Convert WebM to MP4 in your browser
Screen recorders, browsers and Chromebooks save WebM; editing software, phones and most upload forms want MP4. This converter rewrites a VP8 or VP9 WebM as an H.264 MP4 with AAC audio on this page, with ffmpeg compiled to WebAssembly, so the recording never leaves your computer.
- VP8 and VP9 to H.264
- Audio kept as AAC, or removed
- Transparent WebM flattened on white or black
- Opens in Premiere, iMovie, PowerPoint and iPhones
- No upload, no queue
How it works
- Step 1
Drop the WebM on the box above, or click to choose it. Files up to 500 MB can be converted; what fits depends on your device’s memory.
- Step 2
Choose the quality, keep or remove the audio, and pick the background colour in case the video is transparent. Press Convert to MP4.
- Step 3
The converter loads once, about 30 MB, and the encoding runs on your device. When it finishes, the MP4 plays in the preview and downloads with one click.
What happens to the video, the audio and the transparency
- Video
- Decoded from VP8 or VP9 and encoded as H.264 with the
veryfastpreset and the quality you choose: High (CRF 18), Balanced (CRF 23) or Smallest file (CRF 28). AV1 WebM files are not supported by this build. - Audio
- Opus or Vorbis in the WebM becomes AAC at 160 kbps, the codec every MP4 player expects. Choose Remove for a silent file, which many autoplay rules require anyway. A WebM without an audio track gives an MP4 without one.
- Transparent areas
- A WebM can carry an alpha channel, which MP4 cannot. When the file has one, it is decoded with libvpx, which reads the alpha plane, and laid over white or black. Other converters hand you a black background without asking.
- Frame size and rate
- Both are kept. Dimensions are rounded down to even numbers because H.264 in
yuv420pneeds them, and variable frame rate recordings, which browsers produce, keep their timestamps. - File layout
- Written with
faststart, so the index sits at the front of the file and the video begins playing while it is still downloading, which is what streaming and upload previews rely on.
Why WebM keeps turning up
WebM is the format browsers can write: MediaRecorder, the screen recorders built on it, Chromebooks and the recorder behind this site all produce it, often with VP9 video and Opus audio. It is a fine format for the web and a poor one for everything else: Premiere Pro, Final Cut, iMovie, PowerPoint and most phones either refuse it or need a plugin.
What makes an MP4 play everywhere
Not the extension but what is inside: H.264 video in the yuv420p pixel format with even dimensions, AAC audio, and the index at the front. That combination is what this page writes, and it is the one accepted by every editor, player, upload form and social network since the iPhone 3G.
Transparency, done properly
VP9 and VP8 WebM files can hold an alpha channel; MP4 cannot. ffmpeg’s own decoders ignore the alpha plane, and the usual result is a black background. This converter detects an alpha channel, decodes with libvpx so the alpha is read, and composites the video over the colour you chose.
Frequently asked questions
- Is the WebM uploaded anywhere?
- No. The file is read into your browser’s memory and converted there by ffmpeg running as WebAssembly in a web worker. The MP4 is handed back as a download. No server sees the recording.
- How long does it take?
- The converter downloads once, about 30 MB, and stays cached. Encoding runs at about a fifth to a tenth of native ffmpeg speed, so a one minute 1080p recording takes a minute or two on a laptop. Longer or larger files take proportionally longer; the progress bar follows the encode. Keep the tab in front while it runs: browsers give hidden tabs a fraction of the processor, and the same file took twenty times longer in a background tab.
- Will the quality drop?
- The video is decoded and encoded again, so some generation loss is unavoidable; at High quality it is invisible, at Balanced it is hard to find, at Smallest file fine detail softens. Audio is transcoded from Opus or Vorbis to AAC at 160 kbps, which is transparent for speech and music.
- Why does my MP4 have a white or black background?
- Because the WebM was transparent and MP4 cannot be. Pick the colour that matches where the video will sit, white or black, before converting. For a video that must stay transparent, keep the WebM, or ask the recorder for one with an alpha channel.
- The converter says the codec is not supported. What now?
- The WebM holds AV1 video, which this WebAssembly build of ffmpeg does not decode, or it is not a WebM at all. A desktop ffmpeg or the recorder that made the file can write VP9 or H.264 instead.
- Can it convert MP4 to WebM, or MOV, MKV and AVI to MP4?
- Not here; this page goes from WebM to MP4 only. For video made from HTML, the recorder writes MP4, WebM or GIF directly, so there is no conversion to do.
Recording a web page or an HTML5 ad?
RecordHTML records it in a real Chrome browser and writes MP4, WebM or GIF directly, at the exact size, frame rate and length you need, so there is nothing to convert afterwards.
10 credits free every month · No card needed