Convert FLV to MP4
FLV files are Adobe's Flash Video format - what YouTube ran on from 2005 until about 2015, and what a lot of educational-video libraries, corporate training platforms, and personal downloads from the Flash era were saved as. Flash itself has been dead since 2020, but the archives of FLV files are enormous. Modern browsers, phones, and TVs all refuse to play them. Wave converts your FLV to MP4 in your browser - the picture data usually comes through untouched because most FLVs from 2010 onward already contain H.264, the same video format MP4 uses. Older FLVs (pre-2008 Sorenson or VP6 video) need a light re-encode.
What actually changes
Flash Video files come in two eras: pre-2008 FLVs use Sorenson Spark or VP6 video with MP3 or Speex audio, and post-2008 FLVs typically use H.264 with AAC. Wave inspects the streams and picks its path automatically.
When the source is H.264 + AAC (most common for FLVs from 2010 onward), the conversion is a re-mux: streams copied bit-for-bit into MP4, with a fast-start rewrite. When it's a pre-2008 codec, the video re-encodes to MPEG-4 Part 2 and audio to AAC. Both paths run locally in the browser.
Adobe released Flash Video in 2003 as the video format for the Flash Player, which by 2010 was installed on more than 99% of desktop browsers. YouTube ran on FLV from its 2005 launch through 2015; every YouTube video from that decade was originally uploaded as (or transcoded to) FLV before serving. When Apple refused Flash on iOS in 2010, the industry started migrating - first to H.264 in MP4 for HTML5 video, and by 2020 Flash Player had been retired entirely.
What survives is an archive: educational-video libraries, corporate training platforms, and personal downloads from the Flash era all sit in FLV files that no modern browser can play. The 2008-onward FLVs (H.264 inside) are the easy case - Wave's remux turns them into an MP4 in seconds. The older ones need a real decode of Sorenson Spark or VP6, both of which ffmpeg-wasm supports natively; the resulting MP4 plays anywhere.
Three steps.
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Step 1
Drop the FLV onto the card.
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Step 2
Wave shows either Fast copy (H.264 source) or Re-encode needed (older codec).
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Step 3
Convert and save the MP4.
Answers, specifically.
Why bother converting FLV?
Because Flash is gone. No modern browser plays FLV, no phone plays FLV, no video editor imports FLV cleanly. MP4 is the universal replacement.
Will the fast-copy version be identical to the FLV?
Yes. If the FLV's streams are already MP4-compatible, the streams are copied without touching any pixels or samples.
Some of my FLVs are very old - will they still convert?
Usually. Wave supports Sorenson Spark and VP6 through re-encode. Some very old FLVs with corrupted headers may fail during inspect - try re-downloading if you can.
How big can the FLV be?
About 1.5 GB in-browser. FLVs from the era rarely approach this ceiling.
Does the aspect ratio survive?
Yes. Wave preserves the source's stored aspect ratio in the MP4 metadata, including anamorphic cases.
What about F4V files - are those the same as FLV?
No. F4V is Adobe's later H.264 container based directly on ISO Base Media File Format (essentially an MP4 with a different extension). See our <a href="/f4v-to-mp4/">F4V-to-MP4 page</a> for that path - it is a header rewrite rather than the FLV story.
Are old YouTube downloads from 2010 usually FLV?
Yes. Downloader tools of that era saved YouTube's FLV streams directly - format code 5 (240p Sorenson Spark), format 34 (360p H.264), format 35 (480p H.264). All of them convert fine here; the older ones re-encode from Sorenson, the newer ones remux from H.264.
Will the MP4 play in the browser?
Yes. H.264 in MP4 is the exact combination the HTML5 video element handles natively in every current browser. The FLV never did play in a browser without the Flash Player plugin; the MP4 does not need any plugin.