How to Convert MKV, AVI, WebM, or FLV to MP4

Why video compatibility varies so much

A container organizes streams, while codecs compress pictures and sound. MKV can hold rich subtitles, chapters, and multiple tracks. AVI is an older container. WebM commonly carries VP8, VP9, or AV1 video. FLV belongs to an older web-video ecosystem. A player may recognize the container but not a stream inside it.

MP4 with H.264 video and AAC audio is a broadly playable distribution combination. The cost is decoding and re-encoding the input, which takes time and is not a lossless copy.

Check three things before conversion

• Does the file contain multiple audio tracks, such as original language and dubbing?
• Are subtitles a selectable track, burned into frames, or a separate .srt file?
• Must chapters, cover art, attachments, or special metadata survive?

Conversion steps and presets

1. Back up the source and drop it into the tool.
2. Create a baseline MP4 with High quality at original resolution.
3. Play it to check picture, sound, subtitles, and duration.
4. If sharing is the goal, create a lighter Balanced or 720p version.
5. Keep the source container until every destination has been verified.

“Convert to MP4” does not guarantee a smaller file

If the source already uses an efficient codec at low quality, an H.264 MP4 may become larger. If the source bitrate is excessive, a sensible CRF and resolution may reduce it substantially. An extension alone cannot predict size; codec, quality, dimensions, frame rate, duration, and visual complexity determine the outcome.

A representative one-minute segment or small-file sample is therefore more reliable than promising “smaller” from the input extension.