it finds repetition not just within one file but across the whole archive. On typical mixed content that means:

• 10-30% smaller than an equivalent ZIP in most tests
• Solid compression option: similar files (like many small HTML pages) compress together as one block
• Adjustable dictionary size, from fast small-window presets up to heavy archival runs

The trade-off is speed and memory: maximum settings are noticeably slower and need more RAM on both sides.

Encryption and other features

• AES-256 encryption of file contents
• Header encryption, which hides the list of file names until the password is entered
• Split volumes into .7z.001, .7z.002 pieces
• Unicode file names everywhere

For a deeper dive into the security side, see our guide to 7z AES-256 password encryption.

How to open and create 7z files

7-Zip itself is a free desktop download for Windows. But you do not need to install anything for one-off jobs:

• To unpack one: extract 7Z files in your browser - drop the file, preview the contents, download what you need.
• To make one: create 7Z archives here, with optional AES-256 encryption, all processed locally.

Because everything runs on WebAssembly inside your browser, a 7z archive is never uploaded to any server. That makes the browser route the safest option for confidential material: the file simply never leaves your machine.