How FLV Works
FLV (Flash Video) is the format that carried nearly all web video through the 2000s, the era when watching a clip in a browser meant loading Adobe Flash Player. Technically it uses a simple tag-based container: a short file header is followed by a sequence of tags, each carrying audio, video, or scripted metadata along with a timestamp. This lightweight structure was designed for smooth streaming inside the Flash runtime, which is exactly the job it did for years on sites across the web.
Inside that container, the codecs evolved over time. Early FLV files typically carried Sorenson Spark or VP6 video, while later versions added support for H.264 video and AAC audio, the same modern codecs found in MP4. A related format, F4V, was introduced specifically to hold H.264 in a more MP4-like structure. But the codec inside was never FLV's defining feature; its defining feature was its dependence on Flash to play.
That dependence is the whole story of FLV. The format existed to feed the Flash Player, so it was only ever as alive as Flash itself. When Flash thrived, FLV was everywhere; when Flash was deprecated and finally discontinued at the end of 2020, FLV lost its reason to exist along with it.
Why FLV Is Obsolete
FLV is now a legacy, effectively dead format, and the cause is direct: the web moved to HTML5 video. Modern browsers play video natively through the <video> element using MP4 and WebM, with no plugin required, so the entire reason FLV existed, delivering video through Flash, disappeared. YouTube, Hulu, and essentially every major platform that once served FLV moved to HTML5 years ago. Today browsers cannot even play FLV without special software, because Flash has been removed entirely.
The practical implication is simple. If you have old FLV files, you should convert them to MP4 to keep them usable. MP4 plays everywhere, is smaller and better supported, and preserves the video. There is no reason to keep new content in FLV, and no modern workflow that produces it; FLV survives only as archived files from the Flash era.
When You Encounter FLV
You will only meet FLV in old, archived content: downloaded videos from the 2000s and early 2010s, files exported by legacy software, or recordings from systems built around Flash. In every one of these cases, the right move is to convert the FLV to MP4 so it plays on current devices and browsers. There is no scenario today in which choosing FLV for new video makes sense, so think of it purely as a format to migrate away from, not one to adopt.
Limitations
FLV's limitations are now fundamental: it depends on Flash, which no longer exists, so it will not play in any modern browser without special tools, and no current platform accepts it. Even setting that aside, it is an aging container with weaker support for modern features than MP4 or MKV. Its one historical virtue, efficient streaming inside Flash, is meaningless now that Flash is gone. For any FLV file you care about, converting to MP4 is both the fix and the future-proof choice.
