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

Got a video that won’t play because it’s not in MP4 format? Can’t upload it to your phone or social media? FFmpeg can convert virtually any video format — AVI, MKV, MOV, and more — to MP4. This article covers everything you need, from basic commands to advanced options.

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

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

“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

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

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. For precise file size targeting with 2-pass encoding, see Two-Pass Encoding — Precise Bitrate and File Size Control.

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