Will quality drop when I convert my FLV file to MP4?

The answer depends on the video codec inside the file. Late-era FLV files usually carry H.264 video and AAC audio; in that case the conversion is only a container change, the picture data is copied across as it is and there is no quality loss. Older FLV files, however, contain codecs that MP4 cannot carry, such as Sorenson Spark or VP6; those require the picture to be re-encoded and a certain amount of quality loss is unavoidable. Our tool detects which situation you are in by itself when it opens the file.

How long does the conversion take?

The time depends on two things: the size of the file and whether re-encoding is needed. If there is H.264 inside, the operation amounts to nothing more than copying data and usually finishes very fast. With an older file containing VP6 or Sorenson, the picture has to be compressed from scratch; on long, large files this can take minutes. You need to wait without closing the page.

Can I just rename my FLV file's extension to .mp4 by hand?

No, that method does not work for FLV. Changing the extension does not change the structure inside the file, and FLV's internal layout is completely different from MP4's. With related formats such as MOV and MP4 you sometimes see files that happen to open anyway, but FLV does not belong to that group. The player tries to open the file believing it is MP4 and throws an error. You need to do a genuine conversion.

My FLV file looks corrupted, can I still rescue it?

Partial recovery is sometimes possible. Because FLV was designed for streaming, it has a structure that can be read from the beginning even if the end of the file is missing; a recording that was cut off halfway can often be converted up to the moment it was cut. But if the header at the start of the file is corrupted, or the download was interrupted very early, the conversion will fail. There is no harm in trying, because your original file is not altered during the process.

A Guide to Converting an FLV File to MP4: Step by Step, With Fixes

Why are we still dealing with FLV files?

FLV was the video container of the Flash Player era. In the late 2000s almost every video on the internet was streaming in this format. After Flash was removed from browsers entirely, FLV was abandoned as well, but the files did not disappear. They still turn up in old backups, on archive drives, in lecture recordings and in downloaded video collections.

Here is the problem: if you have an FLV file today, you will struggle to find anything that opens it. Windows' built-in player will not open it, your phone does not recognise it, you cannot upload it to social media, and most video editing programs refuse to import it. The solution is to move the file to MP4 — and the good news is that in most cases this operation is far more harmless than it looks.

Before you start: what is inside?

You can skip this step, but we recommend you do not, because it is the only thing that determines how long the conversion will take and whether there will be quality loss.

FLV is a container — that is, a box that holds video and audio data together. What is inside the box changes according to which year and which tool produced the file:

H.264 video + AAC audio: files produced in Flash's final years. These are the lucky group.
Sorenson Spark or VP6 video + MP3 audio: older files. Re-encoding is mandatory for these.

You can see which group a file falls into in a player such as VLC, under the Tools > Media Information > Codec tab. But frankly you do not have to; the converter already reads this itself when it opens the file and picks the right path.

Conversion step by step

1. Upload the file

Open our FLV to MP4 conversion tool and drag and drop or select your FLV file. Its contents are read while the file uploads.

2. Wait for the detection result

The tool determines the video and audio codecs inside. If H.264 + AAC is detected, the fast path is used. If not, re-encoding kicks in.

3. Press Convert and wait

Do not close the tab during the operation. On long files that need re-encoding this can take a few minutes; even if the progress indicator looks stalled, it carries on working in the background.

4. Download the result

The output file downloads with an MP4 extension. Your original FLV file does not change, it stays on your computer as it was.

Always check after conversion

The job is not done just because the file downloaded. Verify these three things quickly:

Audio and video sync. This is the most frequent problem with FLV files. In the Flash era recordings were often produced at a variable frame rate, and when those recordings are carried over to MP4's stricter timing structure the audio can drift away from the picture. Watch a piece from the start, the middle and the end of the video — if there is drift it usually grows as you go, so looking only at the start is misleading.

Is the duration right? The converted file's duration should be the same as the original's. If it is much shorter, the source file may have been cut off halfway.

Is the picture quality what you expected? You need to be realistic here: FLV files are usually low resolution and heavily compressed, because they were designed for the internet speeds of their day. Conversion cannot bring that loss back. A 360p FLV stays 360p once it is MP4. Nobody can sharpen a blurry recording by converting it.

Common problems and their fixes

I get a 'file not supported' error. Even if the extension is .flv, the file may not really be FLV. Some download tools save F4V or fragmented MP4 files under an .flv name. Open the file in VLC and check its real format.

There is no audio at all. There are two possible causes. The source file genuinely has no audio track — common with screen recordings. Or the audio track is encoded with a very old Flash audio codec such as Nellymoser. In the second case the audio needs re-encoding; our tool does this automatically, but with some extremely old codecs the result may not be ideal.

The video plays but the picture stutters. This is usually a problem created by variable frame rate. Switching to a constant frame rate during conversion solves it in most players.

I cannot upload a very large file. Operations performed through the browser have size limits. If you have a very long recording, splitting it into parts first is a safer approach.

What should you do after converting the file?

Do not delete the original FLV file straight away. Watch the new MP4 from start to finish once, make sure the audio and picture are correct, and only then remove it from your archive. This matters especially for files that needed re-encoding — because the quality that was lost does not come back, and the new file becomes your only copy.

If there are dozens of FLVs in the same archive, run a test with one of them before converting them all at once. If that file's result is good, the others were most likely produced from the same source with the same settings and will give a similar result.

In summary

Moving from FLV to MP4 is a less risky operation than most people assume. If your file belongs to Flash's final period and has H.264 inside, the conversion only changes the container without touching quality at all. If it is an older recording, re-encoding is required and there will be a certain amount of loss — but the alternative is not being able to open the file at all. In either case the result is a file that plays on every device today and will still open years from now. That is the real gain: FLV is a dead end, MP4 is still the main road.