ZIP archive

ZIP is the archive format everyone can open. It packs files and folders into one file and usually compresses them, and every current version of Windows, macOS, Android and iOS can open it without extra software. You get ZIPs as e-mail attachments, from cloud downloads ("Download all" in Google Drive or OneDrive produces a ZIP), from school and office portals, and inside other formats: DOCX, XLSX, PPTX, ODT, EPUB, JAR and APK are all ZIP files with a defined internal structure.

History

The format was created by Phil Katz of PKWARE for the DOS program PKZIP at the end of the 1980s; the first version of its public specification, APPNOTE.TXT, is dated 1990. Because the specification was published, other programs adopted it quickly, and ZIP outlived the DOS archivers of its time (ARC, ARJ, LZH). PKWARE still maintains APPNOTE. Milestones: Deflate became the standard method in the early 1990s, ZIP64 (version 4.5, 2001) removed the 4 GB and 65,535-file limits, and version 6.3.0 (2006) added a flag for UTF-8 file names. ISO/IEC 21320-1:2015 standardises a restricted ZIP profile for document containers.

How a ZIP file works

Each file inside is compressed separately and followed by the next one. At the end sits the central directory, a table of contents with names, sizes, dates and CRC-32 checksums. Programs read the directory first, which is why you can list or extract a single file quickly, and also why a ZIP cut off at the end (an interrupted download) often cannot be opened at all.

• Compression: almost always Deflate (stored without compression is also common). The specification also allows BZIP2, LZMA, Zstandard and others, but many tools, including Windows Explorer, read only Deflate.
• Per-file compression means ZIP cannot exploit similarities between files. A folder of similar text files ends up noticeably smaller in 7z or as tar.xz.
• Encryption: the original "ZipCrypto" is weak and should not protect anything sensitive. AES-256 encryption (a WinZip extension) is strong, but not every program can open it. File names stay readable even in encrypted ZIPs.
• Metadata: modification date (in the old format with 2-second precision), optional Unix permissions, a comment for the whole archive.

Opening ZIP files

Windows: double-click opens the archive as a folder, "Extract all" unpacks it. macOS: Archive Utility unpacks it on double-click. Linux: any file manager, or unzip and 7z x in the terminal. Android: Files by Google; iPhone and iPad: the Files app. For ZIPs with AES encryption or unusual methods use 7-Zip (Windows, Linux) or The Unarchiver and Keka (macOS).

Common problems

"The compressed (zipped) folder is invalid"

Usually an incomplete download: download the file again and compare the size. A second cause is a compression method that Windows Explorer does not support (LZMA, Deflate64 or AES encryption from 7-Zip or WinZip). 7-Zip opens such files.

Garbled file names, broken Czech or German characters

Older archivers stored names in the code page of the system they ran on (CP437, CP852, CP932…) without marking it. Another computer then interprets the bytes differently. Programs that guess the encoding, such as The Unarchiver, often fix it. Converting the archive with us also helps: the names are re-saved as UTF-8 with the proper flag; characters that cannot be decoded are replaced, so check names with diacritics afterwards.

ZIP larger than 4 GB will not open

Files over 4 GB need ZIP64. Old tools, some cameras, printers and embedded devices do not support it. Split the content into several smaller archives.

Converting ZIP

ZIP itself is rarely the problem, so conversion usually serves a specific need: 7z for a smaller file, TAR, tar.gz or tar.xz for Linux servers and backups, or ISO for software that expects a disc image. Our converter unpacks the ZIP and packs the content again. The files themselves are copied bit for bit; what changes is the container. Limits worth knowing: the upload can be up to 100 MB and the unpacked content up to 2 GB; empty folders are not carried over; password-protected ZIPs cannot be converted, because there is nowhere to enter the password. We cannot create RAR: the RAR compressor is proprietary and only WinRAR writes it. And if you just want the files out of a ZIP, you do not need a converter at all, your system does that itself.

More detail: ZIP File Format (Library of Congress) and ZIP version 6.3.3.