A browser needs to read the file’s container and decode the tracks it will play. Two files can both end in .webm and use different codecs. If your target browser cannot decode the original video or audio, an MP4 with H.264 video and AAC audio is a common fallback. Changing the file extension alone leaves the original container and codecs intact.
Figure out why the WebM won’t play
WebM and MP4 are containers: they hold tracks. VP9 and H.264 are video codecs; Opus and AAC are audio codecs. Browser playback can depend on the device, operating system, and codec profile as well as the container.
- The file works in one browser but fails in another: Try the same downloaded file in both browsers. If the second browser cannot play it or you get a picture without sound, check the video and audio codecs separately.
- The downloaded file plays, but the embedded video fails: Check that the page loads the same file and that the request succeeds before converting it.
- The file won’t convert even though it plays: Playback and conversion use different browser capabilities. The converter uses WebCodecs; for this sample it needs VP9 and Opus decoders and H.264 and AAC encoders. If it reports a codec error, try another browser or a local desktop converter.
Convert the WebM sample to MP4
The sample is about two seconds long and contains 160 × 90 VP9 video and one Opus audio track. Conversion runs in your browser without uploading the file.
Convert WebM to MP4 Download the sample WebM
- In Transcode Video, select Choose file and open
sample.webm. - Keep Maximum resolution at 720p and Target video bitrate at 3 Mbps. The resolution cap does not enlarge this small sample; the target bitrate is a setting, so the measured bitrate may differ.
- Select Transcode video, then Download MP4.
Check the output codecs
Open the downloaded MP4 in Video Information. Use this step on the MP4 output; the viewer does not inspect the source WebM.
- Video tracks: One H.264 / AVC track. Dimensions should still be 160 × 90.
- Audio tracks: One AAC-LC track, converted from Opus.
- Subtitle tracks: None. The sample started with no subtitles.
Finally, play the MP4 in the browser or device you need to support. The track list confirms the output codecs; playback verifies that the destination can show the picture and play the audio.
What the converter keeps
The converter writes the primary video and, if present, the primary audio track. It leaves out additional tracks and selectable subtitles. The WebM sample has no extra tracks, so it shows the codec changes but cannot demonstrate that omission.
If your source has alternate audio languages or subtitles you need to keep, use a workflow that lets you select and preserve the required tracks. Keep the original file and verify each required track after export.