Guide
Converting WebM recordings to MP4 with H.264 and AAC
Record your screen in a browser, or save a clip from a web tool, and you usually get a WebM file. Chrome and Firefox play it happily, but an iPhone may not, Windows Media Player may not, and many editors and social platforms ask for MP4. The two are containers that can hold different codecs, so converting means reading one set of compressed pictures and sound and writing another. This tool does it in your browser, with H.264 video and AAC sound, which is the combination everything understands.
Containers and codecs
A container like WebM or MP4 is a box that holds a video stream and an audio stream. The codec is how each stream is compressed. A typical WebM from a browser holds VP8 or VP9 video and Opus sound. A typical MP4 holds H.264 video and AAC sound. You cannot just rename one into the other; the streams have to be decoded and compressed again. The page lets your browser decode the source, then re-encodes it frame by frame with the WebCodecs encoders.
A worked example
Press Sample to record a three-second 320 × 180 WebM clip with a beep each second, and press Convert to MP4. The MP4 it writes keeps the 320 × 180 frame size and lasts about 2.9 seconds. In the browser test the downloaded file is opened and searched for the codec identifiers: it contains avc1, the H.264 video entry, and mp4a, the AAC audio entry. The panel then shows its size against the source as a percentage, so you can see whether the conversion made the file larger or smaller.
Getting the best copy
Every conversion costs a little quality, because the picture is compressed twice. To keep it small, leave Size at Original size and choose High quality, which allows 0.15 bits per pixel per frame. Choose Small file when size matters more. The frame rate of the source is measured when the file is opened and kept, up to 60 frames per second, and the sound is encoded at 128 kbps.
Why it can be slower than expected
H.264 encoding is done by your browser, which may use a hardware encoder or software, depending on your device. The conversion goes frame by frame: the progress bar shows the audio first and then the picture. A short clip converts in seconds; a long one can take minutes. Keep the tab open until it finishes. You can cancel at any moment.
Edge cases
A WebM from a browser recorder may lack a length in its header; the page finds it by seeking. Odd frame sizes are rounded up or down to even numbers because H.264 requires them. A source without sound produces an MP4 without sound. Very large pictures, such as 4K, need an H.264 level that some browsers do not offer; if the encoder is unavailable the page says so and suggests a smaller size.
Limits
The conversion re-encodes, so it is not lossless. HDR is not preserved. Chapters, subtitles and several audio tracks are not copied. The tool converts to MP4 only; for WebM output use the other video tools, which have a format switch. Nothing is uploaded.
Related tools
- Screen Recorder creates the recordings that usually need converting.
- Compress Video reduces the size of the new MP4.
- Video Cutter & Trimmer trims the converted file.
- Extract Audio from Video takes only the sound.
Checked against the current version of the WebM to MP4 Converter on .