In general, compression ratios of up to 5:1 are much more likely than
the 10:1 ratio you are describing. For example, see
which is one of several web pages that describe and compare the
performance of file compression on "real world" files.
However, there are a number of special cases in which much higher
compression ratios are possible.
 Audio, video, and images. The ear and eye have a number of
physical characteristics that allow "lossy" compression to 10:1 to
20:1 without any "visible" loss or up to 50:1 or 100:1 with minor
for a nice explanation of the trade offs and limitations of some of
the compression methods.
 Text (e.g., web pages). Most text data is encoded in seven bits
(but stored in eight), giving you a "head start" on the compression.
Ratios over 10:1 have been measured for text (RTF files), see
and scroll down to table 4.
 Databases. For a variety of reasons (mainly related to
performance), database files may have a "lot" of empty space in them.
Most databases also have a utility to "compact" the database files -
but if not done, you could compress the files and get a relatively
large compression ratio (could be ratios of hundreds to thousands to
 "Disk Images". A copy of a CD ROM that does not have much
information can be over 600 Mbyte, but an "image" of that CD ROM can
be much smaller if the disc has lots of empty space and the image is
compressed. One person asked a similar question at
and there are several good responses to describe why the compression was so good.
To find out more information on file compression methods, see
which has several links to specific algorithms or search using phrases such as
file compression ratio
file compression ratio best
file compression (put an algorithm name here)
file compression (type of file here)
and so on.
Good luck with your work.