What is a 7z file?
A 7z file (.7z) is a compressed archive format that bundles many files and folders into one smaller file. Like a ZIP, it is a container you open to recover the originals, but 7z is its own format — built around the LZMA and LZMA2 compression algorithms — and it typically shrinks files smaller than ZIP or even RAR do.
The format was created by Igor Pavlov and released in 1999 alongside the 7-Zip archiver, which remains its reference implementation. Unlike RAR, 7z is an open format: anyone can read and write it, which is why 7-Zip and a wide ecosystem of tools support it. A 7z file always starts with the magic bytes 37 7A BC AF 27 1C (a readable 7z followed by a fixed signature), so a program can recognise one from its first few bytes without trusting the extension.
You can open a 7z right here in your browser with ZipTool: drop a .7z onto the page and it is decoded entirely on your device — the file is never uploaded to a server. You can browse the archive, search by name, and preview text, images, audio, video, and PDFs inside it without extracting anything to disk. For the dedicated browser workflows, see extracting a 7z online and the online 7z viewer. To open one now, go straight to the 7z page.
7z vs ZIP vs RAR: what is different
ZIP, RAR, and 7z all solve the same problem — pack many files into one compressed file — but they sit at different points on the same trade-off curve. 7z usually produces the best compression ratio of the three, especially on text-heavy, source-code, or repetitive data, because its LZMA2 algorithm (with optional BCJ and BCJ2 filters) is more aggressive than ZIP’s default DEFLATE. The cost is speed and memory: 7z takes more CPU and RAM to create and extract than a plain ZIP.
The openness story sets 7z apart from RAR. ZIP is an open standard every operating system handles natively. RAR is proprietary — only WinRAR and official tools can create it. 7z splits the difference: the format is open, so any tool can read and write it, but fewer operating systems handle it out of the box than handle ZIP, so recipients usually need a 7-Zip-compatible extractor. For a deeper comparison, see archive formats explained.
7z also shares some of RAR’s strengths: solid archives (the whole archive compressed as one block for a better ratio) and strong AES-256 encryption. One feature RAR has that 7z users sometimes want is recovery records for repairing minor corruption; 7z has no equivalent.
How to open a 7z file
On Windows, 7-Zip itself is the obvious choice — free, open-source, and the reference implementation. WinRAR also reads 7z. On macOS, Keka and The Unarchiver both open 7z files cleanly. On Linux, 7z (from the p7zip or 7zip package) is the command-line tool.
In a browser: open the .7z in ZipTool and browse its contents with nothing installed. The archive is decoded locally in your tab by a WebAssembly build of 7-Zip — there is no upload and no server copy. This is the fastest way to check what is inside a 7z you just downloaded before you commit to extracting it. The full step-by-step is in how to open a 7z file.
- Windows: 7-Zip (the reference tool, free) or WinRAR for reading 7z.
- macOS: Keka or The Unarchiver (free); double-click to extract.
- Linux: install
7z(p7zip / 7zip), then7z x archive.7zto extract with paths. - In-browser: drop the
.7zonto ZipTool to preview contents with no install and no upload.
What you can preview inside a 7z
Once a 7z is open in ZipTool, the entries render the same way they do for a ZIP. Source code and config files — JavaScript, TypeScript, JSON, Python, Markdown, HTML, CSS, and many more — open in a syntax-highlighting editor. Images, audio, video, and PDFs render inline. An archive stored inside the 7z can be browsed too.
Because the decoding runs through a WebAssembly build of 7-Zip entirely inside your browser, the moment you close the tab the data is gone. There is no upload step for it to persist anywhere. You can even disconnect from the internet after the page has loaded and browsing, searching, and previewing keep working.
Encryption, and what stays out of reach
A 7z archive can encrypt its contents with AES-256 (and optionally encrypt the file-name headers too). ZipTool decrypts an encrypted .7z in your browser once you enter the password — the password never leaves your device — so a password-protected 7z previews the same way a plain one does, with zip.js handling ZIP and the WebAssembly 7-Zip handling 7z.
The honest limits are about creating, not reading. You can open, browse, extract, and decrypt a 7z here, but you cannot create one — ZipTool creates and converts to ZIP, TAR, and TAR.GZ instead, so reach for the desktop 7-Zip program to produce a .7z. Encrypted RAR is the one archive the browser does not decrypt.
Is opening a 7z file safe?
A 7z is just a container — opening one to look at its contents is low-risk, whether in a desktop tool or in a browser. The risk is not in viewing, it is in what you do next. 7z’s strong compression is also why it is sometimes used to hide malware: a small archive can unpack to something large, and a malicious file extracted and then run is still malicious.
Previewing a 7z in ZipTool is a privacy win, not a security guarantee: your file never leaves the browser, so there is no server-side copy, but client-side processing does not make a malicious archive harmless. Treat archives from untrusted sources the same way you always would, and read more in security and privacy.
Frequently asked questions
What is a 7z file?
A 7z (.7z) is an open compressed archive format created by Igor Pavlov and the 7-Zip project. It uses the LZMA/LZMA2 algorithms and usually compresses better than ZIP or RAR, while supporting solid archives, strong AES-256 encryption, and large files. Because the format is open, any tool can read and write it; 7-Zip is the reference implementation.
Is 7z better than ZIP or RAR?
For compression ratio, 7z usually beats both ZIP and RAR, especially on text and source code, thanks to LZMA2 and its BCJ filters. The trade-off is that 7z is slower and uses more memory to create and extract, and recipients need a 7-Zip-compatible extractor (operating systems handle ZIP natively). RAR is proprietary; 7z and ZIP are open.
Can I open a 7z file online without uploading it?
Yes. ZipTool decodes a .7z entirely in your browser using a WebAssembly build of 7-Zip, so the file is never uploaded to a server. Drop the 7z onto the page to browse its contents and preview text, images, audio, video, and PDFs inside it.
What program opens 7z files?
On Windows, 7-Zip (free, the reference tool) or WinRAR. On macOS, Keka or The Unarchiver. On Linux, the 7z command from the p7zip or 7zip package. You can also open a 7z in a browser with ZipTool, which decodes it locally via WebAssembly with no install and no upload.
Why will my 7z file not open?
The common reasons: the download is incomplete or corrupted, a solid archive is large enough to need more memory than the tool allows, the archive is password-protected and the wrong password was supplied (ZipTool decrypts a 7z when you enter the correct one), or the file is not actually a 7z despite the extension. Confirm the file starts with the 7z signature (bytes 37 7A BC AF 27 1C), and try 7-Zip or Keka if an in-browser viewer cannot read it.