Cookies for analytics and advertising
We use cookies for analytics and advertising, both sent to Google. Refusing changes nothing you can see.Read the privacy page
7Z
What is a 7Z file?
Compresses harder than ZIP but needs its own tool on most systems.
What 7Z is
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.
Where 7Z came from
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.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
Nothing is thrown away
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.
It can hold a whole folder
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.
Where its compression ratio comes from
It works with a window of 64 MB: the amount of already-seen data it can refer back to when it meets a repeat.
That single number explains most of the difference between one archive format and another. A larger window finds more repetition and takes longer and more memory to do it, which is the whole trade.
What it does to protect itself
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.
What opens 7Z
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.
Opening it in a browser
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.
It is a working format
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.
What goes wrong with it
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.
The facts, in one place
| Extension | .7z |
|---|---|
| Media type | application/x-7z-compressed |
| First published | 1999 |