Video file formats

Which video format should I use?

For anything you will send to another person, upload or play on a TV, use MP4 with H.264 video and AAC audio. It is the most widely supported combination in existence and every phone, browser, editor and smart TV made in the last decade reads it.

For a film you want to keep, MKV is the better archive: it holds several audio languages, switchable subtitle tracks and chapter marks in one file, and it can wrap the same H.265 or AV1 video at a smaller size. Convert a copy to MP4 when you need to play it somewhere that refuses MKV.

For video embedded on a web page, WebM (VP9 or AV1) avoids patent licensing and compresses well, but Safari support is uneven, so most sites still ship an MP4 fallback. For editing on a Mac, MOV with ProRes keeps quality high at the cost of very large files, and MXF plays the same role in broadcast.

What is the difference between MP4, MKV and AVI?

All three are containers, so none of them determines picture quality on its own. MP4 is the modern, streamable standard: it supports H.264, H.265 and AV1, seeks quickly and plays everywhere. MKV is the most flexible: it accepts any codec and any number of audio, subtitle and chapter tracks, which is why film collections use it, but browsers and many TVs will not open it. AVI is the oldest of the three: it has no proper support for soft subtitles, chapters or streaming, and it typically wraps older codecs such as Xvid or DivX, so the same footage takes noticeably more space.

Practically: if a file will not play, converting an MKV or AVI to MP4 with H.264 fixes it almost every time, because you are re-wrapping and re-encoding into the combination that has the widest support.

How to tell what a video file really is

The extension can be wrong, and the codec inside is invisible from the filename. The reliable check is the file's first bytes: an MP4 or MOV has the ASCII text ftyp at offset 4, a Matroska or WebM file starts with the EBML header 1A 45 DF A3, and an AVI starts with RIFF followed by AVI at offset 8. Renaming a file changes none of that.

Dropping a clip into the identifier on the home page reads those bytes in your browser, without uploading anything, and tells you the real container. VLC's Tools then Codec Information panel goes one step further and names the codec inside.

Container versus codec

A video file has two separate parts: the container (the extension, such as MP4, MKV or MOV) and the codec inside it (such as H.264, H.265/HEVC or AV1). The container bundles the video stream, one or more audio streams, subtitles and metadata. The codec is the maths that actually compresses the pictures. This is why two files with the same .mp4 extension can behave differently: one may use a codec your device supports and another may not.

MP4 is the safe, universal container that plays almost everywhere. MKV (Matroska) is more flexible and popular for high-quality rips because it can hold many audio and subtitle tracks. MOV is Apple's QuickTime container, common out of macOS and iPhones. WebM is the open container built for the web.

Why a video won't play

When a video refuses to open, the container is rarely the problem; the codec usually is. Installing a capable player such as VLC, which bundles the codecs itself, fixes most cases without changing the file. If you need a file to play on a specific device, re-encoding it to H.264 inside an MP4 is the most compatible target.