What is the maxiumum compression ratio for gzip algorythm?

Question

What is the largest size a gzip (say 10kb for the sake of an example) can be decompressed to?

Answer 1

It very much depends ont he data being compressed. A quick test with a 1Gb file full for zeros give a compressed size of ~120Kb, so your 10Kb file could potentially expend into ~85Mbytes.

If the data has low redundancy to start with, for instance the archive contains images files in a format that is compressed natively (gif, jpg, png, ...), then gzip may add not further compression at all. For binary files like program executables you might see up to 2:1 compression, for plain text, HTML or other markup 3:1 or 4:1 or more is not unlikely. You might see 10:1 in some cases but the ~8700:1 seen with a file filled with a single symbol is something you are not going to see outside similarly artificial circumstances.

You can check how much data would result from unpacking a gzip file, without actually writing its uncompressed content to disk, with gunzip -c file.gz | wc --bytes - this will uncompress the file but not store the results, instead passing them to wc which will count the number of bytes as they pass then discard them. If compressed content is a tar file containing many many small files you might find that noticably more disk space is required to unpack the full archive, but in most circumstances the count returned from piping gunzip output through wc is going to be as accurate as you need.

Answer 2

The compression ratio of any compression algorithm will be a function of the data being compressed (besides the length of that data).

Here is an analysis at MaximumCompression,
Look at one of the samples like,

Summary of the multiple file compression benchmark tests

File type : Multiple file types (46 in total)  
# of files to compress in this test : 510  
Total File Size (bytes) : 316.355.757 
Average File Size (bytes) : 620,305
Largest File (bytes) : 18,403,071
Smallest File (bytes) : 3,554

Answer 3

Usually you don't get more than 95% compression (so that 10kB gzipped data would decompress to ~200kB), but there are specially crafted files that expand exponentially. Look for 42.zip, it decompresses to few petabytes of (meaningless) data.

Answer 4

A huge file containing only one symbol will compress very well.