The extension gave you a list: one clip, two entries — MP4 and WebM. Different sizes, seemingly identical picture. Here is the difference and how to choose.
A container is not a codec
MP4 and WebM are containers — boxes. Inside sit the tracks: video, audio, sometimes subtitles. Each track is compressed with its own codec. The confusion starts when people treat the file extension as the video format: two files both ending in .mp4 can hold completely different things.
- MP4 almost always carries H.264 video and AAC audio — a pairing over twenty years old that virtually everything understands.
- WebM carries VP9 video (sometimes VP8 or AV1) and Opus audio — open codecs with no licensing fees attached.
- You will also meet .mkv, a container with no restrictions on what goes inside. Flexible, but consumer players argue with it more often.
The practical takeaway: MP4 is about compatibility, WebM is about size. At the same bitrate the picture quality is comparable.
Why WebM is smaller
VP9 and AV1 are newer than H.264 and pack the same image more tightly — typically 20 to 50 percent smaller at visually indistinguishable quality. That is exactly why large platforms serve WebM to browsers that understand it: it is a direct saving on bandwidth.
The price of that density is CPU load. Older hardware and cheap TVs decode VP9 in software, with no hardware acceleration — hence the stutter and the draining battery. H.264 has a hardware decoder in just about every device ever made.
Which one for which job
- Editing and post — MP4. Premiere, DaVinci, CapCut and Final Cut take H.264 without ceremony; half of them either refuse WebM or silently transcode it first.
- Phone, tablet, sending over a messenger — MP4. iPhone and iPad will not play WebM in the system player, so you would be converting it anyway.
- TV, set-top box, USB stick — MP4. Consumer devices keep a short list of supported formats and WebM is usually not on it.
- Archiving to disk or publishing on your own site — WebM. The space saving is real, and every browser but the truly ancient reads it.
- No idea what happens to the file next — MP4. It is the one that will still open a year from now on somebody else’s machine.
Why the choice is not always there
The list of options comes from the platform, not from the extension. What you see is what actually sits on the server: if the site serves WebM only, there is no MP4 to conjure up. The reverse holds too — plenty of platforms still run on H.264 alone.
Streaming is a separate case. With HLS and DASH the server hands out segments rather than a file, and the container is assembled on your machine. There the result is MP4 regardless of what the site was using.
Is it worth converting
Converting WebM to MP4 works, but it always costs something: the video is already lossy, and a second compression pass adds artifacts on top of what was there. Plus minutes of your time per clip.
So the rule is simple: pick the format at download time, not afterwards. If the platform offers both, grabbing MP4 up front is cheaper than converting WebM later.
The exception is when the platform’s MP4 is noticeably worse than its WebM. Then take WebM at the highest quality and convert: the loss from a second pass beats the gap in source bitrate.