MP4, H.264 and AAC explained
An MP4 file is a box. What matters for playback is what is inside it.
Container versus codec
A container such as MP4, MOV, MKV or WebM holds a video stream, an audio stream, subtitles and timing data in one file. A codec is the method used to compress each stream. The file extension tells you the container, not the codecs.
| Name | Type | Typical use |
|---|---|---|
| MP4 | Container | Plays on nearly every phone, computer and TV. |
| MOV | Container | Camera and Apple device recordings; often holds H.264 or HEVC. |
| MKV, WebM | Containers | Open formats; WebM usually holds VP8/VP9 or AV1 video with Opus or Vorbis audio. |
| H.264 (AVC) | Video codec | The most widely supported video codec in MP4. |
| HEVC (H.265) | Video codec | Smaller files, but patchy support in browsers and older devices. |
| AAC | Audio codec | The standard audio codec in MP4. |
What the converter does
If a file already has H.264 video and AAC or MP3 audio, the converter copies the streams into an MP4 container. Nothing is re-compressed, so quality is unchanged and the job takes seconds. If the video or audio uses another codec, such as VP9 or Opus in a WebM file, the converter re-encodes it to H.264 and AAC. That is slower and loses a little quality.
Why a file may not play after conversion
An MP4 that contains HEVC, VP9 or an unusual audio codec can fail on some devices even though the extension is .mp4. Choosing Re-encode for maximum compatibility produces H.264 and AAC, which plays almost everywhere.
Resolution and upscaling
Converting cannot add detail. A 480p source stays 480p; enlarging it only makes the file bigger. The tool therefore offers only the original size or smaller.