How can we estimate “overhead” of a compressed file?

Question

Suppose we compress for example a .txt file that has 7 bytes size. After compression and convert to .zip file, the size will be 190 bytes.

Is there a way to estimate or compute the approximate size of “overhead”?

What factor affects the overhead size?

The Zlib compute the overhead: They said: “... only expansion is an overhead of five bytes per 16 KB block (about 0.03%), plus a one-time overhead of six bytes for the entire stream.”

I just put this site to tell that it's possible to estimate the "overhead" size.

Note: Overhead is some amount of extra data added into the compressed version of the data.

Answer 1

From the ZIP format ..

Assuming that there is only one central directory and no comments and no extra fields, the overhead should be similar to the following. (The overhead will only go up if any additional metadata is added.)

• Per file (Local file header) - 30+len(filename)
• Per file (Data descriptor) - 12 (to 16)
• Per file (Central directory header) - 46+len(filename)
• Per archive (EOCD) - 22

So the overhead, where afn is the average length of all file names, and f is the number of files:

  f * ((30 + afn) + 12 + (46 * afn)) + 22
= f * (88 + 2 * afn) + 22

This of course makes ZIP a very poor choice for very tiny bits of compressed data where a (file) structure or metadata is not required - zlib, on the other hand, is a very thin Deflate wrapper.

For small payloads, a poor Deflate implementation may also result in a significantly larger "compressed" size, such as the notorious .NET implementation ..

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Examples:

• Storing 1 file, with name "hello world note.txt" (len = 20),

  = 1 * (88 + 2 * 20) + 22 = 150 bytes overhead

• Storing 100 files, with an average name of 14 letters,

  = 100 * (88 + 2 * 14) + 22 = 11622 bytes overhead