The MKV (Matroska) file format is a free and open standard video container that includes audio and subtitles. The specification is similar in concept to AVI and MP4 formats, but is completely open, with most of the implementations being open source software. You can convert between these video formats or compress existing MKV files using various programs.

Compressing MKV Files

Compression will always be a balance between quality and file size. With any compression level, some quality will be lost in exchange for reduced file size. The goal is to maximize compression but with minimal or acceptable quality loss. You can compress existing MKV files by lowering the bit rate, resolution or frame rate for the output file. The following programs let you compress MKV files by modifying these types of settings, which will determine file size and quality. This compression can also be done during conversion of a file into MKV format.

VidCoder

VidCoder is a free and easy to use Windows converter program which can convert DVD, Blu-ray, ISO or really any video file into MKV format, with adjustable bit rate, size and video quality. It can also perform compression of existing MKV files by changing these settings. The software can rip directly from DVD and Blu-ray discs but does not attempt to defeat the CSS/AACS encryption found on these discs. The program requires the .NET 4 client profile and supports Windows XP through Windows 8.

XMedia Recode

XMedia Recode is a free and easy to use converter program for Windows. The program supports an extremely long list of video and audio formats and will convert between most of them. It also includes profiles for many types of devices. This program will convert only unprotected DVDs or Blu-rays -- it will not attempt to defeat the copy protections and encryption on the media. The program will run on Windows XP through 7 and requires DirectX to be installed. The website is in German, but the software itself includes English translation and other languages. This program will also do compression of existing MKV files.

MKVToolNix

MKVToolNix is a cross-platform set of programs for working with MKV files. With the full package of tools you can create, modify and inspect MKV files. The programs are all open source, and there are downloadable versions for Linux and Windows. The main programs are actually command-line but there are GUI front ends available for the mkvmerge tool, which is the creation tool. This is an advanced and highly technical set of programs which may be overwhelming for many users. You can compress the file and change video quality settings with the mkvmerge tool.