Video Format Conversion Basics (MP4, MKV, WebM, MOV)

What You Will Learn

The difference between containers and codecs (essential knowledge before converting)
When stream copy (-c copy) works instantly — and when it fails
Practical commands for MKV→MP4, MP4→WebM, MOV→MP4, AVI→MP4, and MP4→MKV
How to verify codecs before and after conversion using ffprobe

Tested version: Verified with FFmpeg 6.1 (ubuntu-latest / CI-validated) Target OS: Windows / macOS / Linux

About filenames: This article uses placeholder filenames such as input.mkv and input.mp4. Replace them with the actual file paths you want to process.

Containers vs. Codecs

The most common source of confusion when working with video files is the distinction between containers and codecs.

Concept	Description	Examples
Container (extension)	A “wrapper” that holds video, audio, subtitles, etc.	`.mp4`, `.mkv`, `.webm`, `.mov`, `.avi`
Video codec	Compression format for video data	`H.264`, `H.265`, `VP9`, `AV1`
Audio codec	Compression format for audio data	`AAC`, `MP3`, `Opus`, `Vorbis`

Simply renaming the file extension does not convert it. For example, if an .avi file contains H.264+AAC video, you only need to “repack” it into an MP4 container (stream copy). On the other hand, converting a WebM file with VP9 video to MOV may require re-encoding the codec.

Checking Codecs (ffprobe)

Checking codecs before conversion helps you choose the right approach.

ffprobe -v error -show_streams -select_streams v:0 input.mp4

ffprobe -v error -show_entries stream=codec_name,codec_type -of default=noprint_wrappers=1 input.mkv

Commands that use only -hide_banner or -show_streams without an output file are shown in text blocks.

When to Use Stream Copy (-c copy)

If you only need to change the container (keeping the codec as-is), -c copy is the fastest option.

ffmpeg -i input.mkv -c copy output.mp4

Benefits:

Completes without re-encoding (seconds to tens of seconds)
Zero degradation in video or audio quality

Caveats and failure cases:

Will fail if the output container does not support the input codec
Example: Trying to convert an MKV with VP9 video to MP4 using -c copy will error because MP4 does not support VP9
If -c copy fails, switch to re-encoding

Common Conversion Commands

MKV → MP4 (Stream Copy)

If the MKV contains H.264+AAC, a single command converts it instantly.

ffmpeg -i input.mkv -c copy output.mp4

If the content uses a different codec and stream copy fails, re-encode instead.

ffmpeg -i input.mkv -c:v libx264 -crf 23 -c:a aac output.mp4

MOV → MP4 (iPhone / Mac Video Conversion)

MOV files recorded on iPhone or Mac almost always contain H.264/HEVC, so stream copy often works.

ffmpeg -i input.mov -c copy output.mp4

If the file contains HEVC (H.265) video and you need broad compatibility, re-encode:

ffmpeg -i input.mov -c:v libx264 -crf 22 -c:a aac output.mp4

AVI → MP4 (Converting from Legacy Format)

AVI files use a wide variety of codecs, so re-encoding is recommended for safety.

ffmpeg -i input.avi -c:v libx264 -crf 23 -c:a aac output.mp4

MP4 → MKV (Container-Only Conversion)

MKV can hold virtually any codec, so stream copy works reliably here.

ffmpeg -i input.mp4 -c copy output.mkv

MP4 → WebM (For Web Delivery)

WebM uses VP8/VP9 video and Vorbis/Opus audio as its standard. Well-suited for browser delivery.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 -c:a libopus output.webm

Option	Meaning
`-c:v libvpx-vp9`	Use VP9 encoder
`-crf 30 -b:v 0`	Constant quality mode (CRF mode)
`-c:a libopus`	Use Opus encoder (recommended audio for WebM)

How Automatic Codec Selection Works

Simply specifying the output container causes FFmpeg to auto-select default codecs.

ffmpeg -i input.avi output.mp4

This command automatically selects libx264 (video) and aac (audio) — the defaults for the MP4 container. However, since you have no quality control, it is recommended to explicitly specify options like -crf.

Supported Codecs by Container

Container	Common Video Codecs	Common Audio Codecs
MP4	H.264, H.265	AAC, MP3
MKV	H.264, H.265, VP9, AV1, etc.	AAC, MP3, Opus, FLAC, etc.
WebM	VP8, VP9, AV1	Vorbis, Opus
MOV	H.264, H.265, ProRes	AAC, PCM
AVI	H.264, DivX, Xvid, etc.	MP3, PCM, etc.

Common Errors and Solutions

`Could not write header for output file` Error

This is a codec/container compatibility problem. Switch from -c copy to -c:v libx264 -c:a aac.

`Encoder not found` Error

The FFmpeg build you installed does not include that codec. Reinstall a full-featured build from the official site or gyan.dev (Windows).

WebM Conversion Is Extremely Slow

VP9 encoding is time-consuming. Add -cpu-used 4 to prioritize speed.

Tested with: ffmpeg 6.1.1 / Ubuntu 24.04 (GitHub Actions runner) Primary sources: ffmpeg.org/ffmpeg.html / ffmpeg.org/ffmpeg-formats.html / trac.ffmpeg.org/wiki/Encode/H.264