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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 '13 at 8:19
if all compressed file can be compressed again to reduce its size then we don't need storage at all since the size will become 0 eventually – Lưu Vĩnh Phúc Jun 24 '14 at 15:50

5 Answers 5

up vote 8 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
I have a 42k archive that contains 4.5 petabyte of data when decompressed. – Zoredache Mar 31 '12 at 8:45
@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! – SS-3.1415926535897932384626433 Mar 31 '12 at 10:00
That's correct. Zip bombs work same way: . 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

Is there a way I can compress the RAR file again?

Again? No, see this question for why a compressed file cannot be compressed further. If anything, re-compressing an already compressed file may even increase the resulting file.

If not, is there any other way out?

Your best bet is to either tweak the WinRAR settings to the file you were trying to compressing, or to use a better compression algorithm. 7-Zip supports several different powerful algorithms, and by tuning them just right, you could potentially get a big increase in savings over RAR.

I usually like to keep everything set to the maximum (figure 1). It takes longer to compress, but it’s usually worth it.

You can also try nanozip. It is an experimental archiver (read alpha) and runs very slowly, but gives shockingly good compression most of the time.

Of course, as always, compression results will depend on your file(s). You said you are compressing an ISO which is essentially just the contents of the files with little file-system overhead. Depending on the nature of the files, you may get some decent compression, but if it is an installation disk, then the contents of the main software are likely already compressed, so you won’t get that much more from the remaining files.

Figure 1: 7-Zip settings set to maximum compression

7-Zip settings set to maximum compression

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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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It might make a little more sense to look at why you can't compress it further. In an oversimplified statement, compression would work like this:

Document Pre-Compression (110 Characters):

this is a document that has not been compressed. After this document is compressed the length will be shorter. 

Same document, Post Compression (103 Characters):

1 is a 2 that has not been 3. After 1 2 is 3 the length will be shorter.

In this particular example, a moderate level of compression is applied, if you had more time (computer power) to count occurrences of words, you could increase the compression of this statement by pulling out the word "is". But even large amounts of CPU time can only compress documents a fraction further, leaps from 110 MB to 22 MB aren't possible unless there are a lot of things to replace in the first compression pass..

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First use the winrar to compress it with the best mode. Then archive it with 7zip. I tried it with gta sanandreas with a size 5.79 GB and after compression it got reduced to a memory of 12MB. Believe me, it's working.

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Might work with PAQ (see other answer), sure as hell won’t work with 7-Zip and friends. – Daniel B Jun 24 '14 at 15:40
I highly doubt that's possible. – slhck Jun 24 '14 at 15:47
It will work in rare cases. There are compression methods which can decrease size every time you compress the compressed file (PAQ was mentioned by @DanielB). WinRAR and 7Zip don't use such methods by default. So the size can increase in place of decreasing while doing this. – Jet Jun 24 '14 at 16:51

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