FLV to MKV Converter

What happens when you convert FLV to MKV

• If the video inside the FLV is already H.264, the tool first copies the streams straight into the MKV. That takes seconds and loses nothing.
• If a copy is not possible, it re-encodes to H.264 video with AAC audio, which is slow in a browser: expect several times the video's length for HD footage, less at lower resolutions.

How to convert FLV to MKV online

1. Press "Choose FLV files" and select one or more files, or drag them onto the box. You can add files of any size your device can hold in memory.
2. Adjust the settings if you need to: resolution and quality control the size of the result. The defaults suit most people.
3. Press "Convert to MKV". The first audio or video conversion downloads the converter engine, about 32 MB, which your browser then keeps for next time.
4. Save each MKV with its download button. Close the tab and every trace of the files is gone from memory.

Why convert FLV to MKV?

FLV files usually come from old YouTube downloads and Flash-era video sites. People convert them to MKV because MKV keeps several audio and subtitle tracks in one file and is the favourite container of media servers such as Plex and Jellyfin.

In short, FLV is good at what it was made for, and MKV is the more practical choice when the file has to play, upload or edit somewhere else. FLV does not play in browsers since Flash ended in 2020; plays in VLC, while Chrome and Edge play many MKV files; Safari and iPhones do not.

How to convert FLV to MKV on Windows, Mac, iPhone and Android

On Windows

Open this page in Chrome, Edge or Firefox, drag your FLV files in from File Explorer, and press Convert. Each MKV is saved to your Downloads folder. If you prefer a desktop program, VLC can also convert FLV to MKV: choose Media, then Convert / Save.

On a Mac

Safari, Chrome and Firefox all work. Drag the FLV files from Finder onto the drop zone and the MKV files land in Downloads.

On iPhone and iPad

Open the page in Safari and tap Choose FLV files. You can pick from Photos or from the Files app. The converted MKV appears in Files, in the Downloads folder; to put it in your photo library, open it there, tap Share and choose Save Video.

On Android

Open the page in Chrome, tap Choose FLV files and pick them from Files or your gallery. The MKV is saved to the Download folder, where any file manager can find it. Keep the screen on while a long video converts, because Android may pause a tab in the background.

Best settings for FLV to MKV

Leave Resolution on Keep original and Quality on Standard if the video only needs to change container: that is what lets the tool copy the H.264 stream straight into the MKV, which is fast and loses nothing.

If the file is too big, change either setting and the video is re-encoded with H.264. Quality uses the CRF scale: High is 20, Standard 23 and Smaller file 28. As a rule of thumb each step of about 6 on that scale halves or doubles the size. Dropping from 1080p to 720p usually saves more than lowering the quality, and it also makes the conversion much faster.

FLV vs MKV: the differences

About FLV and MKV files

What is an FLV file?

FLV powered YouTube and most web video until about 2010. Since browsers dropped Flash at the end of 2020, FLV files need converting or a desktop player.

What is an MKV file?

MKV can hold unlimited audio tracks, subtitle tracks and chapters in one file, which is why film rips use it. OBS offers MKV recording because a crash part way through does not corrupt the whole file.

FLV to MKV not working? Common problems

• The conversion is slow. Audio and container changes take seconds, but re-encoding video runs on your own processor. Lower the resolution to 720p, close other tabs, and keep the device plugged in.
• The tab stops or reloads on a very large file. Browsers limit how much memory one tab can use, around 2 GB and less on phones. Split long recordings or convert them on a computer.
• The MKV will not play on an old device. Try the other formats offered for FLV, or choose 720p, which older players handle more reliably.

Convert FLV to MKV with ffmpeg

This converter runs ffmpeg inside your browser. If you would rather use the ffmpeg command line on your own computer, this command produces an equivalent MKV with the same kind of settings:

ffmpeg -i input.flv -c:v libx264 -crf 23 -c:a aac -b:a 128k output.mkv

Replace input.flv with the name of your file. For a whole folder of FLV files, run the command in a loop, or drop them all on the converter above, which processes them one after another.