WebM to MP4 Converter
Turn WebM into MP4 that plays everywhere — and fix recordings that cannot be seeked.
What will happen
About the WebM to MP4 Converter
WebM is the web’s open video format — VP8, VP9 or AV1 video with Opus or Vorbis audio. It is what browser-based screen recorders, many meeting and capture tools, and some website downloads produce. MP4 with H.264 video and AAC audio is the format phones, PowerPoint and Keynote, messaging apps and older TVs play most reliably, so this page converts WebM to exactly that.
It also repairs a common problem: recordings made with the browser’s MediaRecorder often have no duration in the file and cannot be seeked. The converted MP4 — or, if you prefer, a rewritten WebM — gets a proper duration and a seek index. If the WebM already contains VP9 or AV1, you can also keep that video as it is inside MP4, which takes seconds. Everything runs in your browser; nothing is uploaded.
How to use it
- Drop the .webm file on the box, or click Choose a WebM file. The page shows the codecs and warns if the duration is missing.
- Choose what you need: Compatible MP4 (H.264 + AAC, re-encoded), Fast MP4 (keep VP9/AV1 video, no re-encoding) or Repair the WebM (same format, now with duration and seeking).
- For Compatible MP4, optionally pick a quality, cap the resolution, or choose a constant frame rate for editors that dislike variable-frame-rate recordings.
- Press Convert, then preview and Download the file.
Examples
demo.webm · VP8 + Opus · no duration in the header · Compatible MP4, quality High
demo.mp4 with H.264 + AAC, a correct duration and working seeking.
talk.webm · VP9 1920×1080 + Opus · Fast MP4
talk.mp4 — the VP9 video and Opus audio copied unchanged, done in seconds.
meeting.webm from a browser recorder · Repair the WebM
meeting.webm (rewritten) that shows its length and can be scrubbed in any player.
Why browser recordings have no duration
MediaRecorder writes WebM as a stream while recording, so it does not know the length when it writes the header, and it usually adds no seek index (Matroska “Cues”). Players then show no total time and may refuse to jump around. Rewriting the file — as WebM or as MP4 — measures the real duration from the packets and writes it, together with a seek index.
Variable frame rate
Screen and browser recordings often have a variable frame rate: frames are only written when something changes. Players handle this fine, but some video editors drift out of sync. Choosing Constant 30 fps (or 25/24/60) while converting to MP4 repeats or drops frames so every second has the same number of frames.
Which option should I choose?
- Compatible MP4 (H.264 + AAC) — for iPhones, PowerPoint, Keynote, messaging apps, social sites and older TVs. It re-encodes, so how long it takes depends on your device; the page shows the time left while it works.
- Fast MP4 (keep VP9/AV1) — for current browsers, VLC and Android, when you just need an .mp4 file. Not possible for VP8 video.
- Repair the WebM — when WebM is fine but the duration or seeking is broken.
Limitations
- VP8 video cannot be stored in MP4, so “Fast MP4” is not available for VP8 recordings; Compatible MP4 converts it.
- Vorbis audio is converted to AAC in MP4 (MP4 players do not support Vorbis).
- Re-encoding needs WebCodecs (Chrome, Edge, Safari 26+, desktop Firefox 130+). Firefox for Android has no WebCodecs, so there only “Fast MP4” and “Repair the WebM”, which copy without re-encoding, work.
- Audio that starts later than the video in the original (common in browser recordings) keeps that gap.
- WebM files with transparency (alpha) lose it in H.264 MP4.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
Is my recording uploaded?
No. The WebM is converted inside your browser and the MP4 is created on your device.
Why does my WebM not show its length or let me skip ahead?
It was most likely recorded by a browser (MediaRecorder), which writes no duration and no seek index. Converting it here — to MP4, or to a repaired WebM — fixes both.
Will converting reduce the quality?
“Fast MP4” and “Repair the WebM” copy the video unchanged. “Compatible MP4” re-encodes it; at High quality the difference is hard to see.
Does the MP4 play on iPhone?
Yes — the Compatible MP4 option produces H.264 video with AAC audio, which iPhones and iPads play and save to Photos.
How long does it take?
Fast MP4 and Repair finish in seconds. Compatible MP4 depends on your device’s encoder; many computers convert faster than real time. The page shows the time left while it works.