FLV vs MP4
Use MP4. FLV was the format of Flash video players, and Flash was removed from every browser at the end of 2020; nothing current plays FLV without extra software. Many FLV files already hold H.264, which moves into MP4 without being re-encoded.
Last updated
Choose FLV when
- Feeding an RTMP live-streaming server, which still uses FLV internally.
- Keeping an old archive exactly as it is.
Choose MP4 when
- Everything else: playing, sharing, editing, uploading.
- Old Flash-era downloads you want to watch again.
What FLV actually is
FLV was built to stream video inside the Flash plugin, and for most of the 2000s it was how video worked on the web. Flash was discontinued at the end of 2020, which left FLV as a format with a great deal of archived content and almost nothing that plays it.
If you have FLV files they are probably old downloads worth rescuing, and converting them to MP4 is the way to keep them watchable.
What MP4 actually is
MP4 is a container: it holds a video stream, usually H.264, an audio stream, usually AAC, and any subtitles or chapter data, and describes how they line up. Almost every device made in the last fifteen years plays that combination in hardware, which is why MP4 became the default.
Two MP4 files can behave completely differently depending on the codecs inside. A file that will not play is usually an unusual codec in an ordinary container, not a broken file.
FLV and MP4 side by side
| FLV | MP4 | |
|---|---|---|
| Extension | .flv | .mp4 |
| Media type | video/x-flv | video/mp4 |
| Inside the file | Sorenson Spark (FLV1) video with MP3 sound | H.264 video with AAC sound |
| Compression | Lossy | Lossy |
| Carries sound | Yes | Yes |
| Plays on | VLC and very little else since Flash was retired in 2020 | every phone, laptop, TV, browser and messaging app |
| We can write it | Yes | Yes |
Questions
Can I still play FLV files?
With VLC, yes. Browsers and the players built into Windows, macOS and phones do not.
Does converting FLV to MP4 lose quality?
Not if the FLV holds H.264 video: it can be repackaged into MP4 without re-encoding. Older FLVs use Sorenson Spark or VP6 video and have to be re-encoded.