FLV vs MP4: How to Rescue Your Old Flash Videos

This one started with a comment on our Windows Movie Maker reviews page: 'I just installed Windows Movie Maker, but I can't see FLV extension movies. What can I do?'

FLV and MP4 file icons side by side, the FLV one greyed out

You have a folder of old FLV files - screen recordings, downloaded clips, things from when that was the only option.

You double-click one and nothing happens.

Here is what happened to FLV, and how to get your videos out of it before the tools that still open them disappear too.

What Is Actually Inside an FLV File

FLV is a container - a box that holds a video stream and an audio stream together. The box is not the problem. What matters is what is inside it.

Most FLV files hold one of two video codecs: Sorenson Spark, an early relative of H.263, or On2 VP6. The audio is usually MP3. Later files, from roughly 2008 onwards, often hold H.264 video and AAC audio - the same codecs used in MP4 today.

That difference decides everything about how you convert the file, so it is worth checking before you start. Our Codec Finder reads the file in your browser and tells you which codec is inside.

Why it matters.

If the FLV holds H.264 and AAC, the video can be moved into an MP4 without being re-encoded at all - nothing is lost and it takes seconds. If it holds VP6 or Sorenson, it has to be re-encoded, and re-encoding always costs a little quality.

Why FLV Died

FLV did not fall out of fashion. It was retired, on a schedule everyone could see coming.

How Flash video ended.

In 2010 Apple refused to put Flash on the iPhone and iPad. In 2015 YouTube switched its default player to HTML5, taking the largest FLV audience on earth with it. Adobe announced the end in 2017, and Flash Player was discontinued in December 2020.

Browsers do not merely ignore Flash now. They block it, so installing Flash Player today would not help even if you could find it.

The reasons were not only commercial. Flash had a long run of security holes that made it a favourite way into people's machines, and it drained batteries on every laptop it touched.

FLV Against the Formats That Replaced It

	FLV	MP4	WebM
Plays in a browser	No, since Flash ended	Everywhere	Everywhere
Plays on a phone	Only in a third-party app	Built in	Built in
Typical codecs inside	VP6 and Sorenson, both two decades old	H.264, HEVC, AV1	VP9, AV1
Made by current tools	No - nothing writes FLV any more	Yes	Yes
File size at the same quality	Largest	Smaller	Smallest

A 720p clip that takes 50MB as VP6 in an FLV would land nearer 30MB as H.264 in an MP4, and lower again with HEVC or AV1. Those are illustrations rather than measurements - the real numbers depend entirely on the footage.

FLV file shown beside the MP4 it converts to, with file sizes compared

Play an FLV Right Now, Without Flash

If you only need to watch the file, do not convert anything. Three routes, in order of effort.

VLC Media Player - drag the file in and it plays. Nothing to install beyond VLC itself.
K-Lite Codec Pack - teaches every player on the PC to open FLV, which is the better answer if you have a lot of old formats around.
Flash Video Splitter - a single small filter for FLV, if you would rather not install a full pack.

Convert It Properly

Playing the file is a reprieve. Converting it is the fix, and which route you take depends on what the Codec Finder told you.

For a handful of small files, the ConvertICO FLV to MP4 converter runs in the browser with nothing to install, which also makes it the only option on a Chromebook or tablet.

For anything bigger, or a folder of them, use a desktop tool. There is no upload wait, and you keep control of the quality.

The Easy Route: HandBrake

Install HandBrake. Free, open source, and it opens FLV without any extra setup.
Drag the FLV into the window. HandBrake reads it and fills in sensible defaults.
Set Format to MP4. The default H.264 preset is the right choice for compatibility.
Click Start Encode. The MP4 appears in the output folder when it finishes.

HandBrake re-encodes, so there is a small quality cost. For VP6 and Sorenson files that is unavoidable - no modern container will take those streams as they are.

The Lossless Route: FFmpeg

If the file holds H.264 and AAC, FFmpeg can move the streams into an MP4 without touching them:

ffmpeg -i input.flv -c copy output.mp4

-c copy is the whole trick: the video and audio are copied across unchanged, so nothing is re-encoded and the job takes seconds rather than minutes.

If that command fails, the file holds VP6 or Sorenson and has to be re-encoded:

ffmpeg -i input.flv -c:v libx264 -crf 20 -c:a aac -b:a 192k output.mp4

The -crf 20 part sets the quality. Lower numbers mean better quality and bigger files, and most people settle somewhere between 18 and 22.

Converting an FLV file to MP4, with the AV1 and H.264 output options shown

Is There Any Reason to Keep the FLV Files?

Two, and neither applies to most people.

Archiving. If you are preserving files exactly as they were distributed, keep the original alongside the MP4. Archivists generally keep the source format whatever else they make from it.

Old Flash projects. Some interactive .swf files load FLV files by name. If you are keeping such a thing alive in an isolated environment, those FLVs have to stay as they are.

What to Convert To

MP4 with H.264 - the safe answer. It plays on phones, browsers, TVs and consoles without asking anything of anyone.
MKV with HEVC - up to about half the size at the same quality, which adds up across a large library, with slightly less universal support.
WebM with VP9 or AV1 - the best choice for the web, royalty free, and AV1 compresses better than anything else in common use.

Quick questions

Will converting FLV to MP4 lose quality?

Not if the FLV holds H.264 and AAC - those streams are copied across untouched. If it holds VP6 or Sorenson, it has to be re-encoded, which costs a little, though at a sensible quality setting you are unlikely to see it.

How do I know which codec my FLV uses?

Open it in the Codec Finder, or right-click it in VLC and look at Media Information. Files from 2008 onwards are often H.264; older ones are usually VP6.

Can I still install Flash Player to watch them?

No, and you should not try. Flash Player was discontinued in December 2020 and modern browsers block it outright. Anything offering it today is not something you want on your PC.

Why does Windows Movie Maker not see my FLV files?

It never supported FLV, and it has not been updated in over a decade. Convert the files to MP4 first and it will open them, or use a current editor instead.

Can I convert a whole folder at once?

Yes. HandBrake has a queue for batches, and FFmpeg handles it in a one-line loop. Online converters are the wrong tool once you get past a handful of files.

The bottom line

FLV ended with Flash in December 2020 and nothing is bringing it back, but the videos inside those files are fine. Convert them once, keep the originals if they matter, and stop saving anything new as FLV.