MyMediaTools

Convert WMV to MKV — Free Online Tool

Convert WMV files to MKV by re-encoding Microsoft's proprietary msmpeg4 video stream into the widely compatible H.264 (libx264) codec inside Matroska's flexible open container. This unlocks subtitle tracks, chapter markers, and broad device support that the Windows Media format simply cannot offer.

FFmpeg Command

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Estimated output:

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How It Works

WMV files use Microsoft's ASF (Advanced Systems Format) container, typically wrapping an msmpeg4 or msmpeg4v2 video stream alongside WMA audio. Neither of these codecs are native to Matroska's ecosystem, so this conversion performs a full transcode: the video is decoded from msmpeg4 and re-encoded into H.264 using libx264 at CRF 23, and the WMA audio is decoded and re-encoded into AAC at 128k. The result is a true MKV file — not just a container swap — meaning every frame is processed. Because WMV is already a lossy format, some generation loss is unavoidable, but libx264 at CRF 23 is a high-quality setting that minimizes perceptible degradation for most content.

What Each Flag Does

Flag What it does
ffmpeg Invokes the FFmpeg tool — in this browser-based tool, it runs as a WebAssembly (FFmpeg.wasm) instance entirely within your browser, with no server upload. The same command works identically in a local FFmpeg installation for files over 1GB.
-i input.wmv Specifies the input WMV file, which FFmpeg reads as an ASF container holding an msmpeg4 (or msmpeg4v2) video stream and typically a WMA audio stream.
-c:v libx264 Selects libx264 as the video encoder, transcoding the msmpeg4 video stream from the WMV source into H.264 — the most broadly compatible video codec for MKV and the format required since msmpeg4 cannot be stored natively in Matroska.
-c:a aac Selects AAC as the audio encoder, transcoding the WMA audio from the source WMV into AAC, which is natively supported by MKV and widely playable across all modern devices and platforms.
-crf 23 Sets the Constant Rate Factor for libx264 to 23, which is the default and represents a good balance between visual quality and file size. Unlike WMV's bitrate-based quality control (-b:v), CRF lets the encoder allocate more bits to complex scenes automatically — lower values like 18 give higher quality, higher values like 28 reduce file size.
-b:a 128k Sets the AAC audio bitrate to 128 kilobits per second, which matches the default audio quality of the WMV source and delivers clear stereo audio suitable for most speech and music content without excessive file overhead.
output.mkv Defines the output filename with the .mkv extension, which instructs FFmpeg to wrap the transcoded H.264 video and AAC audio inside a Matroska container — an open-standard format that supports subtitles, chapters, multiple audio tracks, and rich metadata.

Common Use Cases

  • Sideloading WMV recordings from older Windows systems onto media players like Kodi, Plex, or Jellyfin that prefer MKV with H.264 over ASF-wrapped msmpeg4
  • Preserving old Windows Media Player screen recordings or tutorial videos in a future-proof open format before the original software becomes unavailable
  • Adding subtitle tracks to a WMV file — since WMV does not support embedded subtitles but MKV does, converting first allows SRT or ASS tracks to be muxed in afterward
  • Preparing WMV corporate training videos or presentations for playback on macOS or Linux systems where Windows Media codecs are not natively installed
  • Archiving WMV footage captured by older security cameras or webcam software into a format with broader long-term codec support and no Microsoft DRM entanglement
  • Editing workflows where the NLE or transcoding pipeline accepts MKV/H.264 source files but struggles to decode msmpeg4-encoded WMV streams

Frequently Asked Questions

Some quality loss is unavoidable because both the WMV source and the MKV output use lossy compression, and transcoding always involves decoding and re-encoding. However, libx264 at CRF 23 is a high-quality setting, and for most WMV content — which was already compressed with msmpeg4 at moderate bitrates — the output will be visually very close to the source. If you want to minimize loss further, you can lower the CRF value (e.g., -crf 18) at the cost of a larger file.
WMV files use Microsoft's msmpeg4 or msmpeg4v2 video codec, which is not a supported video codec inside the Matroska container. MKV is designed around codecs like H.264, H.265, VP9, and AV1 — so the msmpeg4 stream must be decoded and re-encoded into a compatible codec (here, H.264) before it can be placed in an MKV wrapper. This is different from, say, an MKV-to-MP4 conversion where H.264 streams can often be copied without re-encoding.
This tool cannot process DRM-protected WMV files. Windows Media DRM (WMDRM) encrypts the media stream so that it cannot be decoded without a valid license, and FFmpeg has no mechanism to bypass that encryption. If you attempt to convert a DRM-protected WMV, FFmpeg will fail to read the stream and produce an error. Only DRM-free WMV files — such as those you recorded yourself or received without protection — can be converted.
Yes — this is one of the primary reasons to convert from WMV to MKV. The ASF container used by WMV has no meaningful support for subtitle tracks or chapter markers, but Matroska was purpose-built for exactly these features. Once your content is in MKV, you can use tools like MKVToolNix to mux in SRT, ASS, or PGS subtitle files and define named chapter points without re-encoding the video again.
The -crf 23 flag controls H.264 quality using a Constant Rate Factor scale from 0 (lossless) to 51 (lowest quality). Lower numbers produce higher quality and larger files — try -crf 18 for near-lossless quality or -crf 28 for a smaller file with more compression. For audio, change the -b:a 128k value to 192k or 256k for higher-fidelity audio, or 96k if file size is a concern. Example: ffmpeg -i input.wmv -c:v libx264 -c:a aac -crf 18 -b:a 192k output.mkv
Yes. On Linux or macOS, run this in your terminal: for f in *.wmv; do ffmpeg -i "$f" -c:v libx264 -c:a aac -crf 23 -b:a 128k "${f%.wmv}.mkv"; done. On Windows Command Prompt, use: for %f in (*.wmv) do ffmpeg -i "%f" -c:v libx264 -c:a aac -crf 23 -b:a 128k "%~nf.mkv". This processes every WMV in the current directory and outputs a matching MKV file. The browser-based tool handles one file at a time, but the FFmpeg command is ideal for bulk conversions on your desktop.

Technical Notes

WMV's ASF container stores metadata such as title, author, and copyright fields using Windows Media-specific attribute names. FFmpeg will attempt to map these to standard Matroska metadata tags during conversion, but some WMV-specific fields may not have direct equivalents and could be dropped or renamed. The msmpeg4 codec family used by WMV is a pre-standard variant of MPEG-4 Part 2 and predates modern codecs significantly — H.264 (libx264) will typically produce smaller files at equivalent or better visual quality. WMV's ASF container supports multiple audio tracks, and this tool preserves that capability since MKV also supports multiple audio streams; however, if your WMV has multiple audio tracks, the default FFmpeg command will map only the first audio track. Add -map 0 before the output filename to include all streams. WMV files with embedded WMA Pro or WMA Lossless audio will also be transcoded to AAC, which is a lossy format — if lossless audio preservation is important, consider switching the audio codec to FLAC by replacing -c:a aac -b:a 128k with -c:a flac in the command.

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