FFmpeg - Concatenate MP4 Files
Concatenating video files in FFmpeg can be achieved in several ways, depending on your input formats, codecs, and whether you want to avoid re‑encoding. This article explains the three official FFmpeg concatenation methods, when to use each one, and common pitfalls.
1. concat Video Filter
Use this method when:
- Your inputs do not share the same properties (resolution, frame rate, codec).
- You want to apply filters, scaling, or other transformations.
- You are comfortable with a full re‑encode of all inputs.
Because the filter merges decoded streams, FFmpeg must re‑encode everything.
Example: Concatenating Three Clips
ffmpeg -i opening.mkv -i episode.mkv -i ending.mkv -filter_complex "[0:v] [0:a] [1:v] [1:a] [2:v] [2:a] concat=n=3:v=1:a=1 [v] [a]" -map "[v]" -map "[a]" output.mkv
Notes
- Ideal for complicated workflows or mismatched sources.
- If you want to avoid full re‑encode, pre‑normalise the odd inputs first, then use the concat demuxer.
2. concat Demuxer
Use this method when you want to avoid re‑encoding and your formats support stream copying. This is the recommended method for MP4 as long as the files are perfectly matching (same codec, profile, level, frame rate, resolution, audio layout).
Create a List File
file '/path/to/file1' file '/path/to/file2' file '/path/to/file3'
Concatenate Without Re‑Encoding
ffmpeg -f concat -i mylist.txt -c copy output.mp4
Windows Examples
Manual list:
(echo file 'first file.mp4' & echo file 'second file.mp4') > list.txt ffmpeg -safe 0 -f concat -i list.txt -c copy output.mp4
Automatically list all MP4 files:
(for %i in (*.mp4) do @echo file '%i') > list.txt ffmpeg -safe 0 -f concat -i list.txt -c copy output.mp4
Notes
- All parameters must match. Even slight differences can break stream copy.
- Safest method for MP4 when conditions are met.
3. concat Protocol
Use this only when formats support raw file‑level concatenation (MPEG‑1, MPEG‑2 PS, DV). Do not use this for MP4.
Example
ffmpeg -i "concat:input1|input2" -codec copy output.mkv
Why This Fails for MP4
The concat protocol performs raw joining without rewriting container metadata. MP4 requires regenerated moov atoms and indices, so raw concatenation corrupts the container.
Summary
| Method | Re-Encode? | MP4 Support | Best Use Case |
|---|---|---|---|
| concat filter | Yes | Yes | Mismatched sources, filtering |
| concat demuxer | No | Yes (if identical) | Safe MP4 joining |
| concat protocol | No | No | MPEG‑1 / MPEG‑2 PS / DV |
