Compress an FLV file

A dead container with a live video inside it

FLV was Macromedia's wrapper for video delivered through Flash Player, and for about a decade it was how video worked on the web. Adobe ended Flash Player on 31 December 2020 and browsers blocked the plugin outright a few weeks later, which left the container with nothing to play it. Nothing in a browser opens an FLV today, and most consumer video software dropped it too.

The container itself was always thin — a small header, then a sequence of tagged packets, with no index worth the name. That thinness is why it streamed well over 2006 connections and also why a truncated FLV is often still watchable. What matters now is that the wrapper is the dead part. The video inside frequently is not.

Which FLV you have decides everything

Three codec eras shipped inside this container, and the one you have determines whether the fix takes five seconds or twenty minutes.

• Sorenson Spark, roughly 2002 to 2005. A variant of H.263. Usually 320×240, soft, and about as efficient as you would expect from a codec designed for dial-up.
• On2 VP6, roughly 2005 to 2008. A clear step up and the codec behind most of the Flash-era video people remember. Still nothing a modern device will decode without help.
• H.264 with AAC, 2007 onwards. Once Adobe added support, everything serious switched. A late FLV holds exactly the same video stream a modern MP4 would, in the wrong box.

Try remux first — it costs seconds and answers the question

If the video inside is H.264, converting it to MP4 means rewriting the wrapper and nothing else. Choose Remux with MP4 as the output: the streams are copied bit for bit, no pixel is decoded, and the job finishes in seconds regardless of how long the video runs. The file will not get smaller, because nothing about it changed except the box it sits in. That is the point.

You do not need to identify the codec first. Remux is the cheapest possible diagnostic — it either produces a working MP4 in a few seconds or it fails almost immediately, and either outcome tells you what you have. Trying it before anything else is strictly better than guessing.

The rough heuristic, if you want one anyway: a video downloaded from a video site after about 2008, or anything at 480p or above, is almost certainly H.264. Something at 320×240 out of an e-learning course or an old screen recorder is almost certainly not.

When the remux fails, and what to do then

A failure means Sorenson Spark or VP6, neither of which has any defined mapping into an MP4. Those need a real re-encode: switch to Best compression, output MP4, and let every frame be decoded and rewritten as H.264. FFmpeg has decoders for all of them and the build in this page is a stock one with the full component set, so it will read the file — it just has to do the work.

Set your expectations accordingly. This footage is typically 320×240 or 400×300, and there is not much detail in it to preserve or to throw away. Leave the resolution on original; downscaling something already that small is pointless, and upscaling it is worse than pointless because you would be spending bitrate on invented pixels. The file may also barely shrink, because a 15 MB FLV from 2004 was already compressed hard for its era. You are converting it so it plays, not to save space.

One case where the saving is real: Camtasia, Jing and the rest of the e-learning tooling of that period exported long screen recordings to Flash formats, and screen content re-encodes to H.264 extremely well. A half-hour tutorial can come down a lot.

Two things that will trip you up

Target-size mode will not work on an FLV. The tool reads clip length from the browser's own video element so it can solve a bitrate before encoding, and browsers no longer open FLV files at all. Target mode will report that it cannot read the length and stop. Use Best compression, or remux, both of which do not need a duration.

The other is audio, and only on very old files. MP3 and AAC both copy into an MP4 without trouble. Nellymoser, the speech codec Flash used for microphone capture, does not — if a remux fails on a file whose video looks fine, that is a likely culprit. Re-encoding with Audio set to Compress solves it.

FLV compression questions

Compressing to a size limit instead?

If what brought you here is a number rather than the fact that it is a FLV file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your FLV:

• 5 MB
• 8 MB
• 10 MB
• 16 MB
• 20 MB
• 25 MB
• 50 MB
• 100 MB
• 200 MB
• 500 MB

Other formats

• Compress MP4 The universal container. Usually already H.264, so the gains come from bitrate and resolution.
• Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
• Compress MKV Long recordings, rips and archives. Big enough that upload-based tools often refuse them.
• Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
• Compress AVI Legacy codecs, dramatic savings. Read in, MP4 out.
• Compress WMV Old Windows exports. Nothing plays them now, and remuxing is not an option.
• Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.
• Compress MTS / M2TS AVCHD camcorder footage. Enormous, usually interlaced, and split into 2 GB pieces.

The formats reference puts FLV beside the other containers and shows which codec combinations play where.