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I had a 326 MB iso file that I compressed using WinRAR, with the best compression method. It got compressed to 144 MB.

I want to compress more and want it to be limited to something around 22 MB.

Is there a way I can compress the RAR file again? If not, is there any other way out?

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What kind of data does the ISO file contain? Programs? Documents? Music? Video? – David Schwartz Mar 31 '12 at 6:55
@ David Schwartz It is a software CD,with autorun – Suhail Gupta Mar 31 '12 at 7:00
It would be nice if we could compress compressed archives again and again. Every archive would be about 1 byte. – totymedli May 1 at 8:19

2 Answers

up vote 6 down vote accepted

If you've already compressed with best compression method, you can't compress further.. Few KB/MB could be digestible, but compressing 144MB compressed data to 22MB: No way!
I'd not say, its impossible. But.. Currently, there's no standard compression algorithm to do this. Sorry!

If 22MB is a limit somewhere (like cloud, storage), you can always use file splitters to split this 144MB file into multiple 22MB chunks. File Joiners (often, built-in with file splitters) reverse this process.

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From information theory there's a limit to how much can be compressed. You can't go beyond that. – slhck♦ Mar 31 '12 at 7:51
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I have a 42k archive that contains 4.5 petabyte of data when decompressed. en.wikipedia.org/wiki/Zip_bomb – Zoredache Mar 31 '12 at 8:45
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@Zoredache You can't apply Zip Bomb example on practical files. Compression algorithms just remove repeating data. So, create a 10GB file fully filled with 1. Now, compress it. What do you expect? Again compress it to hunt down repeating compression meta data. Do it again-n-again. You'll definitely end up with a KB file. For sure, you can't apply this to practical files! – Sachin Shekhar Mar 31 '12 at 10:00
That's correct. Zip bombs work same way: stackoverflow.com/a/1459776/577898 . Our everyday files can't be compressed with that compression ratio. – user178094 Mar 31 '12 at 18:14
why can't the winrar compress the already compressed file ? – Suhail Gupta Apr 3 '12 at 18:20
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up vote 0 down vote

If you have a PC with RAM higher than 3 GB and processor about 3 GHz, use KGB Archiver.

At maximum compression and PAQ6 algorithm you’ll be able to reduce a file's size to a fourth or fifth of the original – but it's a very slow process.

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