Convert MKV to FLV

Why Convert MKV to FLV?

Converting MKV to FLV serves a narrow but important set of use cases centered around RTMP-based live streaming infrastructure and legacy Flash content systems. Although Flash Player reached end-of-life in December 2020, the RTMP protocol — which uses FLV as its native container — remains the dominant ingest protocol for live streaming platforms including Twitch, YouTube Live, Facebook Live, and OBS Studio. If you need to prepare MKV content for RTMP streaming, FLV is the required format.

The live streaming workflow is the primary reason for this conversion today. Broadcasting software like OBS Studio encodes and transmits video to platforms using RTMP, which wraps H.264 video and AAC audio in an FLV container. If you have pre-recorded MKV content that needs to be streamed as if it were live — for scheduled broadcasts, re-streams, or 24/7 content channels — converting to FLV ensures compatibility with the RTMP pipeline without additional transcoding at the server level.

Legacy content management is another reason. Organizations with extensive Flash-based e-learning platforms, interactive training systems, or archived web content from the 2005-2015 era may still maintain FLV files. If new content is produced in MKV format but needs to integrate with these existing libraries, conversion to FLV maintains system consistency. Some legacy content management systems and media asset management platforms only accept FLV input.

Note that FLV is significantly more limited than MKV. It supports only a single audio track, has no subtitle or chapter support, and is limited to H.264/VP6 video codecs. All of MKV's multi-track, attachment, and advanced features are lost during conversion. For any use case not specifically requiring FLV or RTMP, MP4 is the better target format.

Key Benefits of Converting MKV to FLV:

• RTMP Streaming: Native container format for RTMP live streaming protocol
• OBS Compatibility: Ready for broadcast through OBS Studio and similar tools
• Low Latency: Optimized for real-time streaming delivery
• Small Files: Efficient packaging for web delivery
• Legacy Systems: Compatible with Flash-based content management platforms
• Platform Ingest: Accepted by RTMP endpoints on Twitch, YouTube Live, Facebook Live
• Progressive Download: Supports progressive download for legacy web players

Practical Examples

Example 1: 24/7 RTMP Re-Stream Channel

Scenario: A content creator has a library of pre-recorded MKV videos and wants to set up a 24/7 streaming channel on Twitch using RTMP ingest.

Source: episode_library/ (30 MKV files, 1080p, H.264, multi-audio)
Conversion: MKV → FLV (H.264 + AAC for RTMP)
Result: episode_library_flv/ (30 FLV files, 1080p, single audio)

Workflow:
1. Select primary English audio from each MKV
2. Re-encode or remux video to H.264 Main Profile
3. Convert audio to AAC stereo at 128 kbps
4. Package as FLV container for RTMP compatibility
5. Queue FLV files in streaming playlist software
✓ FFmpeg pushes FLV directly to Twitch RTMP endpoint
✓ Seamless loop of pre-recorded content as live stream
✓ Compatible with nginx-rtmp and Wowza servers
✓ Low-latency delivery to viewers

Example 2: Legacy E-Learning Platform Integration

Scenario: A corporate training department has updated course videos in MKV format but their learning management system (LMS) only accepts FLV uploads from the Flash era.

Source: compliance_training_2024.mkv (800 MB, 1280x720, H.264, AAC)
Conversion: MKV → FLV (H.264 for LMS upload)
Result: compliance_training_2024.flv (780 MB, H.264, AAC)

Workflow:
1. Remux H.264/AAC streams from MKV to FLV container
2. Add FLV metadata (duration, keyframes) with yamdi
3. Upload to legacy LMS as required FLV format
4. LMS progressive download player handles playback
✓ No re-encoding needed (H.264/AAC compatible)
✓ FLV metadata enables seeking in legacy players
✓ Existing LMS infrastructure remains functional
✓ Training content accessible without system upgrade

Example 3: OBS Studio Pre-Recorded Broadcast

Scenario: A streamer wants to play pre-recorded MKV content through OBS Studio to multiple streaming platforms simultaneously via RTMP.

Source: gaming_highlights.mkv (2.5 GB, 1920x1080, H.264, Opus audio)
Conversion: MKV → FLV (RTMP-ready for multi-platform)
Result: gaming_highlights.flv (2.4 GB, H.264, AAC 192k)

Workflow:
1. Convert Opus audio to AAC (RTMP requires AAC)
2. Copy H.264 video stream without re-encoding
3. Package in FLV container with proper timestamps
4. Feed FLV to OBS via media source or FFmpeg command
✓ Broadcasts to Twitch, YouTube, Facebook simultaneously
✓ RTMP servers accept FLV natively without transcoding
✓ Audio re-encoded from Opus to AAC for compatibility
✓ Video quality preserved through stream copy

Frequently Asked Questions (FAQ)

Q: Why would I convert to FLV if Flash is dead?

A: While Flash Player is indeed dead, FLV lives on as the native container for RTMP streaming. RTMP remains the dominant protocol for live streaming ingest — Twitch, YouTube Live, OBS Studio, and most streaming servers use RTMP with FLV containers. Converting to FLV is necessary for RTMP-based workflows, even though browser-based FLV playback no longer works.

Q: Can I remux MKV to FLV without re-encoding?

A: If your MKV contains H.264 video and AAC or MP3 audio, you can often remux directly to FLV without re-encoding: ffmpeg -i input.mkv -c copy -f flv output.flv. However, codecs like H.265, VP9, FLAC, DTS, and Opus are not supported in FLV and must be re-encoded. H.264 + AAC is the ideal combination for lossless remuxing.

Q: What happens to my subtitles and multiple audio tracks?

A: All lost. FLV supports only a single audio track and has no subtitle support. You must select one audio stream during conversion. Subtitles can only be preserved by burning them into the video (hardcoding). Chapters, attachments, and all other MKV metadata are discarded.

Q: What's the maximum resolution for FLV?

A: With H.264 encoding, FLV supports up to 1080p. Older FLV codecs (Sorenson Spark, VP6) are practically limited to 720p or lower. For RTMP streaming, 1080p at 30fps or 720p at 60fps are the most common configurations. There's no 4K support in FLV — use MP4/MKV for higher resolutions.

Q: Is FLV smaller than MKV?

A: If you remux without re-encoding, the FLV file will be roughly the same size as the MKV (minus the additional audio tracks and subtitles). The FLV container overhead is minimal. If you re-encode at a lower bitrate for streaming, the FLV will be smaller. The main file size reduction comes from dropping MKV's extra tracks, not from the container itself.

Q: Can I stream MKV directly via RTMP without converting to FLV?

A: Not directly. RTMP protocol requires FLV container format. However, FFmpeg can convert on-the-fly: ffmpeg -re -i input.mkv -c:v copy -c:a aac -f flv rtmp://server/live/key. This remuxes the MKV to FLV in real-time during transmission without creating an intermediate file. The -re flag sends at real-time speed.

Q: Should I use FLV or MP4 for streaming?

A: For RTMP ingest (sending to streaming servers), FLV is required. For HLS/DASH delivery (serving to viewers), MP4 segments (fMP4) or TS segments are used. The streaming platform handles the conversion from RTMP/FLV ingest to HLS/DASH delivery automatically. You only need FLV for the upload/ingest side of streaming.

Q: Can modern media players play FLV files?

A: Yes. VLC, mpv, PotPlayer, MPC-HC, and MX Player all handle FLV playback without issues. The format itself is simple and well-documented. What no longer works is browser-based FLV playback, since Flash Player has been removed from all browsers. For desktop and mobile app playback, FLV remains fully functional.