I know that higher size may lead to better compression ratio and vice verca. But is there a way I can decide better?.. since there are so many choices 7zip

So far I've noticed dictionary size ≈ file size yields optimum compression. file size
Here the ∼8mb file test.avi has same compression ratio for all dictionary sizes greater than 8mb. Then it starts to fall.

  • 3
    Yes, that is since the whole file is in memory. However, this may not be possible if dealing with multi-gig files. The return on investment diminishes the higher you go. If you need that last 1% then size=file size. Note: When you have a much larger data set a 128mb+ dictionary size will increase the time it takes to compress files significantly.
    – cybernard
    Jul 8, 2013 at 4:40
  • if you just want to improve compression ratio then just ditch zip and use another format like 7z, rar or gz, xz...
    – phuclv
    Apr 18, 2021 at 7:50

2 Answers 2


Repeatable items are stored in a dictionary and a code is assigned as a substitute.


aaaaaaaaaaaaaaaaaaaaaaaa  0001
bbbbbbbbbbbbbbbbbbbbbbbb  0002
alsdjl;asjdfkl;asdfjkljj  0003

instead of the whole line it just put the code in its place. The larger the dictionary the more codes it can handle. Normally, when a dictionary becomes full it starts a new one on the fly. When it starts a new one it is blank and new codes are assigned to detected patterns.

Generally, the larger the better to a point. The entire dictionary is held in memory so you need more RAM than the dictionary size.

The dictionary size depends on the compressibility of your data, the number of files, size, and overall size.

Generally, 32mb is more than enough, but if your compressing numerous multi-gig files then a much higher number can be used. Larger dictionaries often make the process slower, but the results in a smaller file.

  • 2
    Is the size that you set a limit for the dictionary size, or the actual size it will be? Do programs (7-zip in particular) normally determine intelligently whether they really need to fill the whole dictionary that you've allowed?
    – Stan
    Feb 24, 2016 at 9:27
  • 1
    Yes, it is a limit. When full they either start a new dictionary, or intelligently push out old data. Unless the data to compress is greater that the size of the dictionary it will get filled.
    – cybernard
    Feb 24, 2016 at 12:55
  • @cybernard "it will get filled"? To be clear, does the dictionary size remain less than the limit when it is not filled?
    – LonnieBest
    Sep 6, 2019 at 3:09
  • 1
    @LonnieBest Yes, the dictionary starts out completely empty. Every so many bits/bytes makes a new dictionary entry until it gets full.
    – cybernard
    Sep 9, 2019 at 17:02

Compression is dependent on what you are compressing as well as how much RAM you have. Pre-compressed files like pictures and videos are more difficult to compress than files like text, or directories where there is a lot of very similar or same files.

I have six backup directories of a personal php website (xampp) with some mostly minor differences in them. Each main directory is about 600M to 1Gig give or take. Totaling 6gig for all directories together, again, all holding similar files.

Dictionary size to compressed size for all directories in a single package.

4M dictionary = 1,687,995KB
24M dictionary = 1,685,337KB
128M dictionary = 1,685,336KB
512M dictionary = 1,685,336KB (no change from 128) 
1024M dictionary = 315,224KB 

since every directory is roughly the same, and are larger than 512M the 1024M dictionary seems to be the best in this situation.

Compressing 6gigs worth of folders of different music using the same 1024M dictionary size resulted in only 96% ratio of compression. 5.76 gigs instead of 6gigs.The best thing to do for compression of video is lossy compression where you use a program to convert the video. Try lowering the bit-rate to something you don't notice, or can accept the video quality of. Handbrake is a decent video tool, but there are many. VLC is capable of video compression as well using the convert option. Both programs are free to use.

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