Convert FLV to MP4
Turn Flash-era FLV files into MP4s that iPhones, Android phones, TVs and editors play, without uploading them.
- Nothing is uploaded
- Works offline after first visit
- Open source
- No watermark · limit is your device’s memory; phones hold less
Converts FLV videos that hold H.264 picture, with AAC or MP3 sound, to MP4. The picture is copied into the MP4 without re-encoding, so it is fast and loses no quality. It is free and needs no account, and your videos are not uploaded: everything runs in your browser on your device, and after one visit and a short wait the page works offline.
Done
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Open the network tab. Your files never appear in it.
Add files and run the tool to see every request this tab makes.
Your files are processed on this device and never sent to a server. Reload the page to clear everything from memory.
FLV is the Flash Video container. Sites that streamed video in the Flash era saved clips as FLV, screen and stream recorders still write it, and a lot of old downloads are sitting in folders as .flv files that nothing on a phone or a modern computer will open.
Most FLV files from 2008 onward hold H.264 video, the same picture format an MP4 uses. This page reads the FLV in your browser and copies that picture into a new MP4 as it is, with the sound alongside it. Nothing in the picture is re-encoded, so a long recording converts in seconds and looks exactly like the original.
The file never leaves your device. Recordings and downloads are exactly what you should not hand to a stranger’s server just to change the wrapper, and here there is no upload to wait for and no copy kept anywhere.
Which FLV files this converts
- Picture: H.264, which is what Flash Player 9 and later, YouTube’s old FLV files, OBS and most stream recorders write.
- Sound: AAC is copied as it is. MP3 is converted to AAC, and low-quality MP3 is brought up to 48 kHz so every player accepts it.
- Not converted: older FLV files whose picture is Sorenson Spark or VP6 (the codecs of Flash Player 6 to 8), or whose sound is Nellymoser or Speex. The page names the codec it found, so you know what you have.
What this page loads
Most video runs on your browser's WebCodecs through Mediabunny. For old formats the browser cannot read, the page loads a decoder built from FFmpeg's libraries (LGPL) only when you convert such a file.
For FLV that module is small, well under 1 MB, and comes from stayput.dev, not from anyone else’s server. It only reads the container: the picture is not decoded, and the licenses page links its exact source and how it was built.
How it works
- Drop one or more FLV files, or tap to pick them.
- Tick Remove the sound if you only need the picture.
- Convert, then open or share the MP4. Several files download together as a zip.
In your browser, not on a server
Mediabunny (MPL-2.0), a media toolkit written in TypeScript, reads the MOV, MKV, WebM or MP4 container in your tab. H.264 video and AAC or MP3 sound are copied into the new MP4 packet by packet, which takes seconds and changes nothing in the picture. Other video (VP8, VP9, AV1) is decoded and re-encoded by your browser’s own video encoder through WebCodecs, H.264 where available, and other sound becomes AAC or Opus. An AVI, which no browser opens, is decoded in a background worker by a separate decoder module and re-encoded the same way. An FLV is opened by a small separate reader module, and its H.264 picture is copied into the MP4 as it is. Most video runs on your browser's WebCodecs through Mediabunny. For old formats the browser cannot read, the page loads a decoder built from FFmpeg's libraries (LGPL) only when you convert such a file. The MP4 is written in memory.
Compared with upload-based tools: Online converters upload the whole video just to rewrap it, which can take longer than the conversion itself, cap free files at 100 MB to 1 GB, and keep a copy on their server. Here the file never leaves your device, and a large MOV from your phone converts in seconds because nothing is re-encoded that does not need to be.
Verify it yourself. Give the page a short while after one visit to save itself for offline use, then turn off Wi-Fi or switch to airplane mode and run the tool again. Or open your browser's developer tools (F12, or Cmd-Option-I on a Mac), pick the Network tab, and watch it while you convert. The panel above does the same count for you.
Questions
Does converting FLV to MP4 lose quality?
Not for the picture. The H.264 video in the FLV is copied into the MP4 as it is, so nothing is re-encoded. AAC sound is copied too. MP3 sound is converted to AAC, which is the one step that touches the audio.
Why does it say the video uses VP6 or Sorenson Spark?
FLV is only a wrapper. Files from the early Flash years hold Sorenson Spark or On2 VP6 picture, which this page does not open, and it tells you which one it found instead of failing without a reason. Files made since about 2008 are nearly always H.264.
Why does the message mention the sound?
The sound inside the FLV is in a format this page does not read, such as Nellymoser or Speex. Tick Remove the sound to convert the picture alone.
Will the MP4 play on my iPhone and TV?
Yes. It holds H.264 picture and AAC sound, which iPhones, Android phones, Windows, macOS and current smart TVs play. The line under the result says the picture was copied.
Is the FLV uploaded anywhere?
No. The file is read from your disk in slices by code running in a background worker in your tab, and the MP4 is written in memory. The reader module downloads from stayput.dev when you first convert, and it carries none of your data.
Can I convert a whole folder of FLV files?
Select them all at once and they are converted one after another; several download together as a zip. Each MP4 keeps the name of its FLV with the new extension.
Is the file uploaded anywhere?
No. There is no server in this process. The page is a static file and the work is done by code running inside your browser tab. Give the page a short while after one visit to save itself for offline use; then you can turn off Wi-Fi and it still works.
More video and audio tools
Video to MP4
Turn MOV, MKV and WebM files into MP4s that play everywhere. Converted on this device, never uploaded.
Video to MP3
Pull the sound out of a video, or turn any audio file into an MP3. Your files are converted on this device and never uploaded.
Video to GIF
Trim a clip, pick a size and frame rate, and get a looping GIF. The video is decoded by your browser and never leaves this device.
GIF to MP4
Turn animated GIFs into small MP4 videos that post and play anywhere. Converted on this device, never uploaded.
Other conversions
Guides
- How to convert MOV to MP4MOV plays fine on Apple devices but not everywhere else. Here is how to convert it on a Mac and on Windows.