Video formats, and which one is smallest

The default answer to "which format should I use?" — and the right output for almost everything.

MP4 is a container: a wrapper that holds a video stream, an audio stream and the timing information that keeps them together. What makes it universal is not the container itself but what is usually inside it — H.264 video and AAC audio, a pairing that every browser, phone, TV, editor and social platform in current use can decode.

Compressing to MP4 here re-encodes the video at a bitrate derived from your quality setting, using H.264 by default. H.265 is offered alongside it and lands roughly a third smaller for the same picture, but it takes several times as long inside a browser tab and drops off the compatibility list that made MP4 worth choosing in the first place. If your source is already H.264 in some other container, and you only need the wrapper changed, use Remux instead — it swaps the container and copies the video stream untouched, so it finishes in seconds with zero quality loss.

Compress MP4 — settings, expected savings and the gotchas

Apple's QuickTime container. Structurally close to MP4, which is why converting between them is cheap.

MP4 was standardised from the QuickTime file format, so MOV and MP4 are near relatives. That is why a MOV can usually be turned into an MP4 by remuxing rather than re-encoding: the streams inside are frequently already compatible, and only the wrapper needs rewriting.

MOV files off an iPhone are often large for their length — phones favour encoding speed and quality headroom over file size, and newer devices may record HEVC rather than H.264. Re-encoding to H.264 here both shrinks the file and makes it play on hardware that has no HEVC support. You can also keep it as H.265, which the tool tags hvc1 so QuickTime and Safari will open it, and that is the sensible pick when the file is going back onto Apple hardware and nowhere else.

Compress MOV — settings, expected savings and the gotchas

Matroska: an open container that will hold almost any combination of streams you throw at it.

MKV's strength is flexibility. It can carry multiple audio tracks, multiple subtitle tracks, chapters and attachments in one file, which is why media servers such as Plex and Jellyfin, and most home archives, standardise on it. It is also the only output here that will take any of the three video codecs, so it is where an H.265 or VP9 stream can go without arguing with the container.

Its weakness is browser support: Safari and Chrome will not play an arbitrary MKV natively, so an MKV destined for a web page needs converting. If the video inside is already H.264, Remux to MP4 is the fast route — no re-encoding, no quality loss, seconds rather than minutes.

Compress MKV — settings, expected savings and the gotchas

The open, royalty-free container built for browsers, normally carrying VP9 video and Opus audio.

VP9 is a newer codec than H.264 and generally reaches the same perceived quality at a lower bitrate, which makes WebM attractive for video you are embedding on a page and serving repeatedly. Opus, its usual audio partner, is similarly efficient at low bitrates.

A WebM written here is a real VP9 file, encoded with libvpx-vp9, with its audio always converted to Opus because an AAC track has no legal home in the container. Two things to weigh before picking it. VP9 in WebAssembly is markedly slower than H.264, so a long clip will keep the tab busy. And WebM remains a bad choice for a file you are emailing to someone or handing to an unknown device, however efficient it is.

Remux is unavailable for WebM output for the same containment reason: a copied H.264 stream cannot be muxed into it. If you want the streams left alone, remux to MKV instead.

Compress WebM — settings, expected savings and the gotchas

A container from the early 1990s. Accepted as input; convert it to MP4.

AVI predates modern codecs and typically carries older, far less efficient video such as MPEG-4 Part 2 or DivX. That is why AVI files feel enormous for their resolution: the compression inside them is a generation or two behind.

AVI is not offered as an output format, deliberately — there is no situation where writing an AVI is the better choice today. Convert to MP4 and the same footage will usually be dramatically smaller at equal quality.

Compress AVI — settings, expected savings and the gotchas

Windows Media Video. Still turns up in old corporate archives and screen recorders.

WMV playback outside the Windows ecosystem is unreliable, and browsers do not support it at all. If you have been handed a WMV, converting it to MP4 is almost always the first step.

Compress to MP4 with the quality slider left near the default. WMV sources are frequently low-bitrate to begin with, so aggressive settings have little room to work before artefacts become obvious.

Compress WMV — settings, expected savings and the gotchas

Adobe Flash Video. Obsolete as a delivery format, but plenty of archives still hold it.

Flash Video is what much of the web streamed before HTML5 video, so old downloads, course recordings and archived clips are often FLV. Nothing modern plays it without help.

Later FLV files actually contain H.264 video, in which case Remux to MP4 will rescue them instantly with no re-encode. Older ones carry H.263 or Sorenson Spark and need a real conversion — pick MP4 and accept that a low-resolution source will still look low-resolution.

Compress FLV — settings, expected savings and the gotchas

The mobile container from the 3GPP standard, designed for early phone networks.

3GP was built for bandwidth that no longer exists as a constraint: small frames, low bitrates, aggressive audio compression. Files are tiny, and there is usually nothing to gain by compressing them further.

The reason to run a 3GP through here is compatibility rather than size — convert to MP4 so it plays in a browser or a modern editor. Do not expect quality to improve; nothing can add detail that was never captured.