Video Converter

A video file has two identities that people routinely conflate. The container is the extension — MP4, MOV, MKV, AVI, WEBM — and it is a box holding streams plus metadata. The codec is how the picture and sound inside are actually encoded, usually H.264, H.265/HEVC, VP9, or AV1. A device can support the box and still fail on its contents, which is why an MP4 from a modern phone sometimes will not play on an older TV: the container is fine, the H.265 inside is not.

This is also why renaming a file does nothing. Changing .mkv to .mp4 relabels the box and leaves the streams untouched, so the player fails in exactly the same way. Real conversion decodes and re-encodes, which is why it takes time roughly proportional to the length of the video rather than completing instantly.

For anything that has to work everywhere, the answer has not changed in a decade: MP4 with H.264 video and AAC audio. It plays on essentially every phone, browser, smart TV, and editing program in use, and it is what upload forms expect when they say "video file". It is not the most efficient option available, and that is the point — compatibility is the feature.

WEBM produces noticeably smaller files at the same quality and is well supported in browsers, so it is a good choice for a video embedded on a website and a poor one for a file you are emailing to someone. MOV is Apple's container and behaves best inside Final Cut and QuickTime. AVI is a legacy format worth converting away from rather than to; it has no modern advantage and produces larger files.

When a video is too large, the instinct is to change format. The bigger levers are elsewhere. Length is first: trimming thirty seconds off a two-minute clip removes a quarter of the file with zero quality cost. Resolution is second, and it is the one most often left wrong — exporting at 4K for something that will be watched in a 600-pixel-wide web player wastes most of the file. Dropping 4K to 1080p typically cuts size by around 75 percent with no visible difference at normal viewing sizes.

Bitrate is what finally determines how it looks. A 1080p file at a low bitrate looks worse than a 720p file at a generous one, because the encoder runs out of data to describe motion and falls back on blocky approximations, most visible in fast movement, water, and smoke. Re-encoding at a higher bitrate than the source never helps: the detail is already gone and you are only storing the same loss in a larger file.