Convert WMV to MP4 — Free Online Tool
Convert WMV files to MP4 by re-encoding Microsoft's proprietary ASF-wrapped MPEG-4 video into the universally compatible H.264/AAC format. This unlocks playback on virtually every modern device, browser, and platform that refuses or struggles with WMV's legacy Windows Media codecs.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your WMV file here
or click to browse
Free — no uploads, no signups. Your files never leave your browser.
Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
WMV files use Microsoft's ASF (Advanced Systems Format) container with either the MS-MPEG4v2 or msmpeg4 video codec and WMA (Windows Media Audio v2) audio. Neither of these streams can be directly remuxed into an MP4 container — both must be fully re-encoded. The video is decoded from the legacy Microsoft MPEG-4 variant and re-encoded to H.264 (libx264) using CRF-based quality control, which produces a visually consistent result rather than targeting a fixed bitrate. The WMA audio track is simultaneously decoded and re-encoded to AAC at 128k, which MP4 natively supports. The -movflags +faststart flag then relocates the MP4 metadata atom to the beginning of the file, enabling progressive streaming before the full download completes.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg binary. In this browser-based tool, it runs as a WebAssembly module (FFmpeg.wasm) entirely within your browser — no file data leaves your machine. |
-i input.wmv
|
Specifies the input WMV file. FFmpeg detects the ASF container format and identifies the internal msmpeg4 video and wmav2 audio streams for decoding. |
-c:v libx264
|
Re-encodes the video stream using the libx264 H.264 encoder, replacing the source's Microsoft MPEG-4 (msmpeg4) codec with the industry-standard codec supported natively by every modern device, browser, and platform. |
-c:a aac
|
Re-encodes the audio stream to AAC using FFmpeg's native AAC encoder, replacing the WMA v2 (wmav2) audio that is incompatible with the MP4 container format. |
-crf 23
|
Sets the H.264 Constant Rate Factor to 23, FFmpeg's default quality level. This produces visually high-quality output for most WMV sources while keeping file sizes reasonable — lower values like 18 increase quality and file size, higher values like 28 reduce both. |
-b:a 128k
|
Sets the AAC audio output bitrate to 128 kilobits per second, matching the WMV default audio quality and producing clear stereo audio suitable for voice, music, and general video content. |
-movflags +faststart
|
Moves the MP4 container's metadata (moov atom) to the start of the output file, enabling the video to begin playing in a browser or streaming player before the full file has downloaded — a critical optimization that WMV's ASF format does not offer in the same way. |
output.mp4
|
Specifies the output filename and tells FFmpeg to write an MP4 (MPEG-4 Part 14) container, which will hold the newly encoded H.264 video and AAC audio streams. |
Common Use Cases
- Upload old Windows Media Player recordings or screen captures to YouTube, Vimeo, or other platforms that reject or poorly handle WMV uploads
- Play WMV corporate training videos or webinar recordings on macOS, iOS, or Android devices where Windows Media codecs are not installed
- Embed legacy WMV marketing or product videos into a website using an HTML5 video player, which does not support the ASF container
- Archive WMV files recorded by older Windows-based DVR or security camera systems into the more future-proof MP4/H.264 format
- Send WMV screen recordings or presentations to colleagues who use iPhones or non-Windows machines where native WMV playback fails
- Edit WMV footage in video editors like DaVinci Resolve, Final Cut Pro, or Adobe Premiere that have limited or no support for the msmpeg4 codec
Frequently Asked Questions
Yes, some generation loss is unavoidable because WMV uses the legacy msmpeg4 codec and the conversion requires a full re-encode to H.264 — there is no lossless remux path between these formats. However, the default CRF 23 setting for libx264 is widely considered visually transparent for most source material, meaning the quality difference is typically imperceptible on standard-definition or moderate-bitrate WMV files. If your WMV source has a high bitrate or you need maximum fidelity, you can lower the CRF value (e.g., to 18) in the FFmpeg command to retain more detail at the cost of a larger output file.
Stream copying (-c:v copy) is only possible when the source codec is natively supported by the destination container. MP4 supports H.264, H.265, and VP9, but it does not support Microsoft's proprietary msmpeg4 or msmpeg4v2 codecs used in WMV files. Attempting to copy the stream would produce an unplayable or invalid MP4. Full re-encoding to H.264 is the only correct approach for this specific conversion.
The WMA v2 (wmav2) audio codec used in WMV files is not supported inside an MP4 container, so it is fully decoded and re-encoded to AAC at 128k bitrate. AAC is the standard audio codec for MP4 and is natively supported by every major device, browser, and streaming platform. At 128k, AAC generally sounds equivalent to or better than WMA at the same bitrate, so audio quality is well preserved. If your WMV has multiple audio tracks, the default command encodes all of them; you can target a specific track with -map flags.
Change the -crf value to control output quality. CRF (Constant Rate Factor) ranges from 0 (lossless) to 51 (worst quality), with 23 as the default. Lowering the number — for example, -crf 18 — increases quality and file size, while raising it — for example, -crf 28 — reduces file size at the cost of some sharpness. For archiving a high-quality WMV source, CRF 18-20 is a good target. For web delivery where file size matters more, CRF 26-28 is often acceptable.
The -movflags +faststart flag moves the MP4 moov atom (the metadata index that tells players how to decode the file) from the end of the file to the beginning. Without it, a web browser or streaming player must download the entire MP4 before it can start playing. With +faststart, playback can begin almost immediately as the file streams. It has no effect on file quality or size, and it is harmless for local playback, so it is recommended to leave it in the command.
Yes. On Linux or macOS, you can loop over all WMV files in a directory with: for f in *.wmv; do ffmpeg -i "$f" -c:v libx264 -c:a aac -crf 23 -b:a 128k -movflags +faststart "${f%.wmv}.mp4"; 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 -movflags +faststart "%~nf.mp4". Each WMV file will be converted sequentially and saved as a matching MP4 filename in the same folder.
Technical Notes
WMV files use Microsoft's ASF container, which is signaled to FFmpeg via the -f asf flag internally, though FFmpeg typically auto-detects it from the .wmv extension. The video codec inside WMV is almost always msmpeg4 (a non-standard MPEG-4 Part 2 variant that predates and differs from the MPEG-standardized version) or the older msmpeg4v2, neither of which is muxable into ISO Base Media File Format containers like MP4. The libx264 encoder used in the output is significantly more efficient than the source codec — a WMV file re-encoded at CRF 23 will often be smaller than the original despite being visually comparable, especially for older high-bitrate WMV files. WMV's DRM (Digital Rights Management) protection, if present, cannot be bypassed by FFmpeg and will cause the conversion to fail; the tool only works on unprotected WMV files. Metadata such as title, author, and copyright fields embedded in the ASF container may not fully transfer to the MP4 output without explicit -map_metadata 0 flags. WMV does not support subtitle tracks, so no subtitle data is lost in this conversion. The MP4 output gains chapter and subtitle support that can be added in post-processing if needed.