What is the FLV file format?
A .flv file is a container that holds encoded audio and video data. The format stores its stream in a structure similar to that used by .swf, a multimedia container based on Adobe Flash technology, and supports the same set of compression methods, namely:
- audio data:
- MP3,
- ADPCM,
- Nellymoser,
- Speex,
- AAC (with some restrictions);
- video data:
- Sorenson Spark (H.263),
- On2 VP6,
- Screen Video, Screen Video 2,
- H.264 (with some restrictions).
The .flv file structure consists of a 9-byte header and a stream of tagged packets. The header opens with the three-byte ASCII signature FLV, followed by a version byte, a flags byte indicating whether audio and/or video tracks are present, and a 4-byte DataOffset field; the remainder of the file stores the actual audio, video, and script-data packets.
.flv was introduced in 2002 with Macromedia Flash MX and Flash Player 6; Adobe Systems later published a partial specification after acquiring Macromedia. The format became the dominant choice for streaming video on the web - YouTube relied on it for years - and an .flv file can be embedded directly inside an .swf file. The FLV format is supported by many media player tools, and .flv files can be converted into other multimedia formats such as .mp4. In 2007, Adobe introduced the successor format .f4v, based on the MPEG-4 Part 12 standard with .aac audio. Following Adobe's end-of-life of Flash on 31 December 2020, browser plugins for .flv playback have been globally removed, though desktop players such as VLC and tools like ffmpeg continue to open and convert .flv files.
Security & safety
RISK: LOWAn FLV file is plain video/audio data - it does not run code and is not the interactive, exploit-prone Flash (that was SWF + the Flash Player plug-in, both now dead). Opening an FLV in a modern player like VLC is safe. The real-world risks are indirect: 'FLV player/codec' download sites and bundled converters (RealPlayer, KMPlayer, some 'free' converters) often carry adware/PUPs - stick to VLC, MPC-HC (clsid2), HandBrake and FFmpeg. As always, keep your media player updated so a malformed file can't exploit a decoder bug.
Format details
in a nutshellPrograms that open FLV files
Windows4 apps
VLC media player
Open-source File > Open the .flv - VLC has the codecs built in and plays it with no Flash needed. Also converts via Media > Convert/Save. 
HandBrake
Open-source Load the FLV and convert to MP4/MKV; the standard free transcoder for getting FLV into a modern format. 
MPC-HC (clsid2 fork)
Open-source Open the FLV; lightweight player with bundled decoders. The actively maintained fork (the original mpc-hc.org project ended). 
macOS2 apps
VLC media player
Open-source Open the FLV to play; QuickTime can't, but VLC can. Convert via File > Convert/Stream. 
Linux2 apps
FFmpeg
Open-source Command line: 'ffmpeg -i in.flv -c copy out.mp4' remuxes without re-encoding when streams are H.264/AAC; otherwise transcode. The engine most converters use.
CloudConvert
Freemium Upload the FLV in a browser and convert to MP4 - no install. Good for a one-off file. Technical details
deep spec| MIME type | video/x-flv |
| Magic bytes | 46 4C 56 01 ('FLV' + version byte 0x01) at byte offset 0 |
| Header size | 9 bytes - 3-byte ASCII signature 'FLV', 1-byte version, 1-byte TypeFlags, 4-byte DataOffset |
| Tag types | Audio (tag 8), Video (tag 9), Script/AMF metadata (tag 18) |
| Video codecs | Sorenson Spark (H.263), On2 VP6, Screen Video, Screen Video 2, H.264/AVC |
| Audio codecs | MP3, ADPCM, Nellymoser, Speex, AAC, PCM (linear) |
| Metadata encoding | AMF0 (Action Message Format 0) carried in Script Data tags |
| Timestamp format | 24-bit millisecond field per tag; extended to 32-bit with an ExtendedTimestamp byte for streams longer than ~4.6 hours |
| Byte order | Big-endian (network byte order) for all multi-byte integer fields |
| Streaming support | HTTP progressive download; RTMP (Real-Time Messaging Protocol) for live streaming |
| SWF embeddability | An FLV stream can be embedded directly within an .swf file |
| Developer | Macromedia; specification later maintained and partially published by Adobe Systems |
| Flash Player EOL | Adobe terminated Flash Player support on 31 December 2020; browser plugins removed globally |
| Successor format | .f4v - ISO Base Media File Format (MPEG-4 Part 12) with AAC audio, introduced 2007 |
| Common conversion | ffmpeg -i input.flv output.mp4 (container re-wrap; no re-encode if codecs are compatible) |
| Released | 2002 (Macromedia Flash MX / Flash Player 6); spec later published by Adobe, then frozen after Flash EOL |
| Open standard | Yes · royalty-free |
| Specification | web.archive.org |
FLV conversions
Community Q&A
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