Video files come in a confusing range of formats: MP4, MOV, MKV, WEBM, AVI. Sometimes a file plays perfectly on one device and not at all on another. Once you understand one simple idea, the difference between a container and a codec, all of this starts to make sense.
Containers vs codecs
Think of a video file as a box with things inside.
- The container is the box. It's what the file extension tells you:
.mp4,.mov,.mkv,.webm. It holds the video, the audio, and sometimes subtitles and chapters, plus information about how they fit together. - The codec is how the contents are compressed. Common video codecs are H.264, HEVC (H.265), VP9 and AV1. Common audio codecs are AAC, MP3 and Opus.
Whether a file plays depends on both: the device has to understand the box and what's inside. That's why two MP4 files can behave differently: one might contain H.264 (plays everywhere) and the other HEVC (doesn't play on some older devices).
The common formats
MP4
The universal format. MP4 usually contains H.264 or HEVC video with AAC audio. It plays on virtually every phone, computer, TV, browser and social network. If you're not sure which format to use, use MP4 with H.264.
MOV
Apple's format, used by iPhones and Macs. MP4 was actually based on it, so they're close relatives. MOV files from iPhones usually contain HEVC video, which is why they sometimes won't play on Windows or older TVs. Our guide to converting MOV to MP4 covers the fix.
MKV (Matroska)
A flexible open format that can hold almost anything: several audio tracks (for different languages), multiple subtitle tracks, chapters and many codecs. It's popular for films and archives. The downside: phones, browsers and social networks often don't support it.
WEBM
A format designed for the web, usually containing VP9 or AV1 video with Opus audio. Browsers play it natively, and it's used by many websites behind the scenes. Some editing apps and Apple devices are less friendly to it.
AVI
An old Windows format from the 1990s. It still turns up in old camera footage and archives, but it lacks modern features and compresses poorly by today's standards. Convert AVI files to MP4 for everyday use.
Quick comparison
| Format | Plays almost everywhere | Best for |
|---|---|---|
| MP4 | Yes | Sharing, uploading, everyday use |
| MOV | Mostly (Apple devices best) | iPhone and Mac recording and editing |
| MKV | No | Films with several audio and subtitle tracks |
| WEBM | Browsers, yes; others, partly | Web playback |
| AVI | Partly | Old footage (convert it) |
Which format should you use?
- Sharing with anyone, on any device: MP4 (H.264 + AAC).
- Uploading to social networks: MP4.
- Archiving a film with several languages: MKV.
- Editing on a Mac: MOV is fine; export MP4 when sharing.
How to convert between formats
Our MOV to MP4 converter accepts MOV, MKV, WEBM, AVI and FLV and outputs a universal MP4. It looks inside your file first:
- If the video is already H.264, it repackages it into MP4 without re-encoding: instant and lossless.
- If it's HEVC, VP9 or something else, it re-encodes to H.264 with high-quality settings.
Need the file smaller as well? Run it through the video compressor. Just need the sound? Use the video to MP3 converter.
A note on audio formats
The same container/codec idea applies to audio. An .m4a file is usually AAC audio in an MP4-style container, while .mp3 is both the container and the codec. Our guide on MP3 vs M4A explains which to choose.
Once you see files as "box plus contents", format problems become easy to solve: when something won't play, put the contents into a box the device understands.