Where do WEBM files come from?
WEBM is an open video format Google introduced in 2010 for the web. It's based on the Matroska container and carries VP8, VP9 or AV1 video with Vorbis or Opus audio, all royalty-free. You mostly meet it in four places: browser screen recorders and extensions, video exported from web apps, clips saved from websites, and recordings from tools built on the browser's MediaRecorder API, such as online meeting and webcam recorders.
For streaming in a browser it's excellent. Outside the browser, support is patchy: iPhone Photos won't import it, PowerPoint and Keynote won't embed it, many video editors and most smart TVs won't open it, and WhatsApp and Instagram reject it as an upload.
Why this conversion re-encodes
Some conversions can just rewrap the video. WEBM to MP4 usually can't. MP4 can technically hold VP9 and AV1, but the devices and apps you're converting for expect H.264, so the converter decodes your VP8/VP9/AV1 video and encodes it as H.264 (CRF 23, the encoder's standard high-quality setting). The Opus or Vorbis audio is converted to AAC at 192 kbps.
Re-encoding takes longer than a rewrap, roughly the length of the video on a laptop and two to three times that on a phone, but the result plays on practically every device made in the last 15 years.
Will the MP4 be bigger than the WEBM?
VP9 and AV1 are more efficient than H.264, so an MP4 made from a well-compressed WEBM can come out somewhat bigger. Screen recordings are the exception: they're usually recorded at a generous fixed bitrate and often get smaller.
If the MP4 needs to be small, for an email or an upload limit, run it through the Video Compressor afterwards and pick Smallest file or Balanced.
Screen recordings: a few specifics
Screen recordings are the most common WEBM files, and they have a few quirks worth knowing:
- Browser recordings often have a variable frame rate. The MP4 keeps the original timing, so audio stays in sync.
- Odd pixel dimensions, common when recording a single window, are rounded to even numbers, which H.264 requires. You lose at most one pixel row or column.
- Text and interface edges come out sharp at the default quality. If small text looks soft, it was usually soft in the recording too.
- Recordings without a microphone have no audio track; the MP4 is simply silent.
Can I convert WEBM to MP4 on iPhone or Android?
Safari on iPhone can run the converter, so you can convert a WEBM from Files without a computer. Save the MP4 to Files, then use Share → Save Video to put it in Photos. On Android, Chrome saves the MP4 to Downloads, where it shows up in Google Photos and the gallery.
Long recordings are easier on a computer, both for speed and because phones have less memory to work with.
Nothing is uploaded
Screen recordings often show email, documents, dashboards or chat windows. Converting them with an upload service means handing all of that to a stranger's server. Here, ffmpeg runs inside your browser and the file never leaves your device, which makes it safe for work recordings too.
How do I record screen recordings as MP4?
If you make screen recordings regularly, it can be easier to record in MP4 from the start. On Windows 11, the Snipping Tool's screen recorder saves MP4. On a Mac, the built-in recorder (Shift-Command-5) saves MOV, which our MOV to MP4 converter rewraps in seconds. Many browser-based recorders and extensions now offer MP4 as an option in their settings, since newer versions of Chrome can record MP4 directly.
For WEBM files you already have, or ones you receive from others, converting here remains the quickest fix.