How to Convert FLV to MP4 on Windows
Flash Video is a retired web format. The files still exist — old tutorials, screen recordings, downloaded clips, game captures — but modern browsers, phones and editors do not want them. Converting to MP4 is the practical way to keep the content usable.
Fastest Windows method: add the FLV files to Raize All-in-One Video Converter, choose MP4, and convert. The MP4-only product does not accept FLV input. Free method: HandBrake, VLC or FFmpeg can open many FLV files; a remux may work if the video is already H.264.
What FLV is
FLV stands for Flash Video. It was the container used by Adobe Flash Player for web video. Adobe ended Flash Player support on 31 December 2020, and browsers stopped running Flash content in January 2021. The format is not coming back for new web video.
FLV files can contain different codecs. Newer FLV/F4V files often use H.264 video and AAC audio. Older files may use VP6 or Sorenson Spark video and MP3 audio. That matters: an H.264 FLV may be remuxed to MP4 without re-encoding, while a VP6/Sorenson file must be re-encoded.
FLV should not be confused with SWF. SWF is an animation and interactive format; FLV is a video container. This guide is about FLV video files.
Before you start
- Collect the files. Copy the FLV files to a folder on your PC. Keep the originals until the MP4 outputs are verified.
- Identify the codec. Use MediaInfo or VLC’s codec information to see whether the video is H.264, VP6 or Sorenson Spark. It determines whether a remux is possible.
- Check the resolution. Old FLV files are often 320×240 or 640×360. Do not upscale them to 1080p; it will not add detail.
- Test one file. Check audio sync and aspect ratio before converting a folder.
Method 1: Raize All-in-One Video Converter (Windows)
- Add the FLV files. Drag them into the All-in-One window, or use the Add button. Add multiple files for a batch.
- Choose MP4. Select MP4 output. Keep the source resolution unless you have a reason to change it.
- Choose the output folder. Keep the converted files separate from the originals.
- Convert and check. Play one output in VLC to confirm audio and aspect ratio. Then convert the rest.
The MP4 product does not list FLV as a supported input. The All-in-One accepts FLV, which is why the retired Flash converter is no longer needed as a separate product.
Method 2: HandBrake, VLC or FFmpeg (free)
HandBrake can open many FLV files. Choose MP4, H.264 and AAC audio. If the file uses VP6 or Sorenson Spark, HandBrake may or may not decode it depending on the build; VLC is often more forgiving for playback, and FFmpeg is the most flexible for conversion.
If the FLV video is already H.264, try a remux first:
ffmpeg -i input.flv -c copy output.mp4If the video is VP6 or Sorenson Spark, re-encode:
ffmpeg -i input.flv -c:v libx264 -crf 20 -preset medium -c:a aac -b:a 192k output.mp4Test the remux output in VLC. If it plays correctly, it is faster and avoids generation loss. If it does not, use the re-encode command.
Quality and codecs
- H.264 FLV: try a remux first. It can be fast and lossless if the stream is compatible.
- VP6 or Sorenson Spark FLV: re-encode. The old codec is not supported by most modern players, so a conversion is unavoidable.
- Keep the source resolution. A 320×240 clip should stay 320×240. Upscaling adds file size, not detail.
- Use AAC audio. MP3 audio in FLV can be converted to AAC for better compatibility. 128–192 kbps is usually enough for old web video.
- Check the frame rate. Old screen recordings may have variable frame timing; use a constant frame rate if the output stutters.
Troubleshooting
The file will not open
Some FLV files are truncated or use unusual codecs. Try VLC for playback first. If VLC cannot read it, the file may be incomplete or corrupted.
No audio after conversion
FLV audio can be MP3 or AAC. Choose an AAC output and test in VLC. A truncated file may have no usable audio stream.
The output stutters
Old web video may have variable frame timing. Re-encode with a constant frame rate, or use HandBrake’s constant-framerate option.
The output looks soft
The source is probably low-resolution. Do not upscale; keep the original size and let the player scale it. If the source is already low quality, conversion cannot recover detail.
Can I convert SWF?
SWF is an animation/interactive format, not a video container. Some SWF files embed video, but the workflow is different. This guide is about FLV video files.
This guide is based on the current version 3.10 product documentation and the format specifications above. We update it when the product or format support changes. Free alternatives are listed for comparison.
FAQ
Is Flash Video dead?
Adobe ended Flash Player support on 31 December 2020 and began blocking Flash content in January 2021. Browsers no longer run Flash. FLV files still exist as old recordings, but new web video uses MP4, WebM or streaming formats.
Can a browser play FLV directly?
Modern browsers do not play FLV natively. Some players, such as VLC, can open FLV files. For phones, editors and cloud services, convert to MP4.
Does converting FLV to MP4 lose quality?
If the FLV uses H.264, a remux can sometimes avoid re-encoding. If it uses VP6 or Sorenson Spark, the video must be re-encoded. A converter that re-encodes will not be lossless.
Can I convert SWF files?
SWF is an animation and interactive format, not a simple video container. This guide is about FLV video files. SWF may contain animation or embedded video and needs a different workflow.
Why is there no audio after conversion?
Some FLV files use MP3 audio, others AAC. Choose an AAC output and test the file in VLC. A corrupted or truncated FLV may have no usable audio stream.
Can I batch convert a folder of FLV files?
Yes. Add the FLV files to the queue, choose one MP4 profile, and convert the batch.
Related guides
- 3GP to MP4 — another obsolete format worth rescuing.
- AVCHD to MP4 — camcorder footage rather than web video.
- Raize All-in-One Video Converter — the Windows tool used above.
Rescue old Flash videos without uploading them.
Try one FLV file, check the audio, then batch the rest. The All-in-One also handles MKV, AVCHD and audio extraction.