Complete Guide to Converting Video to MP4 (H.264/AAC) with FFmpeg

What You’ll Learn in This Article

Commands to convert any video format (AVI, MKV, MOV, WMV, etc.) to MP4
How to adjust video quality (CRF parameter)
How to convert only the container without re-encoding the video (stream copy)
The trade-off between encoding speed and quality (preset)

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

Note on filenames: This article uses placeholder filenames such as input.avi and input.mkv. In actual commands, replace these with the path of the file you want to process.

Background: The Difference Between Codecs and Containers

Container (extension): .mp4, .mkv, .avi, etc. The “wrapper” for video and audio data
Codec: The data compression format. For MP4, the standard is H.264 (libx264) for video and AAC for audio

The main codecs that can be placed in an MP4 container are H.264 or H.265 (HEVC) for video and AAC for audio.

Basic Command

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

Option	Meaning
`-i input.avi`	Specify the input file
`-c:v libx264`	Specify H.264 (libx264) as the video codec
`-c:a aac`	Specify AAC as the audio codec
`output.mp4`	Output filename (the `.mp4` extension determines the container)

Primary source: FFmpeg Wiki - H.264 Encoding Guide

Adjusting Video Quality (CRF)

Use -crf to specify video quality. Lower values mean higher quality (larger file size).

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

CRF value	Quality guide	Use case
18	Near visually lossless	Archiving
23	Default. Good balance	General use
28	Stronger compression	File size priority
51	Lowest quality	Almost never used

“For general viewing, a range of 18–28 is recommended” — FFmpeg H.264 Guide

Adjusting Encoding Speed (preset)

Use -preset to adjust the trade-off between encoding speed and compression efficiency.

ffmpeg -i input.avi -c:v libx264 -preset slow -crf 22 -c:a aac -b:a 128k output.mp4

preset	Speed	Compression efficiency
ultrafast	Fastest	Low
fast	Fast	Somewhat low
medium	Default	Balanced
slow	Slow	High
veryslow	Slowest	Highest

At a constant CRF, slower presets produce smaller file sizes at the same quality. Primary source: H.264 Guide - preset

Stream Copy (No Re-encoding)

When the input is already H.264/AAC, you can convert only the container without re-encoding.

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

Advantage: Completes instantly with no quality loss
Note: Fails if the input codec is not compatible with the MP4 container. In that case, switch to -c:v libx264 -c:a aac.

Specifying Audio Bitrate

ffmpeg -i input.avi -c:v libx264 -crf 23 -c:a aac -b:a 192k output.mp4

`-b:a` value	Use case
96k	Low quality, audio only
128k	General video
192k	Music / audio quality priority
320k	Highest quality (approximate AAC upper limit)

Summary of Commonly Used Command Examples

ffmpeg -i input.avi -c:v libx264 -c:a aac output.mp4
ffmpeg -i input.mkv -c:v libx264 -crf 22 -preset slow -c:a aac -b:a 192k output.mp4
ffmpeg -i input.mkv -c copy output.mp4
ffmpeg -y -i input.avi -c:v libx264 -crf 23 -c:a aac output.mp4

The final -y flag skips the overwrite confirmation if the output file already exists (useful for batch processing).

Troubleshooting

`Encoder libx264 not found`

Cause: The installed FFmpeg build does not include libx264 Solution: Reinstall FFmpeg from the official site or your package manager. On Windows, the Full build from gyan.dev is recommended.

Output File Is Too Large

Increase the CRF value (28–32) or change to -preset fast to reduce the file size.

Stream Copy Fails

-c copy only works when the input codec is compatible with the output container. If it fails, re-encode with -c:v libx264 -c:a aac.

Primary sources: FFmpeg H.264 Wiki / FFmpeg AAC Wiki