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ZIP
What is a ZIP file?
The archive format built into every operating system.
What ZIP is
ZIP is a container: a wrapper that holds streams encoded by something else. It is used for moving data between programs and handing a finished file over.
The extension is .zip, and the full name is ZIP Archive. Both matter less than what the file can hold, which is what the rest of this page is about.
Where ZIP came from
PKWARE published it in 1989. The specification is APPNOTE.TXT.
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
ZIP 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
ZIP 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 32 KB: 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
ZIP 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 ZIP
Windows Explorer, Finder and 7-Zip 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
ZIP 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 facts, in one place
| Extension | .zip |
|---|---|
| Media type | application/zip |
| Published by | PKWARE |
| First published | 1989 |
| Specification | APPNOTE.TXT |
Converting ZIP to something else
Converting something else to ZIP
Formats worth considering instead of ZIP
Where these figures come from
The claims on this page are checkable, and these are the documents that settle them.