Convert · runs on your device

Convert FLV to MP4 in your browser

Flash Player reached end of life on 31 December 2020, so FLV files need converting to stay watchable — and where the FLV already holds H.264, Video Mender rewraps it to MP4 in seconds with no quality loss.

Nothing is uploaded

Drop video files here

FLV

Choose video files to convert

Output format

Plays on effectively every browser, phone, TV and editor when it holds H.264 and AAC.

Video codec

The universal baseline. Hardware decoding exists on essentially every device sold since 2010.

How to size the output

A constant quality target. File sizes vary with how demanding the footage is.

Quality — 8 of 10

Resolution limit

Files smaller than the limit are left alone. Capping usually looks better than lowering the bitrate.

Frame rate

Audio

Fast rewrapCopy streams without re-encoding when the codecs already fit

Lossless and near-instant when it applies. It cannot reduce file size, so turn it off if you need a smaller file.

FLV was built for Flash Player, and Flash was switched off at the end of 2020. Nothing current plays FLV natively. If you have an archive of Flash-era video, converting it is not an optimisation but the difference between keeping it and losing access to it.

What happens next depends on the file’s age. Later FLV files — roughly 2008 onwards — hold H.264 video with AAC or MP3 audio, all of which MP4 accepts, so those rewrap in seconds with the frames copied untouched. Earlier files hold Sorenson Spark or VP6, which MP4 cannot carry and this build can only decode, so those are re-encoded to H.264.

Video Mender inspects each file and shows which path it will take before you commit. Given how much Flash-era video is irreplaceable, it also validates every output and refuses to report success on a file it cannot verify.

What this actually does to your file

Rewrap or re-encode

Rewrap runs when the FLV holds H.264 with AAC or MP3 audio. FLV1 and VP6 always re-encode, because MP4 cannot carry either.

Quality

H.264 FLV files rewrap losslessly. Sorenson Spark and VP6 sources are re-encoded, which is lossy — but those codecs were so inefficient that H.264 reproduces them closely at a fraction of the bitrate.

FLV files that were streamed rather than downloaded are often truncated or missing their metadata; those may convert only partially.
Flash interactivity, cue points and ActionScript have no equivalent in MP4 and are not preserved.
Very early FLV files may be 320×240 or smaller. Conversion preserves that resolution rather than upscaling it.

How to use this page

Add your FLV files, including whole folders from an old archive.
Check each badge: "Fast rewrap" for H.264 files, "Re-encode" for Sorenson or VP6.
Keep MP4 as the output and press Start.
Download the MP4 files and keep them alongside the originals until you have checked them.

Where the result plays

Checked against the containers and codecs this page produces, stated conservatively.

Target	Works	Detail
Browsers	Yes	No browser has a Flash runtime any more. MP4 plays inline everywhere.
Phones	Yes	MP4 opens in the gallery; FLV never worked on phones.
Video editors	Yes	MP4 imports cleanly. FLV import support has largely been removed.
Archival storage	Yes	MP4 with H.264 is a far safer long-term format than a dead plugin format.

The formats involved

Format	Read	Write	What it is
FLV	Yes	No	Flash Video. Flash Player reached end of life on 31 December 2020, so FLV files now need converting simply to stay watchable.
MP4	Yes	Yes	The MPEG-4 Part 14 container. It is a wrapper, not a codec — an MP4 usually holds H.264 or HEVC video with AAC audio.
MKV	Yes	Yes	Matroska. The most permissive common container: it will carry almost any codec, plus multiple audio tracks, chapters and subtitles.
WebM	Yes	Yes	A deliberately restricted Matroska profile for the web: VP8/VP9/AV1 video with Opus or Vorbis audio, and nothing else.

Tested limits

Real ceilings measured on real devices, not a marketing claim of unlimited.

Limit	Desktop	iPhone / iPad	Why
File size — rewrap	1.3 GiB	384 MiB	Rewrapping never decodes a frame, so it tolerates far larger files.
File size — re-encode	1.0 GiB	256 MiB	Decoding holds uncompressed frames in memory, which is the real constraint.
Clip length — re-encode	4:00:00	30:00	Beyond this, progress reporting and memory behaviour stop being dependable.
Resolution — re-encode	4096 × 2160	1920 × 1080	A 4K frame is about 12 MB decoded, before the encoder allocates anything.

If something goes wrong

The FLV converted but stops early.

The source is truncated, which is very common for files captured from a stream. Video Mender keeps the valid portion and reports the truncation rather than pretending the whole file converted.

The duration is reported as unknown.

FLV stores duration in an optional metadata tag that streamed files often lack. The conversion still works; the duration is measured from the output afterwards instead.

The re-encoded video looks soft.

Sorenson Spark and VP6 discarded a great deal of detail at the bitrates Flash used. H.264 reproduces what is there faithfully but cannot recover what was never encoded.

Your video never leaves this device

This is a structural property of the application, not a policy we ask you to take on trust. There is no upload endpoint, no API route and no server action anywhere in the codebase, so there is nowhere for a video to be sent even by mistake.

Conversion runs in WebAssembly inside this browser tab. The engine is served from this domain and loaded only when you start a job.
The Content-Security-Policy restricts network connections to this origin, so the page is not permitted to contact a third party at all.
Nothing is stored. Close the tab and every queued file and finished output is gone — which is also why you should download results before you leave.
There are no accounts and no AI. Anonymous page views are counted — which page, roughly what country, what kind of device — and nothing whatsoever about your files.

The repository contains a test that fails the build if any media byte is passed to fetch, XMLHttpRequest, sendBeacon or a WebSocket. See the privacy page for how that is verified.

Questions

Why do I need to convert FLV at all?

Because Flash Player reached end of life on 31 December 2020 and was removed from every browser. FLV has no native playback path left, so converting is how the video stays accessible.

Will the conversion be lossless?

For later FLV files holding H.264, yes — the stream is copied into MP4 unchanged. Older Sorenson Spark and VP6 files must be re-encoded, which is lossy.

Can I convert a whole archive at once?

Yes, and that is the usual case here. Queue the folder, let the memory-aware scheduler work through it, and download results individually or as a ZIP.

Do you keep a copy of my videos?

No. There is no server involved in the processing. The conversion runs in WebAssembly in this tab and no media bytes are sent anywhere.

Is MP4 a safe format to archive in?

It is the safest widely available choice today: an open ISO standard, decoded by hardware everywhere, and unlikely to become unplayable the way a proprietary plugin format did.