Convert ASF to MP4 Online — No Upload, No Watermark
ASF is the legacy Microsoft container that wraps WMV/WMA — incompatible with macOS, iOS, and most browsers. Converting to MP4 gives you a file that plays on every phone, browser, and editor without re-encoding at playback time.
Drag & drop your video here, or click to browse
Max file size: ~2 GB (memory permitting)
Where ASF files come from
ASF is the Advanced Systems Format, the Microsoft container underneath both WMV and WMA. Files with an .asf extension usually come from old Windows Media encoders, streaming server archives, or security camera systems that standardised on Microsoft tooling.
What changes when you convert ASF to MP4
| Container | Rewritten to MP4 with the moov atom at the front, so it starts playing before it finishes downloading |
|---|---|
| Video | Re-encoded to H.264 at CRF 23, which is visually lossless for most footage |
| Audio | Re-encoded to AAC, the codec MP4 players expect |
Worth knowing first
ASF is a container, so what is inside varies more than the extension suggests. Most hold Windows Media Video, but some carry MPEG-4 or even audio only. The conversion handles whichever it finds, though an audio-only ASF produces a video file with no picture; use the Audio Converter for those instead.
FAQ
What is the difference between ASF and WMV?
ASF is the container and WMV is the video codec that usually sits inside it. A .wmv file is an ASF file whose contents are Windows Media Video. A .asf file might hold that or something else, which is why the extension alone tells you less.
My security camera exports ASF. Will the timestamp overlay survive?
Yes, if the timestamp is burned into the picture, which is how most camera systems do it. A timestamp stored as separate metadata does not survive, since MP4 has no matching field.
Why does my ASF have no video?
Some ASF files contain only audio, which makes them Windows Media Audio in disguise. Run those through the Audio Converter to get an MP3 or WAV rather than a picture-less video.
Is this ASF to MP4 converter really free and without upload?
Yes. The tool runs entirely in your browser using FFmpeg compiled to WebAssembly. Your ASF file never leaves your device — open the DevTools Network tab to verify zero uploads. No signup and no watermark. The practical size limit is the memory your browser gives this tab, which is generous on a computer and a few hundred megabytes on a phone.