Where FLV is still used
FLV is best known as the old Flash Video format, and browsers no longer play it. It is still alive in one important place: live streaming. RTMP, the protocol used to send live video to many streaming platforms and media servers, carries video in FLV form. Some tools that replay a prepared video as a live stream, or that feed a recorded file to an RTMP server, expect an FLV input. Other reasons to convert MKV to FLV:
- an older e-learning system or kiosk with a built-in Flash-type player,
- a company video archive that only accepts FLV uploads,
- test files for streaming software and media servers.
For anything else, such as phones, editors and social media, MP4 is the better format. Use MKV to MP4.
Speed and quality
FLV supports H.264 video with AAC sound, the same combination found in most OBS recordings and many MKV films. When your MKV contains exactly these codecs and you keep the original settings, the conversion can be much quicker, because less work is needed. Other codecs, such as HEVC, VP9, AV1, DTS or Dolby sound, are re-encoded to codecs FLV supports. FLV is written by the built-in converter engine, which runs ffmpeg in your browser and is slower than the modern fast path.
What FLV cannot hold
FLV is a simple container: one video track and one audio track. Extra audio languages, subtitles and chapters from the MKV are not carried over. HEVC and 10-bit video are not part of standard FLV, which is why they are re-encoded. If the target system is old, it may also have limits on resolution and bitrate. In that case 720p or 480p is a safe choice.
Tips for streaming use
Many streaming setups work best with a constant frame rate, such as 30 or 60 fps. If your MKV comes from a screen recorder with a variable frame rate, choose a fixed frame rate here, which re-encodes the video with even timing. Keep the file reasonably small for older servers: the FLV compressor can reduce it further. For the reverse direction, see FLV to MKV. All processing happens on your computer.