What is a 7Z file?

7Z is a container: a wrapper that holds streams encoded by something else. It is used for archiving.

The extension is .7z, and the full name is 7-Zip Archive. Both matter less than what the file can hold, which is what the rest of this page is about.

It dates from 1999.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

7Z stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.

7Z stores a directory: names, folders and the structure between them. That is what lets it stand in for a folder in an email attachment.

It works over 16 MB at a time: the span of data repetition has to fall inside before it can be compressed away.

That single number explains most of the difference between one archive format and another. A larger span finds more repetition and takes longer and more memory to do it, which is the whole trade.

7Z carries a checksum, so a damaged file is detected rather than silently mis-read and optional encryption.

An encrypted file has to be unlocked before anything can convert it, here or anywhere else — a password is not something a converter can work around, and one that claimed to could not be trusted with the file either.

7-Zip, Keka and PeaZip read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

7Z is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

The recurring complaints: support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

Nobody arrives at 7z by accident. It is what somebody picks after deciding that ZIP is producing files that are too large, and the improvement is real: on documents, source code and text, 7z commonly lands thirty to fifty per cent smaller than ZIP on the same content.

The format was created by Igor Pavlov in 1999 for the 7-Zip utility, and both are free and open-source with the specification published. That is the opposite arrangement from RAR, where the decompressor is open and the compressor is licensed — anyone may implement 7z in either direction.

ZIP compresses each file separately. 7z, by default, concatenates everything into one continuous stream and compresses that — a solid archive. The compressor can then notice that file 400 is nearly identical to file 12 and store the difference rather than the whole thing.

On a folder of similar files the effect is dramatic. Two hundred versions of a document, a source tree, a set of log files: the second and subsequent copies cost almost nothing. On a folder of photographs it changes nothing at all, because JPEGs are already compressed and share no recognisable structure.

Extracting one file from the middle means decompressing everything before it, because the stream has no independent starting points. Pulling a single document out of a large solid archive can take as long as extracting the whole thing, where ZIP would have handed it over instantly.

Damage is worse. A corrupted region in a ZIP costs you the files in that region and leaves the rest readable. In a solid archive, corruption early in the stream can render everything after it unrecoverable. For an archive being stored long-term on unreliable media, that trade deserves thought — and 7-Zip can be told to limit solid block size, which gives back most of the resilience for a little ratio.

The default compressor is LZMA2, which searches a very large window — up to 64 MB or more — for repetitions. That search is what finds the redundancy ZIP misses, and it is also why compressing can take many times longer than ZIP on the same folder.

Decompression is fast and roughly constant regardless of how hard the compression worked, which is the right shape for the usual case: an archive is written once and read many times. If compression time matters more than size, the utility offers faster modes, and the fastest of them lands close to ZIP in both respects.

7z uses AES-256 and does it properly. Unlike ZIP, where the default password scheme dates from 1989 and is broken, a password-protected 7z is genuinely protected — the contents are not recoverable without the password.

It can also encrypt the file names, so an observer cannot see what the archive contains. That option is off unless someone turns it on; with it off, the list of names is readable even though the contents are not. If the names themselves are sensitive, that distinction is the whole point.

Windows 11 has extracted 7z natively since 23H2, and creates one since 24H2. Nothing else does: macOS does not, Windows 10 does not, and a phone generally does not. Outside a current Windows the recipient must install something — 7-Zip, or one of several utilities elsewhere — before they can read what you sent.

That is the entire argument against it for anything going to a person whose setup you do not know. The best archive format is the one the other end can open, and a 40 per cent size saving is worth nothing against an attachment that will not open at all.

The 7-Zip utility reads ZIP, RAR, TAR, GZ, BZ2, XZ, ISO, CAB, DMG, WIM and a long list of others. A great many people have it installed not to make 7z archives but because it is the one tool that opens whatever arrives.

Worth separating the two: the format and the program share a name and are not the same thing. Someone saying they use 7-Zip is usually describing how they open archives, not what they create.

Unix permissions, ownership and symbolic links are handled inconsistently and frequently lost. For moving a project between Unix machines, `.tar.gz` remains the right answer — and `.tar.xz` gets much of 7z’s ratio while keeping everything TAR preserves, since xz uses the same LZMA family underneath.

There is also no recovery record, which is RAR’s one distinguishing feature. Combined with solid compression, that makes 7z a poorer choice than RAR for an archive expected to sit on unreliable storage for years.

Use it for large archives going somewhere you control or know: a backup on your own disk, a transfer to a colleague who has the tool, a download from a site that can put installation instructions next to the link. The saving is genuine and on big collections of similar files it is substantial.

Use ZIP for anything else. That is not a criticism of 7z — it is the same judgement the format itself invites, since choosing it is already a decision about the other end. Where you cannot make that decision confidently, the universal format is the correct one.