Manatee

How to convert WEBM to MP4 on a Mac

· 6 min read

Converting a WEBM to MP4 on a Mac means re-encoding it, because WEBM files use video codecs (VP8, VP9, or AV1) that QuickTime, Photos, iPhones, and most editing apps do not play, and the MP4 that works everywhere needs H.264. You can do that with one ffmpeg command in Terminal or with a drag into a converter app, and at a sensible quality setting the result is visually identical to the original. Nothing built into macOS will do it; QuickTime Player cannot even open a WEBM.

What a WEBM is and why your Mac balks at it

WEBM is a container built for the web, designed around royalty-free codecs: VP8 or VP9 for video (more recently AV1) and Vorbis or Opus for audio. Browsers play it natively, which is why web-based screen recorders, video chat tools, and many download sites produce it. Outside the browser, Apple’s media stack has never supported those codecs, so QuickTime Player reports the file is not compatible, Quick Look shows a blank icon, and Photos refuses to import it.

The file itself is almost never the problem. If you want to confirm it is intact before converting, open it in Safari (drag it onto a Safari window), which has played WEBM since macOS Big Sur, or in an open-source media player. what-is-a-webm-file-and-how-do-you-open-it-on-a-mac goes into more detail on just viewing one.

Why this conversion is always a re-encode

Some conversions between containers, MKV to MP4 or MOV to MP4, can copy the video stream untouched because the codec inside is already one MP4 supports. WEBM is different. MP4 can technically hold VP9 and AV1, but an MP4 with VP9 inside is no more playable on an iPhone than the WEBM was. To get a file that opens everywhere, the video has to be decoded and re-encoded as H.264 (or HEVC), and the audio as AAC.

That has two practical consequences. It takes time, proportional to the length of the video: a few minutes for a ten-minute clip on Apple silicon, longer on Intel. And it is a lossy step, so the quality setting you choose matters. The good news is that a single re-encode at a reasonable setting is not something you can see, and the original WEBM stays on disk if you ever need to redo it.

Method one: ffmpeg in Terminal

ffmpeg is the open-source tool that most converters are built on. It does not come with macOS; a package manager such as Homebrew installs it in one command, or you can download a build from the ffmpeg project.

  1. Open Terminal.
  2. Type ffmpeg -i and drag the WEBM file onto the window so its path is filled in.
  3. Type the following after the path, then a name for the output ending in .mp4:

-c:v libx264 -crf 20 -preset medium -pix_fmt yuv420p -c:a aac -b:a 160k -movflags +faststart ~/Desktop/output.mp4

  1. Press Return and wait for the progress lines to stop.

What each part does: -c:v libx264 picks H.264; -crf 20 sets quality (lower is better, 18 is visually lossless, 23 is the default, 26 is noticeably smaller with a slight softening); -pix_fmt yuv420p forces the pixel format that QuickTime and iPhones require, which matters because some WEBMs use one they will not play; -c:a aac converts the audio; -movflags +faststart puts the index at the front so the file streams properly when uploaded.

If the WEBM has no audio (common with screen recordings), ffmpeg handles that without complaint. If you want HEVC instead of H.264 for a smaller file, swap libx264 for libx265 and add -tag:v hvc1 so Apple players recognize it; keep in mind that older devices will not play HEVC.

Method two: a drag into Manatee

If you would rather not install anything, Manatee bundles its own copy of ffmpeg and does the conversion from a drop:

  1. Open Manatee and choose Convert.
  2. Pick MP4 as the target.
  3. Drop the WEBM into the window or onto the menu bar icon. Drop a folder of them to convert a batch; each file gets a line in the job list with progress.
  4. The MP4 is written alongside the WEBM. The original is never modified.

Manatee picks the codec and pixel format settings so the MP4 opens in QuickTime, imports into Photos, and plays on a phone. Everything runs on the Mac; nothing is uploaded, which matters if the recording is a meeting or a screen capture of something private. Two limits to be clear about: it does not trim or edit, and it does not fix a WEBM that is actually damaged (a recording that was cut off mid-save, for example). For a broken file, the Repair mode is the right place to start, and it reports honestly whether the file was fixed, partly recovered, or not fixable.

Common sources of WEBM files and what to watch for

Browser-based screen recorders and meeting tools. These write WEBM as they go, and many never write a proper duration into the file. QuickTime will not show it; even players that open the file may show a length of 0:00 and refuse to seek. Converting fixes this: the MP4 gets a correct index and duration.

Video chat recordings. Often Opus audio at a low bitrate and VP8 video at whatever resolution the call was. The conversion will not improve what was recorded, so do not expect sharpness that was never there; it will make the file shareable.

Downloaded clips. Usually VP9 at good quality. Use a CRF of 18 to 20 to keep that quality through the re-encode.

Animated content from the web. A short looping WEBM converts fine; if the goal is to post it somewhere that wants a GIF, how-to-convert-a-video-to-gif-on-a-mac is the better route.

Getting the size right afterwards

A WEBM re-encoded to H.264 is often larger than the original, because VP9 compresses a bit more efficiently at the same quality. If the MP4 needs to fit a limit (an email attachment, a messaging app, an upload form), do not run a second conversion at a lower CRF; each lossy pass compounds. Instead, go back to the original WEBM and convert once at the setting that fits, or use Manatee’s Shrink mode, where you name a size in megabytes and it finds the quality that lands under it in a single pass from the source. how-to-make-a-screen-recording-smaller-on-a-mac covers the size question for recordings specifically.

Questions

Can I just rename the .webm file to .mp4?

No. WEBM is a different container with different codecs inside; renaming changes the label and nothing else. QuickTime will still refuse it, and a file that a tolerant player happened to open will break the moment it is sent to a phone.

Will the MP4 look worse than the WEBM?

Not at a CRF of 18 to 20, or with Manatee’s defaults. The re-encode is lossy in the technical sense, but a single pass at those settings is not visible. What degrades quality is converting, then compressing, then converting again.

The MP4 plays but the audio is out of sync.

This happens with WEBMs from browser recorders that had a variable frame rate. Redo the conversion and add -vsync cfr -r 30 (constant frame rate at 30 fps) to the ffmpeg command before the output name. If a file converted with Manatee comes out of sync, send it to hey@everydaymactools.com.

Can I convert WEBM to MOV instead?

Yes. Manatee offers MOV as a target, and in ffmpeg simply end the output name in .mov with the same settings. For sharing, MP4 is the better choice; see mov-vs-mp4-which-should-you-send.