MyMediaTools

Compress WMV Online — Free File Size Reducer

Compress a WMV file into a smaller WMV by re-encoding with the msmpeg4 video codec and wmav2 audio codec at reduced bitrates. This is ideal for reducing file size while keeping the output fully compatible with Windows Media Player and ASF-based streaming workflows.

FFmpeg Command

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Estimated output:

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How It Works

Because both the input and output are WMV files sharing the same codec family (msmpeg4/wmav2 inside an ASF container), this tool performs a full re-encode at lower target bitrates rather than a lossless remux. The video stream is decoded and re-encoded using the msmpeg4 codec at 2000 kbps by default, and the audio is decoded and re-encoded using wmav2 at 128 kbps. Lowering these bitrates is what achieves compression — the encoder discards information that is less perceptually significant, so some quality loss relative to the original is expected. The output is written back into an ASF container with the -f asf flag, ensuring the .wmv file remains playable in Windows Media Player and compatible with Windows-native media workflows.

What Each Flag Does

Flag What it does
ffmpeg Invokes the FFmpeg program, the open-source multimedia processing engine that powers this conversion. In the browser this runs as FFmpeg.wasm compiled to WebAssembly.
-i input.wmv Specifies the input WMV file. FFmpeg reads the ASF container and identifies the msmpeg4 video stream and wmav2 audio stream inside it for decoding.
-c:v msmpeg4 Sets the video encoder to msmpeg4, Microsoft's MPEG-4 Part 2 variant. Using this codec ensures the output WMV remains compatible with Windows Media Player and other ASF-based players that expect Microsoft's specific codec variant rather than standard MPEG-4.
-b:v 2000k Sets the target video bitrate to 2000 kilobits per second. This is the primary lever for compression — lowering this value below the source bitrate forces the encoder to discard more visual detail, producing a smaller file at the cost of some quality.
-c:a wmav2 Sets the audio encoder to wmav2 (Windows Media Audio version 2), Microsoft's proprietary lossy audio codec. This keeps the audio stream in the format natively expected inside an ASF/WMV container.
-b:a 128k Sets the target audio bitrate to 128 kilobits per second for the wmav2 stream. At this bitrate wmav2 delivers reasonable audio quality for speech and general content; lowering it to 64k reduces file size further but may introduce noticeable artifacts on music or complex audio.
-f asf Explicitly forces the output container format to ASF (Advanced Systems Format), which is the underlying container used by WMV. This flag ensures FFmpeg writes a properly structured ASF file even when it might otherwise make a different container assumption based on the output filename alone.
output.wmv The filename for the compressed WMV output. The .wmv extension signals to Windows and media players that this is a Windows Media Video file backed by the ASF container, consistent with the -f asf flag.

Common Use Cases

  • Reducing the file size of a screen recording captured in WMV format so it can be attached to an email or uploaded to a company intranet with a strict file size limit
  • Compressing WMV lecture or training videos before distributing them on a USB drive or CD/DVD where storage capacity is constrained
  • Shrinking WMV files recorded by older Windows-based security camera or DVR systems to free up disk space without changing the format
  • Preparing WMV content for low-bandwidth Windows Media Services streaming by targeting a specific video bitrate (e.g., 500k or 1000k) suited to the expected connection speed
  • Archiving a large library of legacy WMV home videos or corporate presentations at a smaller size while retaining ASF container compatibility for Windows-based playback software
  • Reducing the bitrate of a WMV file received from a client before embedding it into a Windows-only kiosk or presentation application that has strict memory or storage constraints

Frequently Asked Questions

Yes, because WMV uses lossy compression at every stage, re-encoding introduces a second generation of quality loss on top of whatever was already present in the source file. At the default 2000 kbps video bitrate the result is generally acceptable for most standard-definition or moderate-resolution content, but fine detail and fast motion may show increased blocking artifacts compared to the original. If your source was already encoded at a low bitrate, compressing it further will make degradation more visible. Setting the video bitrate to 4000k or higher preserves more quality at the cost of a smaller file size reduction.
This tool is specifically designed to produce a compressed WMV output for workflows that require the ASF container and Windows Media codec compatibility. If you do not have a strict requirement to stay in WMV, converting to MP4 with H.264 video and AAC audio would typically achieve better compression efficiency at the same perceptual quality, because H.264 is a significantly more modern codec than msmpeg4. Use the WMV to MP4 conversion tool on this site if cross-platform compatibility or smaller file sizes are a higher priority than staying within the Windows Media ecosystem.
The ASF container format used by WMV does support multiple audio tracks, and FFmpeg can handle them during re-encoding. However, this tool's default command maps the primary audio stream only. If your source WMV contains multiple audio tracks and you need all of them preserved, you would need to add -map 0:a to the FFmpeg command to explicitly include all audio streams in the output.
Standard metadata fields such as title, author, and copyright that are stored in the ASF container header are generally preserved by FFmpeg during re-encoding. However, WMV does not support embedded subtitles or chapter markers in the way that formats like MKV or MP4 do, so those are not relevant concerns for this format. DRM (Digital Rights Management) protection, if present on the source file, cannot be processed — FFmpeg will not be able to read a DRM-protected WMV, and the output will not carry any DRM either.
To adjust video quality, change the value after -b:v in the command. Available options range from 500k (smallest file, lowest quality) to 8000k (largest file, highest quality), with 2000k as the default. For audio, change the value after -b:a — options range from 64k to 320k, with 128k as the default. For example, to produce a highly compressed version suitable for slow streaming you might use -b:v 500k -b:a 64k, while for near-source quality you would use -b:v 6000k -b:a 192k.
The command shown compresses a single file, but you can adapt it for batch processing on your desktop using a shell loop. On Windows (PowerShell) you could run: Get-ChildItem *.wmv | ForEach-Object { ffmpeg -i $_.Name -c:v msmpeg4 -b:v 2000k -c:a wmav2 -b:a 128k -f asf ("compressed_" + $_.Name) }. On Linux or macOS use: for f in *.wmv; do ffmpeg -i "$f" -c:v msmpeg4 -b:v 2000k -c:a wmav2 -b:a 128k -f asf "compressed_$f"; done. Batch processing is particularly useful for files over 1GB, which exceed the browser tool's limit.

Technical Notes

WMV files use the Advanced Systems Format (ASF) container, and this tool targets it explicitly with the -f asf flag to ensure correct container structure in the output regardless of the .wmv extension. The msmpeg4 codec used here is Microsoft's own MPEG-4 Part 2 variant — it is distinct from standard MPEG-4 (libxvid) and is not the same as H.264 or H.265. This means compressed WMV files will not benefit from the superior compression efficiency of modern codecs. The wmav2 audio codec is Microsoft's Windows Media Audio version 2, a proprietary lossy format that is well-supported within the Windows ecosystem but has limited compatibility outside of it. Because both input and output use the same codec family, FFmpeg must fully decode and re-encode both streams — there is no stream-copy shortcut available. Transparency, subtitle tracks, and chapter markers are not supported in the ASF/WMV format, so none of those features are a concern. Files that carry Microsoft DRM protection cannot be read by FFmpeg and must be stripped of DRM through licensed tools before processing.

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