What is an MP4 file?

MP4 is a container: a wrapper that holds streams encoded by something else. It is used for handing a finished file over, the web and phones.

The extension is .mp4, and the full name is MPEG-4 Video. Both matter less than what the file can hold, which is what the rest of this page is about.

MPEG published it in 2001. The specification is ISO/IEC 14496-14.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

It usually holds H.264, HEVC, AV1 and AAC. This is the distinction behind most "why will this not play" questions: two files with the same extension can contain entirely different encodings, and a player that refuses one may be perfectly happy with the other.

It also explains why changing container is often fast and lossless while changing codec is neither. Moving the same stream into a different box rewrites the box; re-encoding rewrites the picture.

MP4 can hold several audio tracks, subtitle tracks and a layout that lets it start playing before it has finished arriving.

That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.

VLC, HandBrake and Adobe Premiere Pro read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

Every current browser reads it.

That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.

MP4 can hold document properties.

Whether any of it survives a conversion depends entirely on the target, and the honest answer is usually "some of it".

MP4 is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

This is the fact that explains nearly every confusing thing about MP4. The file does not define how the picture is compressed; it defines how compressed streams are stored side by side, indexed and kept in sync. Video, audio, subtitles and chapter marks each sit in their own track, and MP4 is the box they travel in.

So "MP4 quality" is not a property of MP4. Two files with the same extension and the same size can look completely different because one holds H.264 and the other AV1, and a player that refuses one may play the other perfectly. When something goes wrong with an MP4, the useful question is almost never about the container.

For video, H.264 is the overwhelming default and has been for fifteen years: every phone, camera, browser and television decodes it, often in dedicated hardware. H.265 appears where the file needed to be smaller — 4K recordings from recent phones especially — and AV1 is arriving from streaming services and newer capture tools.

For audio, AAC is the near-universal answer, occasionally MP3 in older files and rarely uncompressed PCM. The practical consequence of that spread is that H.264 with AAC is the combination that plays everywhere without thought, and anything else is a deliberate trade of compatibility for size.

Because the player has a decoder for what is inside the first file and not for what is inside the second. A 2016 television handles H.264 in hardware and has never heard of AV1. An older editing program opens H.264 and rejects H.265 because the licence to decode it was never bought. The error message usually says something unhelpful about an unsupported file.

The diagnosis is quick: find out which codec the file carries. If it is H.265 or AV1 and the destination is old, re-encoding to H.264 fixes it, at the cost of a larger file and one generation of quality. If it is already H.264, the problem is elsewhere — a damaged index, an exotic audio track, or a player that wants a different extension.

An MP4 keeps a table of contents — historically called the moov atom — describing where every frame lives. Many tools write it at the end of the file, because its contents are not fully known until encoding finishes. That is harmless on a local disk and a real problem over a network: a player cannot begin until it has that table, so it downloads the entire file before showing a frame.

The fix is a step usually called fast start or web optimisation, which moves that table to the front. Nothing about the picture changes. If a video on a web page shows a long blank pause before playing while the same file plays instantly from the desktop, this is almost always why.

MP4 was standardised out of Apple’s QuickTime format, and the two share their structure to the point that many files are valid as both. A great deal of software will play a MOV renamed to .mp4 and vice versa, which is a hint about how close they are rather than a technique worth relying on.

The differences that survive are at the edges: MOV can carry the editing-oriented codecs Apple’s tools produce, such as ProRes, which no MP4 player expects. As a rule, MOV is what recording and editing on Apple hardware produces, and MP4 is what gets delivered to everyone else.

MP4 can hold a timed-text track, and support for reading it is inconsistent enough that most people never rely on it. Desktop players usually find it; browsers, televisions and set-top boxes frequently ignore it, so subtitles that work on the machine they were made on vanish everywhere else.

Two workarounds are common and both are honest about the trade. A separate SRT or VTT file beside the video is universally understood and can be switched off, but has to travel with the video. Burning the text into the picture guarantees it appears everywhere and makes it permanent, unswitchable and untranslatable.

Compressed video is built from occasional complete frames with many frames in between describing only what changed. That is why an MP4 can be trimmed at those complete frames without touching the picture, and why a cut anywhere else requires decoding and re-encoding the section — which costs a generation of quality.

Rotation is the useful exception. Orientation is stored as a flag rather than baked into the pixels, so turning a video the right way up is a metadata change and instant. Resizing, changing the frame rate, adjusting the audio and joining clips with different settings are all genuine re-encodes.

Rather more than most people expect. Recording devices commonly write the make and model, the date and time, and — if location services were on — the GPS coordinates. Editing software adds its own name and often the project’s. None of it is visible during playback.

That matters for the ordinary case of a clip filmed at home and posted publicly. Metadata can be stripped, and it is worth doing deliberately rather than assuming a platform has done it: some strip everything on upload, some strip only the location, and some keep the file as it arrived.

MP4 became the default around 2010 and has stayed there through several attempts to replace it, because compatibility compounds. Every device that ships with an H.264 decoder makes the next file more likely to be H.264, and hardware decoding means playback costs almost no battery.

Newer containers and codecs are genuinely better at compression — WebM with VP9 or AV1 will produce a smaller file at the same quality. They win where the audience is known, such as a streaming service that controls its own players. MP4 wins where the audience is everyone, which is why it remains the right answer for a file being handed to a person.