Free online FLV to MPEG4 converter

Legacy web clips and OBS recordings are often saved as FLV, but phones, TVs, editors, and upload systems commonly expect an .mp4 file. Converting the file to an MP4 container with H.264 video and AAC audio provides the widest practical playback compatibility.

FLV and MPEG-4 formats

FLV means Flash Video, a container developed for Adobe Flash-based web delivery. Video sites widely used it before browser support for Flash ended; it also remains an OBS Studio recording option because an interrupted FLV recording is less likely than an interrupted MP4 recording to leave the entire file unusable.

FLV may contain Sorenson Spark, VP6, H.264/AVC, or other video streams, with MP3 or AAC audio. Codec support varies by player, so an FLV file that opens on a desktop computer may not import into an editor or play on a television or mobile device.

MPEG-4 is an umbrella term covering multiple media standards. In conversion tools, it usually means an .mp4 file: the MPEG-4 Part 14 container. MP4 can hold video, audio, subtitles, chapters, and metadata. For general use, select H.264/AVC video and AAC audio; this combination is broadly accepted by current operating systems, hardware players, editors, and publishing services.

Do not equate the MP4 container with the older MPEG-4 Part 2 video codec, often shown as mpeg4, Xvid, or DivX. A file can have an .mp4 extension while using different codecs. Choose MPEG-4 Part 2 only when a documented legacy-device requirement calls for it.

Convert an FLV file on 101convert.com

1. Open the FLV-to-MPEG4 converter on 101convert.com and upload the .flv file.
2. Select MPEG4 or MP4 as the output format. If codec controls are available, choose H.264/AVC for video and AAC for audio.
3. Start the conversion, then download the resulting .mp4 file.
4. Play the downloaded file before deleting the source. Test it on the intended television, phone, editor, or upload service when compatibility is the reason for conversion.

Use the original frame rate and resolution unless the destination specifies a limit. Lowering resolution reduces file size, but it cannot improve source quality.

Local conversion with FFmpeg

FFmpeg provides repeatable local conversion without uploading the source file. From a terminal in the folder containing the FLV, run:

ffmpeg -i input.flv -c:v libx264 -crf 20 -preset medium -pix_fmt yuv420p -c:a aac -b:a 128k -movflags +faststart output.mp4

-crf 20 is a high-quality starting point for H.264 encoding. Use 18 for less compression and larger files, or 23 for smaller files and lower quality. -pix_fmt yuv420p avoids a common compatibility problem with older hardware decoders, and -movflags +faststart places MP4 metadata at the beginning of the file so web playback can begin before the whole file downloads.

If the FLV already contains H.264 video and AAC audio, first try remuxing instead of re-encoding:

ffmpeg -i input.flv -c copy -movflags +faststart output.mp4

This copies compatible streams without quality loss and normally completes quickly. It is unsuitable when the source uses VP6, Sorenson Spark, or another codec that MP4 players do not support; it can also produce a file that some devices reject when the source audio or stream metadata is unusual. Test the output, then use the re-encoding command if necessary.

Desktop application option

HandBrake can convert many FLV files locally through a graphical interface. Open the FLV as the source, set the container to MP4, choose H.264 video and AAC audio, choose an output filename ending in .mp4, and start the encode. HandBrake re-encodes video rather than providing a general stream-copy workflow, so FFmpeg is preferable when preserving already compatible H.264/AAC streams matters.

Quality and compatibility limits

Re-encoding is lossy: it cannot restore detail missing from a low-bitrate, low-resolution, or damaged FLV source. Keep the original FLV when it is the only master copy, and avoid repeatedly converting the resulting MP4.

Do not change frame rate unless required by the destination. Frame-rate conversion can cause duplicated frames, motion judder, or synchronization issues. If an MP4 plays audio but shows a black screen on an older device, re-encode using H.264 with yuv420p and AAC audio rather than relying on the file extension alone.