How to Compress ISO File to Smaller Size

As it often happens, you are running low on space on your computer, or you feel the urge to move those huge ISO files or perhaps, you want to constrict it into a lesser size. However, most compression software only compresses it by a few MB, even though it took many hours to compress. Oh! that frustration! In view of this post, I will explain how to easily compress large ISO files into a smaller size.


How Does ISO Compression Work?

An ISO disc image can typically be described as an uncompressed archived data copy available on an optical disc and similar to the DVD or CD stored inside a binary file. In other words, it is an exact copy of your disc, be it a hard drive, CD or DVD. They are mostly employed as a substitute for optical discs, or for allocating bulky file sets that are intended to be burned into an optical disc. Typically, the binary data/ file included in the ISO files is the determinant of the amount of compression. As an example, if such ISO files doesn’t contain a large sequence of repeated bytes, or the original disc contains a large number of compressed files (i.e. it has ZIP, RAR, 7z files or somewhat, a compressed image file like jpg). In this case, there may be little to no compression, or perhaps, compression might even increase the mass of the ISO file. Thus, assuming you have 700GB of music, videos, photos, documents and more, chances are; they are already compressed and recompressing them is a lesson in futility.

However, on the other hand if the ISO file contains much of redundant bits, say a 10GB .txt file with abundance of 1’s in it. Compressing it will find reoccurring patterns and replace them with smaller symbols, i.e. replacing 11111… with 10¹⁰⁰ 1’s or replacing repeated words like ‘ctrl’, ‘print ()’ with ‘z’, ‘y’. Therefore, the txt file can be compressed to 10MB.

It really depends on what you want to compress. Think of a .txt file with plenty of 1’s in it. This would appear to be a really huge .txt file, say 6GB. What exactly tools like 7zip, WinRAR, … do is, they look for patterns within the file and replace them with smaller symbols, e.g. ‘z’ becomes the pattern ‘print ()’ and keep their frequency, etc. For the .txt example, it would only keep that is a .txt file with 10¹⁰⁰ 1’s (or something of such).

Furthermore, compression itself can also be divided into two methods — lossy and lossless compression algorithm. Lossy compression degrades the quality and can also corrupt such file while Lossless compression doesn’t degrade the quality of the files. On the other hand, lossless compression rarely causes a large change in the size of the file while lossy compression can cause up to 50% reduction in the mass of the ISO file. Needless to say, peradventure you don’t want to degrade this ISO file, never pick lossy compression.

Finally, every compression algorithm involves a trade-off between the amount of compression and the time taken to actually do the compression step. Therefore, it can sometimes be possible to squeeze a condensed file by another few percent if you would be willing to run it through a much slower compression algorithm.

Compressing ISO Files Into Smaller Sizes Without Loss in Quality

If it happens that your ISO file is mildly compressible (such as MP3, MP4, FLAC, JPG), no amount of compression no matter what the level of compression or archive format you choose the best option to use is the “split” function of your compression software to slash the ISO file into 400MB sections. This will enable you can fit it into CDs and DVDs more effortlessly as well as “merge” it back when needed.

Alternatively, if the ISO image contain a mishmash of compressed along with uncompressed files, then a pretty more compression may be gotten by simply removing the contents of the ISO image file, and abridge it using another format such as RAR or 7z. This method is mostly suited for software as they often contain a mixture of compressed as well as uncompressed files.

However, if the ISO you wish to compress consists of mostly text files, worksheets, and more documents, then it is possible for you to still compress it greatly. Open the WinRAR archiver software and use the following steps below:

· Open the winrar application.

· Go to Options > Settings or just hold the Ctrl + S.

· In the settings window go to the “Compression” tab and under compression profiles, click on the “Create Default…” button.


· Click the “Compression method” and select “Best” in the dropdown menu of “Set default compression” options.

· Go to the “Archiving options”, check “Create solid archive” and “Lock archive”.

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· Thereafter, check through the advanced tab and select the compression option.

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· In the “Text Compression” option, click “Auto”, change the “Prediction order” to “63” and “Memory to use” to “128 mb”.

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· Afterwards, select OK and close Winrar.

· In order to compress the ISO, right click on the ISO file you wish to compress and choose the Add to “filename.rar”


Await the conclusion of the compression. You are guaranteed of having your highly compressed file once you are done with this.


Again, compression of most files is impossible as they have already been compressed. Forcible compression will simply result in degradation of quality, if such file is not corrupted outrightly. Nevertheless, the methods above will still help to shave off further needed space.

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