WebM to MP4, for phones and editors

Why a WebM has to be re-encoded

WebM is a web format built on Matroska, and every WebM player must handle VP8 and VP9 video with Vorbis and Opus sound; AV1 turns up in WebM files too (MDN). Those codecs are rare outside browsers. Windows Media Player, for one, plays VP8 out of the box but needs Store extensions for VP9 and AV1, and Opus is not on its list of built-in sound formats (Microsoft). Simply moving VP9 into an MP4 box would not fix that, so this tool decodes the picture and encodes it again as H.264, and does the same for the sound with AAC.

Screen recordings and browser recordings

Screen recorders that run in a browser can use the browser's own recorder, and the web standard leaves the container and codecs of a recording up to the browser (W3C). In our test, Chrome's recorder wrote a WebM with VP8 video and Opus sound. We converted that four-second recording here and got an H.264 MP4 with AAC sound of the same length, which FFmpeg decoded without a single error.

Recordings of a window can have an odd picture size, such as 1365 x 767 pixels. H.264 encoders only take even sizes, so the tool trims one row or column of pixels at the edge instead of stretching the picture, and says so in the panel: 1365 x 767 becomes 1364 x 766.

Picture quality and file size

H.264 is older than VP9 and AV1 and needs more data for the same detail, so the MP4 is often bigger than the WebM. The tool starts from the WebM's own bitrate and raises it for the codec: about 1.15 times for VP8, 1.5 times for VP9 and 1.8 times for AV1. It never goes below a floor for the picture size and frame rate, which matters for screen recordings that spend very little on still screens:

Higher multiplies the bitrate by 1.5 and Smaller by 0.6. Encoders spend less on simple pictures than they are allowed, so a recording of a mostly still screen often comes out smaller than the estimate.

How long it takes

Re-encoding is real work for your device, so it takes longer than copying. In our tests on a laptop in Chrome and Edge, ten seconds of 720p VP9 became an MP4 in 2.5 to 6.5 seconds, eight seconds of 360p VP8 in under 1.5 seconds, and six seconds of 360p AV1 in 1.4 to 3.5 seconds. At those speeds an hour of 720p takes roughly 15 to 40 minutes, so keep the tab open and the device plugged in.

A 4K WebM has nine times the pixels of 720p, so expect it to take several times longer again. Before it starts, the tool asks this browser whether it can encode H.264 at that size at all, and says so plainly if it cannot.

No limit on files or length

You can convert as many WebM files as you like, as long as they are, with no account, no daily quota and no mark added to the picture. The only limits are your device's own: the minutes a long re-encode takes, and the memory the finished MP4 needs. A computer running Chrome or Edge saves an MP4 above 500 MB directly into a file on its disk; anywhere else the MP4 waits in memory for the download, which is why about 200 MB is the sensible ceiling on a phone.

If your browser cannot do part of it

The re-encode relies on WebCodecs, which caniuse lists in Chrome and Edge 94 and later, Firefox 130 and later on computers, and Safari 26 and later, on a Mac as on an iPhone (caniuse). Before anything runs, the tool checks that this browser can decode the WebM's picture, encode H.264 at that size and make AAC sound. If one of them is missing, it tells you which, and you can tick Leave the sound out when only the sound is the problem.