FLV to MKV Converter

Move old Flash video into an open container that holds anything. Convert FLV to MKV free in your browser, with no signup and no watermark.

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FLV to MKV Converter Features

EchoWave is used by creators, marketers, educators, and businesses to convert and edit media online.

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Updated June 2026

MKV is an open container that holds almost any codec, plus several audio tracks and subtitles in one file, which makes it a strong home for archiving old Flash video. Because many FLV files already carry H.264 video, the converter can often repack the streams into MKV without re-encoding, so the picture stays identical. It runs in your browser with no signup.

Free, no sign-upNo account, no trial
Runs in your browserNothing to install
Works on every devicePhone, tablet, laptop
No watermarkClean output, every time

How it works

How to convert FLV to MKV

  1. 1

    Upload your FLV

    Drag your FLV onto the converter or pick it from your device. Nothing to install.

  2. 2

    Choose MKV and convert

    Select MKV as the output and start. Free, with no sign-up and no watermark.

  3. 3

    Download your MKV

    Save the finished MKV, or open it in the editor to trim, crop, or resize it first.

Try it

Estimate your MKV

Pick a video bitrate and a length to see the MKV file size before you convert.

Video bitrate
5 min
1 min30 min

Drag to set the clip length in minutes, from 1 to 30.

Estimated MKV size183MBSharp 1080p

An estimate. Real size varies with resolution, frame rate, and how much the picture moves.

The formats

FLV vs MKV

FLV

Flash Video

Legacy Flash video from the pre-HTML5 web

Type
Flash video container
Holds
H.263, VP6, or H.264 video with MP3 or AAC audio
Typical size
Small, made for streaming
Plays on
VLC, since browsers dropped Flash in 2020

Best for:Recovering old Flash web video

MKV

Matroska Video

An open container that holds almost anything

Type
Open video container
Holds
Almost any codec, plus several audio and subtitle tracks
Typical size
Large, depends on the codec inside
Plays on
VLC and most modern players, less on default phone apps

Best for:Movies with multiple audio tracks and subtitles

Will it lose quality?

  • When the codecs already match MKV, the converter can change the container without re-encoding, so the picture stays identical.
  • When the video has to be re-encoded for MKV, a high bitrate keeps it visually close to the FLV.
  • Converting cannot add detail the FLV never had. The output is at best as good as the source.

Use cases

What people use it for

Play anywhereMake a clip your device supports
Video editorsGet footage your software accepts
Web videoUse a format browsers play
CourseworkHand in a standard video file
SharingSend a clip people can open

Troubleshooting

If something looks off

The MKV will not play
Some players are picky about codecs. Try it in VLC, which opens almost anything, or convert again and let the converter re-encode the video.
The file got larger
A format change can shift the size, and re-encoding at a high bitrate adds overhead. Lower the bitrate or trim the clip for a smaller file.
Audio and video drift out of sync
Re-download the source and convert again. If a long recording drifts, splitting it into shorter pieces usually fixes the sync.

FAQ

FLV to MKV questions

Why convert FLV to MKV?
MKV is an open, flexible container that can store almost any codec along with multiple audio tracks and subtitles in a single file. That makes it well suited to archiving old Flash video, where you want one tidy file that VLC and most modern players open.
Is the FLV to MKV converter free?
Yes. EchoWave converts FLV to MKV with no signup, no file-count limit, no trial, and no watermark. Upload your FLV, convert it, and download the MKV from your browser.
How do I convert FLV to MKV?
Upload your FLV, choose MKV as the output format, and start. When it finishes, download the MKV or open it in the editor to trim or adjust it first.
Will converting FLV to MKV lose quality?
Often not. When the FLV already uses an MKV-friendly codec like H.264, the streams can be repacked into the new container with no re-encoding, so the quality is identical. If a re-encode is needed, a high bitrate keeps the result close to the source.
Does MKV really support multiple audio tracks and subtitles?
Yes, that is one of its main strengths. MKV can hold several audio and subtitle tracks at once, which is useful for multilingual playback. A single FLV usually carries just one audio track, but the MKV container is ready if you add more later in the editor.
After converting FLV to MKV, what plays the file?
VLC plays MKV everywhere, and most modern media players support it. Some default phone apps and TVs are pickier, so if MKV will not open on a device, convert to MP4 instead for the widest compatibility.
Does the FLV to MKV converter work on Mac, Windows, iPhone, and Android?
Yes. The converter runs in any browser on macOS, Windows, Chromebook, Linux, iPhone, iPad, and Android with nothing to install. To watch the MKV, VLC is the simplest free player on any of them.
Can I convert large FLV files or a whole batch?
Yes. There are no file-count limits on these pages, so you can convert your Flash archive one file after another. Large videos are supported and simply take a little longer to upload and convert.

What people say after converting with EchoWave

Convert FLV to MKV for free Free online converter

Archive old Flash video in an open MKV container that holds multiple tracks. No signup, no file limits, and no watermark.

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