Zlib Compress and Decompress

What is zlib?

zlib is a small wrapper around DEFLATE compressed bytes. It adds a two-byte header and an Adler-32 checksum. PNG image data and many network messages store their compressed bytes this way.

This tool compresses bytes into a zlib stream, or decompresses a zlib stream back to the original bytes.

zlib options

Input

Pasted text up to 100 MiB, read as UTF-8, or a file up to 2 GiB. A file is read in pieces and is not held whole.

Action

Compress (the default) or Decompress. Decompress reads zlib only. It does not guess gzip or raw DEFLATE.

Level

A whole number from 0 to 9. 0 stores the bytes with almost no compression, so the file can come out slightly larger than the input. 9 spends more time for a smaller result. The default is 6, and the level is ignored when you decompress.

Filename

Optional. Compress adds .zlib. Decompress removes one trailing .zlib or .zz. Left blank, an upload keeps its own name and pasted text downloads as output.zlib or output.bin.

• The result is a file download. Expanding pasted text stops at 200 MiB and expanding a file at 4 GiB.

Why zlib shows up so often

Gzip is a file you can email. zlib is the form DEFLATE takes inside something that already has a length: a PNG chunk, a PDF stream, or a protocol message. The header is two bytes, so a short payload does not pay for a gzip file header.

The checksum is Adler-32. It is faster to compute than the CRC-32 in a gzip footer, and it still rejects a truncated or damaged stream.

zlib alternatives

Gzip is the same DEFLATE data with a file header and a CRC-32 footer, and it uses the .gz suffix. Raw DEFLATE has no header and no checksum. This page will not open either of those, and a .br (Brotli) or .zst (Zstandard) file will not open here either.

The gzip and zlib commands on your computer do the same job for files larger than 2 GiB.

Things to know about zlib

Jean-loup Gailly and Mark Adler defined the zlib wrapper. The format is RFC 1950, and the compressed bytes inside are DEFLATE, RFC 1951. There is no widely used file suffix for a bare zlib stream, so this tool writes .zlib and also accepts .zz when you decompress.

The checksum catches accidental damage. It does not tell you who created the stream, and it is not a signature.