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WebM to MP4

Details

How to use WebM to MP4

What the tool does, how to run it, and what to expect from the result.

How to convert WebM to MP4 in the browser

The tool loads a WebAssembly build of ffmpeg into a worker in your tab, decodes the WebM, and re-encodes the picture with libx264 and the audio with AAC into an MP4 that is written with the faststart flag so it starts playing before it finishes downloading.

Because it is a full re-encode rather than a remux, quality is set by a constant rate factor rather than a bitrate target. Balanced (CRF 27) is the right default for screen recordings and social uploads; drop to Smaller file or Aggressive only when the file has to fit a hard size cap.

  • Drop one WebM file onto the upload area, or click it and pick the file.
  • Check the duration and dimensions in the panel; a duration of 0 means the recorder omitted the header field and is not an error.
  • Choose a quality preset: Balanced for anything you care about, Smaller file or Aggressive when a size cap matters more than detail.
  • Press Convert to MP4 and leave the tab open while the encoder runs.
  • Download the MP4 and play it once end to end to confirm audio sync before you upload it anywhere.

Why a generic app launch draft can be more useful here

Alternative app stores and third-party marketplaces often have less standardized promo workflows. In practice, teams just need a clean, reusable short video draft from screenshots.

This page keeps the defaults generic on purpose, so the same local builder can adapt to different store surfaces without pretending to be an official platform tool.

What this page should help with

It is useful for launch drafts, screenshot storyboards, and short promo videos built from screenshots when the publishing destination is more flexible or mixed.

  • Generic app launch promo drafts
  • Alternative Android store visuals
  • Third-party marketplace screenshot videos
  • Local browser preview and export with no server rendering
Tips

Getting a better result out of WebM to MP4

Specific settings and thresholds, not general advice.

  • The export is a real re-encode, not a container swap: the page runs ffmpeg compiled to WebAssembly and encodes with libx264 at preset veryfast, CRF 27 on Balanced, CRF 31 on Smaller file, and CRF 35 on Aggressive, with AAC audio at 128 kbps.
  • WebM carries VP8, VP9, or AV1 video, and none of those can live inside an MP4 that a phone will open, so unlike MOV to MP4 there is no lossless remux path here. The pixels have to be decoded and re-compressed whichever tool you use.
  • If the WebM has a transparent background (VP8 or VP9 with an alpha channel, the usual output of a Lottie or After Effects web export), the alpha is gone in the MP4. H.264 has no alpha channel, so transparency flattens against black.
  • WebM recordings made by a browser MediaRecorder, which covers most screen-capture extensions and web apps, ship without a duration field in the header, so the panel shows 0 for duration and no bitrate. The conversion still runs on the whole file.
  • Opus is the usual WebM audio codec and it is already lossy, so the AAC re-encode is a second generation. At 128 kbps AAC the loss is inaudible on speech and screen-recording narration, which is what most WebM files contain.
Limits

What WebM to MP4 does not do

The honest boundary, so you do not lose time finding it yourself.

  • Only the first video track and the first audio track survive: the command maps 0:v:0 and 0:a?, so extra audio tracks, subtitle tracks, and chapters are dropped.
  • Output is 8-bit 4:2:0 (yuv420p) H.264 only. There is no 10-bit, no HEVC, no AV1, and no way to keep a 4:4:4 or 4:2:2 source at full chroma resolution.
  • The encoder is single-threaded WebAssembly, so a long or 4K clip can take several times its own runtime and will hold a lot of memory. Short clips are the realistic use case.
  • The tab has to stay open and in the foreground. Navigating away or closing the tab kills the worker, and there is no resume.
Reference

Terms used on this page

Short, plain-language definitions for the formats and settings above.

Files & formatsWebMWebM is an open, royalty-free video container designed for the web, built around the VP8/VP9 (or AV1) video codecs and Opus/Vorbis audio. It is widely used by browsers and sites like YouTube for efficient streaming.GeneralPhotos & videoScreen recordingA screen recording is a video that captures everything happening on your display, saved to your photo library. These clips are easy to make and often forgotten, so they quietly pile up and consume storage like any other video.iOSAndroidPhotos & videoScreenshotA screenshot is an image captured of whatever is on your screen, saved into your photo library like a regular picture. Individually small, screenshots accumulate fast and clutter the camera roll because most are saved once and never needed again.iOSAndroidGeneral
At a glance

Who WebM to MP4 is for

A quick way to understand who this helps, what it solves, and where it connects next.

Best fit

People with a WebM recording that will not play on iOS or import into an editor that expects MP4

Ideal for

Turning a WebM screen recording or web download into a universally playable MP4

FAQ

Common questions

Short answers for the questions people usually have before trying a utility like this.

Is this tied to one store?

No. This page is intentionally generic and is meant for alternative Android stores or third-party app marketplaces that do not fit a more specific store flow.

Can I still switch the canvas and pacing?

Yes. The page opens with a generic app launch setup, but the same arrange, canvas, pacing, and export controls remain available.

Does export remain local-only?

Yes. Preview and export stay on the device in the browser, without server-side video rendering.

Why does the MP4 come out larger than the WebM?

Because VP9 and AV1 are more efficient codecs than H.264. The same picture quality costs more bits in H.264, so a VP9 WebM re-encoded to MP4 at CRF 27 is often bigger than the source. You are trading size for the ability to play the file anywhere.

What do the three quality presets actually change?

They set the x264 constant rate factor: Balanced uses CRF 27, Smaller file uses CRF 31, Aggressive uses CRF 35. Lower is better quality and a bigger file. Each step of about 6 in CRF roughly halves or doubles the size.

My WebM screen recording shows a duration of 0. Is it broken?

No. WebM files written by a browser recorder leave the duration element empty, so the browser reports it as unknown. The converter still reads the whole stream and the MP4 gets a correct duration.

Can I keep the transparent background?

No. MP4 with H.264 has no alpha channel. If you need transparency, keep the WebM, or export a PNG sequence or an animated WebP or GIF instead.

Does anything about my video get sent anywhere?

The video is decoded and encoded in your browser and is never uploaded. The one network request is for the WebAssembly build of the encoder itself, which is fetched from a public CDN on first use and then cached.

Will the audio drift out of sync?

Variable frame rate WebM, which is what most screen recorders produce, is re-timed to constant timestamps during the encode, and that is usually what fixes sync problems rather than causing them. If the source itself has broken timestamps, no browser tool can repair it.

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