“This video format isn’t supported” is one of the most confusing errors on a computer, because two files ending in .mp4 can behave completely differently. One plays everywhere; the other shows a black screen with sound. The reason is that a video file has two layers, the container and the codecs, and the extension only tells you about the first.
The short answer
- The container (MP4, MOV, MKV, WebM) is the file format: a box holding video, audio and subtitle streams. It’s what the extension names.
- The codec (H.264, HEVC, AV1, VP9) is how the video inside is compressed. It decides whether a device can actually play the picture.
- For a video that plays everywhere, use MP4 with H.264 video and AAC audio.
Containers: the box
A container bundles the streams together, keeps them in sync, and stores an index so players can jump to any point. It can also hold chapters, subtitles, cover art and several audio languages.
| Container | Extensions | Typical use |
|---|---|---|
| MP4 | .mp4, .m4v |
Sharing, uploading, phones, the universal default |
| MOV | .mov |
iPhones, Macs, Apple editing |
| MKV | .mkv |
Films, recordings, archiving with many tracks |
| WebM | .webm |
Websites, browser screen recorders |
| MPEG-TS | .ts, .mts |
Broadcasting, live streams, camcorders |
Changing only the container is called remuxing. The compressed streams are copied unchanged into a new box, so it’s fast and loses no quality. Converting MOV or MKV to MP4 is usually a remux.
Codecs: what’s inside
A codec compresses video so it fits in a reasonable file. Raw 1080p video would need about 6 GB per minute; compressed, it’s often under 100 MB. Codecs differ in how efficiently they compress and in how widely they’re supported.
| Video codec | Also called | Efficiency | Support |
|---|---|---|---|
| H.264 | AVC | Baseline | Practically every device since about 2010 |
| HEVC | H.265 | About twice H.264 | Newer devices; Windows needs an add-on; patchy in browsers |
| VP9 | — | Similar to HEVC | All major browsers, Android, YouTube |
| AV1 | — | Better than HEVC | Recent browsers, phones and TVs; slow to encode |
Audio has codecs too. AAC is the standard in MP4 and plays everywhere. Opus is the modern open codec used in WebM. Films often carry Dolby Digital (AC-3) or DTS surround audio, which many apps and all browsers can’t decode. That’s the usual cause of a video playing without sound.
Why a video plays on one device but not another
The device has to support both layers. Some common cases:
- iPhone video on a Windows PC: the MOV container is fine, but the HEVC codec isn’t installed. Sound plays, the picture is black. Fix: convert the video to H.264 with HEVC to MP4, or install Microsoft’s HEVC Video Extensions.
- MKV film on a TV or iPad: the device doesn’t accept the MKV container, or can’t decode its DTS audio. Fix: remux to MP4 with MKV to MP4; surround audio may need a desktop app.
- Screen recording won’t open in PowerPoint or iMovie: it’s WebM with VP9. Fix: WebM to MP4, which re-encodes to H.264.
- Upload form rejects a MOV: the site only checks for
.mp4. Fix: MOV to MP4, usually a quick remux.
Remux or re-encode?
Converting is fast and lossless when the codecs inside are already right for the target, and slower when they’re not:
| Situation | What happens | Quality | Speed |
|---|---|---|---|
| H.264 MOV or MKV to MP4 | Remux | Identical | Seconds |
| HEVC, VP9 or AV1 to H.264 MP4 | Re-encode | Very slightly lower | About playback speed |
| H.264 MP4 to VP9 WebM | Re-encode | Very slightly lower | Slower than playback |
Every re-encode loses a little detail, as photocopying a photocopy does, so it’s best to convert from the original rather than from an already-converted copy.
Which should you choose?
- Sharing with anyone, uploading, editing: MP4 with H.264 and AAC.
- Keeping iPhone videos small: leave them in HEVC, and convert copies only when someone can’t play them.
- Videos on your own website: WebM with VP9 for smaller downloads, plus an MP4 fallback. See MP4 to WebM.
- Archiving films with several languages and subtitles: MKV.
- Long-term storage of 4K footage: HEVC or AV1 save the most space, as long as you can still play them.
Checking what’s inside a video
The extension won’t tell you the codec, but these will:
- VLC (Windows, Mac, Linux): open the video, then Tools › Codec Information (on Mac, Window › Media Information).
- Windows: right-click the file, Properties › Details shows some information; the free MediaInfo app shows everything.
- Mac: open in QuickTime Player and choose Window › Show Movie Inspector.
Once you know both the container and the codecs, it’s clear whether a quick remux will do or a full conversion is needed.