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Rescreen / Free tools / WebM to MP4

WebM to MP4 converter

Turn a WebM recording into an MP4 with H.264 video and AAC audio, the combination that plays on iPhones, Macs, Windows and the social sites. The conversion runs in your browser: the video is never uploaded. No account, no watermark.

Convert a file

Drop a WebM file here

or choose one from your device

Up to 2.0 GB. Stays on your device.

How to convert WebM to MP4

From your original recording to a ready-to-share file.

  1. Choose your WebM

    Choose or drop a .webm file. The page reads which video and audio it contains and checks that this browser can decode them and encode the MP4, before anything starts.

  2. Convert to MP4

    Press Convert to MP4. A progress bar shows how far it has got; Cancel stops it at any point.

  3. Preview and download

    When it finishes, the page opens the new file again to check it really is H.264 and AAC and is the same length as the original. Play it in the preview, then press Download MP4.

What you get

WebM files usually hold VP8, VP9 or AV1 video and Opus or Vorbis audio. Renaming the file to.mp4 does not change what is inside, so most players still refuse it. This tool decodes the picture and sound and encodes them again as H.264 video and AAC audio in an MP4. The size and frame timing are kept: a recording with an uneven frame rate, which browser recordings often have, stays in sync with its sound.

If the WebM already contains H.264 or AAC, that track is copied across untouched instead of being re-encoded, which is faster and loses nothing. Everything else is re-encoded at a bitrate based on the original’s. H.264 needs more data than VP9 for the same picture, so the MP4 is often a little larger than the WebM; a two-minute 1080p test file went from 32 MB to 45 MB.

Worked example

A two-minute screen recording saved by a browser-based recorder is typically a WebM with VP9 video at 1920×1080 and Opus audio. Drop it in and the page shows VP9 video, 1920×1080 · Opus audio, stereo, and that it will become H.264 video, 1920×1080 · AAC audio. On our test Mac a file like that converted in under 20 seconds; a phone or an older computer will take longer. When it is done you get recording.mp4, with a line confirming the codecs and that both files are 2:00 long.

A file with no sound, such as a screen share recorded without a microphone, converts to a silent MP4. A picture with an odd width or height, like 1281×721, is scaled by one pixel to 1280×720, because H.264 needs even dimensions; the page tells you when that happens.

Which browsers it works in

The converter uses WebCodecs, the browser’s own video encoder and decoder, so it needs a browser that has them. We have tested it in a Chromium-based browser (the engine behind Chrome and Edge) on a Mac, at desktop and phone screen widths, with VP8, VP9 and AV1 files, with and without audio, a variable frame rate file, an odd-sized one and a real recording made with the browser’s own recorder. In each, the picture and sound lined up to the millisecond against the original. Safari and Firefox also provide WebCodecs, but we have not tested them yet. Where a browser has no built-in AAC encoder, as in Firefox, the page downloads a small open-source one (about 1 MB, FFmpeg’s encoder built for the web) the first time it is needed; it runs on your device like everything else.

If a browser cannot handle a particular file, the page says which part is the problem — for example that it cannot decode AV1 video — before you start, rather than producing a broken file.

Limitations

  • Files up to 2.0 GB. The finished MP4 is held in your browser’s memory until you save it, so a phone may run out of memory on a long recording well before that.
  • Only the first video track and first audio track are kept. Subtitles and chapter marks are not carried over.
  • Very large pictures (beyond roughly 4K) may be more than a browser’s H.264 encoder can take; the page checks the exact size first and tells you.
  • Transparency in a WebM is dropped: H.264 has no alpha channel.
  • Converting is real work for your computer. Keep the tab open until it finishes; closing it cancels the conversion.

Is my video uploaded?

No. The file is read, converted and checked inside this page and never sent anywhere. We count anonymous page views and whether a conversion started, finished or failed, and why it failed from a fixed list of reasons; those events never include the video, its name or its size. See the privacy policy.

Frequently asked questions

Making a tutorial from the recording? Read how to make a tutorial video, or time your narration first with the script timer.

Why won’t my WebM play on an iPhone or in QuickTime?

Support depends on the player and operating-system version. Safari supports WebM on iOS and iPadOS 17.4 and later, so not every iPhone needs a conversion. If the intended player or upload destination rejects your WebM, try an MP4 with H.264 video and AAC audio, then check the saved result there.

Can I just rename .webm to .mp4?

No. The extension is only a label. The video and audio inside stay VP9 and Opus, and most players that reject WebM will reject the renamed file too.

Does converting lose quality?

Re-encoding can lose detail. The tool chooses a bounded H.264 bitrate based on the source, but no bitrate rule guarantees readable screen text or an imperceptible difference. Compare small labels and moving sections in the saved output before discarding the original. Tracks that are already H.264 or AAC are copied without re-encoding.

Is there a watermark or a time limit?

No watermark and no time limit. The only limit is the 2.0 GB file size, set by how much your browser can hold in memory.

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