Verdict: MP4 (H.264) wins on compatibility — it plays on virtually every device, app, and social platform. WebM (VP9) wins on file size for web streaming. Publish MP4 as the default; reach for WebM for self-hosted web video where you control the player.
| Criterion | MP4 | WebM |
|---|---|---|
| Typical codec inside | H.264 video with AAC audio | VP9 video with Opus or Vorbis audio |
| Compatibility | Plays virtually everywhere, including old devices | Modern browsers; weak on Apple devices and messaging apps |
| File size at equal quality | Larger — H.264 is an older codec | Smaller — VP9 compresses more efficiently |
| Platform uploads | Accepted by every social network and messenger | Frequently rejected by social and messaging apps |
| Editing support | Universal in desktop and mobile editors | Patchier — some editors need transcoding first |
| Best for | Default sharing, uploads, and archiving | Web embeds and bandwidth-sensitive streaming |
Every tool below runs entirely in your browser — your files never leave your device.
At the same file size, WebM (VP9) usually looks slightly better than MP4 (H.264) because VP9 is a newer codec. At the same visual quality, WebM files are smaller.
Most social and messaging apps accept only MP4 (H.264) uploads. Convert to MP4 first — the converters above do it locally in your browser.
Safari added WebM/VP9 support, but playback in apps, AirDrop shares, and older iOS versions is unreliable. Send MP4 to iPhone users.
Serve WebM first with an MP4 fallback in the same video tag: modern browsers get the smaller file, everyone else still plays the video.