Compress a WMV file

What a WMV actually is

The container is ASF, the Advanced Systems Format, which Microsoft designed in the late nineties for streaming over dial-up and then used for everything. The extension tells you what is inside it rather than what it is: .wmv is an ASF file with video, .wma is an ASF file with only audio, and .asf is the generic name for both.

The video inside is almost always WMV3, which Microsoft marketed as Windows Media Video 9 and then took to SMPTE for standardisation, where it became VC-1. Those are the same lineage: a codec roughly contemporary with MPEG-4 Part 2, one full generation behind H.264. Older files hold WMV1, WMV2, or MSMPEG4v3, which is the codec the original DivX was built by patching. The audio is WMA in one of its several varieties.

None of this is exotic and all of it decodes. FFmpeg has carried ASF demuxing and the whole WMV and WMA decoder family for twenty years, and the build running in this page is a stock one with the full default component set, so it opens these files without complaint.

Why nothing plays it any more

No browser ships an ASF demuxer or a VC-1 decoder. Chrome, Firefox and Safari all refuse a WMV outright, which is why one embedded in a page or dropped into a web app does nothing. macOS lost the ability years ago when the QuickTime component that provided it stopped being maintained, and Apple never replaced it. On Windows the format still works, which is exactly the problem: the person who exported it had no way of knowing it would not travel.

Microsoft stopped pushing WMV as a delivery format around 2010 in favour of H.264 in MP4, and everything downstream followed. The result is a format that is fine on the machine that made it and useless everywhere else, so the files sit in folders until somebody needs to send one.

There is one case where no conversion helps. WMV was the main vehicle for Windows Media DRM, used by old rental stores and by a lot of corporate training video. A protected file carries an encryption header, and FFmpeg will say so and stop. That is not a limitation of this build, and nothing in this tool is going to change it.

How much you actually gain

Two things pull in opposite directions here, and it is worth knowing which one applies to your file.

The codec generation works in your favour. WMV3 needs meaningfully more bitrate than H.264 for the same picture, so re-encoding is a genuine efficiency gain rather than a straight quality trade, in the same way it is for an old AVI. Expect something in the region of half, on footage that was encoded reasonably in the first place.

The era works against you. Most surviving WMV files are small already, because they were made when 640×480 was a normal size for video and a hard drive was 80 GB. Halving a 30 MB file is a real percentage and an uninteresting number of megabytes. If your reason for being here is that something rejected the file, check the size first — you may find the problem was the format, not the bytes.

Remux cannot work here, and that is the whole story

On most pages on this site the fast answer is remuxing: copy the video stream untouched, rewrite the wrapper, finish in seconds. For WMV that route does not exist. MP4 has no defined way to carry VC-1 or WMV3, and no way to carry WMA either, so a stream copy fails at the muxer rather than producing a file you cannot play. There is no clever flag that fixes it.

So every WMV conversion is a full decode and re-encode of every frame. That costs time proportional to the length of the video, and it costs one generation of quality. Both are unavoidable, and both are the price of a file that plays on something other than the computer it was made on.

Settings for a WMV, and two things that will trip you up

Output MP4, mode Best compression, resolution left on original unless the source is 720p or larger. Old WMV footage is usually low resolution to begin with and downscaling it further leaves very little.

The first trap is the audio setting. By default the tool keeps the original audio track by copying it, and a WMA track cannot be copied into an MP4 any more than the video can. Set Audio to Compress (128k) so the sound is re-encoded to AAC as well. If a WMV job fails with the video settings apparently fine, this is almost always why.

The second is target-size mode. The tool reads clip length using the browser's own video element, which is how it can show you a bitrate budget before encoding anything — and no browser can open an ASF file. So target mode will tell you it cannot read the length of the clip and stop. Use Best compression instead, and check the reported before-and-after sizes, because without a duration the tool cannot compute a bitrate cap and encodes at a default quality that will not always be smaller.

WMV compression questions

Compressing to a size limit instead?

If what brought you here is a number rather than the fact that it is a WMV file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your WMV:

• 5 MB
• 8 MB
• 10 MB
• 16 MB
• 20 MB
• 25 MB
• 50 MB
• 100 MB
• 200 MB
• 500 MB

Other formats

• Compress MP4 The universal container. Usually already H.264, so the gains come from bitrate and resolution.
• Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
• Compress MKV Long recordings, rips and archives. Big enough that upload-based tools often refuse them.
• Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
• Compress AVI Legacy codecs, dramatic savings. Read in, MP4 out.
• Compress FLV A dead container, often with a perfectly modern video inside. Try remux first.
• Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.
• Compress MTS / M2TS AVCHD camcorder footage. Enormous, usually interlaced, and split into 2 GB pieces.

The formats reference puts WMV beside the other containers and shows which codec combinations play where.