Convert WebM to MP4 on a Mac (and AV1 to H.264), free

Why your Mac won’t play the file

A video file is a wrapper (WebM, MP4, MKV, MOV) around a video codec (H.264, HEVC, VP9, AV1) and a sound codec. QuickTime, Photos, Keynote and most Mac apps use macOS’s own media framework, which reads MP4 and MOV wrappers with H.264 or HEVC inside, plus AV1 on newer Macs. It doesn’t read WebM at all, or VP9 in any wrapper.

That’s why the same “.mp4” can play on one Mac and not another, or open with sound but a black picture: the wrapper is fine, the codec inside isn’t.

Check what’s inside

Select the file in Finder and press Command-I: under More Info, Codecs lists what macOS can read. If the video codec is missing, macOS can’t decode it. For the full picture, the free MediaInfo shows Format under Video: VP9, VP8, AV1 or AVC (that’s H.264). With ffmpeg installed: ffprobe "in.webm".

What we measured

On an M5 MacBook with macOS 27 (3 October 2026) we took a 62-second 1080×1920, 60 fps VP9 clip and asked macOS’s media framework (the one QuickTime uses) whether it could play each version:

Converting that clip to H.264 took 23.8 seconds with ffmpeg’s x264 encoder (43.6 MB out of 16.4 MB in) and 13.0 seconds with the Mac’s hardware encoder (49.0 MB). H.264 files are bigger than VP9 or AV1 for the same picture, but they open everywhere.

Convert in Terminal (ffmpeg)

Install Homebrew if you don’t have it, then brew install ffmpeg. In Terminal, go to the file’s folder (type cd and drag the folder in) and run:

ffmpeg -i "in.webm" \
  -c:v libx264 -crf 18 -preset medium -pix_fmt yuv420p \
  -c:a aac -b:a 192k -movflags +faststart \
  "out.mp4"

-crf 18 sets the quality (lower is better and bigger; 18–23 is the usual range). -pix_fmt yuv420p keeps it 8-bit 4:2:0, which every player decodes. +faststart lets it start playing before it’s fully loaded.

Faster, with the Mac’s hardware encoder. About twice as fast in our test, with slightly bigger files:

ffmpeg -i "in.webm" -c:v h264_videotoolbox -q:v 65 \
  -pix_fmt yuv420p -c:a aac -b:a 192k "out.mp4"

A whole folder of WebM files:

for f in *.webm; do
  ffmpeg -i "$f" -c:v libx264 -crf 18 -pix_fmt yuv420p -c:a aac "${f%.webm}.mp4"
done

Convert with a free app

• HandBrake: open the file, choose Production › Production Standard for editing (or General › Fast 1080p30 for a smaller file to share), click Start.
• Shutter Encoder: drop the file in, pick H.264 as the function, start. Made by video editors; no limits or ads.
• VLC plays WebM without converting, and its File › Convert / Stream can convert too, with fewer settings.

Skip online converters for anything private or big: you upload the whole file to someone else’s server, and most cap the size.

AV1 vs H.264: which to keep

AV1 is the newer codec: much smaller files for the same picture. Macs with M3 or later chips decode it in hardware, so it plays in QuickTime there. But Final Cut Pro and Premiere Pro don’t list AV1 as supported, older Macs can’t play it smoothly, and many apps and phones still can’t play it.

H.264 is older and bigger, but every Mac, editor and phone plays it. Keep AV1 for watching on a new Mac; convert to H.264 for editing, sharing or older Macs. To convert AV1 and keep the sound untouched:

ffmpeg -i "in.mp4" -c:v libx264 -crf 18 -pix_fmt yuv420p -c:a copy "out-h264.mp4"

If Premiere Pro imported only the sound, it’s the same problem. See Premiere Pro only imports audio.

Avoid it next time

If you download video files you have the right to download, get them as H.264 MP4 in the first place. Lynx, our download manager for Mac, saves H.264 MP4 when the site offers it at the quality you pick. Above 1080p some sites offer only AV1, so pick 1080p for editing. Clips you cut in Lynx come out as H.264 with Exact frames on (the default).