FLV to MKV Converter

FLV to MKV — Rescue a Flash Video Into an Open Container

FLV (Adobe Flash Video) is a dead format: Adobe ended support for Flash Player on December 31, 2020, and no current browser will play an FLV file. MKV (Matroska) is the opposite — an open, royalty-free container that holds almost any video and audio codec plus subtitles and chapters, and it's the format media servers like Plex and Jellyfin expect. Converting rescues your Flash-era clip into a container that desktop players and home-media libraries actually read. One thing to set expectations: the picture can't end up sharper than the original — early FLV is often low-resolution, and the conversion re-encodes that source, it never adds detail.

FLV vs MKV — Side-by-side

When to Convert FLV to MKV

• You're building a Plex, Jellyfin, or Kodi library — MKV is the de-facto container for those servers, and it can carry a subtitle track and chapters alongside the video in a single file.
• You want a long-term archive in an open format — Matroska is an open, royalty-free spec, so an MKV is a safer bet for files you want to still open in ten years than a proprietary, end-of-life FLV.
• Your FLV already holds H.264 video — MKV keeps that codec, so the re-encode stays at near-source quality while giving you a modern, open container.
• You need to bundle multiple audio tracks — MKV can hold several language or commentary tracks where FLV is limited to one.

When to Convert FLV to MP4 Instead

• You need the file to play on phones, in browsers, or on social platforms — MKV is not natively supported by most browsers, iOS, or QuickTime; MP4 plays nearly everywhere. For maximum device compatibility, convert FLV to MP4 instead.
• You're uploading to YouTube, Instagram, or TikTok — those accept MP4 cleanly and often reject or poorly transcode MKV.
• You only care about the sound — if the video is disposable, extract the audio to MP3 and skip the container question entirely.

How to Convert FLV to MKV

1. Upload Your FLV File: Drag and drop your .flv file onto the page or click "+ Add Files." You can queue several clips and convert them all in one batch.
2. Pick a Quality Preset: MKV defaults to the H.264 video codec and AAC audio. Leave the Preset under Constant Quality on "Very High (Recommended)" for near-source quality, or switch to Constant Bitrate, Variable Bitrate, or Specific file size to hit a target size.
3. Resize or Trim (Optional): Use Preset Resolutions or Resolution Percentage to scale, Width x Height for an exact box, or Trim to a Time Range to keep only the segment you want — useful since you can't upscale a low-res source anyway.
4. Convert and Download: Click "Convert" and save your MKV. No sign-up, no watermark.

What's Inside Your FLV — and What the MKV Gets

FLV was in service for about fifteen years, so "an FLV file" could hold any of several generations of codec. What it holds decides how much the picture changes, because every conversion here decodes the source and encodes fresh output — there is no stream-copy shortcut, even when the codec on both sides is the same.

Frequently Asked Questions

Should I convert FLV to MKV or MP4?

Pick MKV if the destination is a media server (Plex, Jellyfin, Kodi) or a long-term archive, or if you want subtitles and multiple audio tracks in one file — Matroska is an open, royalty-free container built for that. Pick MP4 if the file has to play in a browser, on a phone, or get uploaded to a social platform, because MKV is not natively supported by Chrome, Safari, iOS, or QuickTime. Both rescue the content out of the dead FLV container; they differ only in where the result will play.

Will I lose quality converting FLV to MKV?

It depends on what's inside the FLV. If the source already uses H.264 video, MKV keeps that codec and the re-encode stays at near-source quality — the "Very High" preset keeps any loss invisible. If the FLV uses an older codec like Sorenson Spark or On2 VP6, the video is re-encoded to H.264, a bigger lossy pass; the same "Very High" preset keeps that loss visually small. Either way the result can't look better than the source, so a low-resolution Flash clip stays low-resolution.

Why won't my FLV file play anymore?

FLV depended on Adobe Flash Player, which Adobe stopped supporting on December 31, 2020, and began blocking from running on January 12, 2021. No mainstream browser ships a Flash runtime today, so the container is effectively dead for playback. The video and audio inside are usually intact — re-encoding them into MKV (or MP4) is how you get the content playing again.

Can MKV keep the subtitles or multiple audio tracks my FLV had?

MKV can carry soft subtitles, chapters, and many audio tracks — that flexibility is one of the main reasons to choose it. But FLV itself is limited to a single audio track and has no native subtitle support, so there usually isn't a second track or subtitle stream in the source to preserve. The benefit is forward-looking: once it's an MKV, you can add external subtitle files or extra audio tracks to your media-server copy.

Does MKV play on my TV or in VLC?

MKV plays in VLC, MPV, Kodi, Plex, Jellyfin, and many modern smart TVs and media-streaming boxes, which is why it's the standard for home-media libraries. It does not play natively in most web browsers, on iPhones, or in Apple QuickTime. In our testing, an MKV produced here with the default H.264 + AAC codecs opened cleanly in VLC and played through a Plex server; if you need it on an iPhone or in Safari, convert to MP4 instead.

Can you just remux the FLV into MKV without re-encoding?

No. Matroska could technically hold the FLV's existing stream, but this page has no stream-copy path — the video is always decoded and re-encoded to the codec you choose, so even an FLV that already carries H.264 comes out with a fresh H.264 encode. In practice that costs one generation of loss, which the "Very High (Recommended)" preset keeps hard to see, and produces a file that is usually a little larger than a straight remux would have been. The upside is a predictable, standards-clean MKV that media servers will direct-play rather than trip over.

Which video codec should I leave selected for a media server?

H.264, the default. Every Plex, Jellyfin, and Kodi client and effectively every direct-play smart TV decodes it in hardware, so the server hands the file over untouched instead of burning CPU transcoding it. The page also offers H.265/HEVC, AV1, VP9, VP8, MPEG-2, MPEG-4, Xvid, Theora and several legacy codecs inside the MKV — H.265 will make a smaller file but forces older clients to transcode, and the rest mostly narrow where the file will play. Given that a Flash-era source is low resolution to begin with, the space you would save with a newer codec is small; the compatibility you would lose is not.

Is the upload private?

Yes. Your file is uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — no sign-up, no watermark, never shared or made public. The main practical limit on a very large FLV is upload time, not your device.