Concatenating Video — When to Use concat demuxer vs. concat filter

What You Will Learn

• The differences between the two concat approaches (demuxer vs. filter) and how to choose
• Fast concatenation with the concat demuxer (text file method)
• Flexible concatenation with the concat filter (filter_complex)
• Things to watch out for when joining files with different frame rates or resolutions

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

Comparing the Two concat Approaches

concat demuxer (Fast, Same-Format Files)

Overview

The concat demuxer lists file paths in a text file and joins them in order. It operates without re-encoding, making it very fast.

Creating the File List (list.txt)

The concat demuxer requires a dedicated text file. Create list.txt in the following format:

# Example list.txt (comment lines start with #)
file 'part1.mp4'
file 'part2.mp4'
file 'part3.mp4'

Creating it from the command line on Linux:

printf "file 'part1.mp4'\nfile 'part2.mp4'\nfile 'part3.mp4'\n" > list.txt

Creating it from Windows Command Prompt:

(echo file 'part1.mp4' & echo file 'part2.mp4' & echo file 'part3.mp4') > list.txt

Running the concat demuxer

ffmpeg -f concat -safe 0 -i list.txt -c copy output.mp4

The concat demuxer command that uses list.txt is shown in a text block because it is skipped in automated CI testing.

Notes on Using concat demuxer

• All files must share the same codec, resolution, and frame rate
• Mismatched formats can cause audio/video desync or errors
• Without -safe 0, files with non-relative paths will error

concat filter (Flexible, Supports Mixed Formats)

Overview

The concat filter in filter_complex can join files even when their formats or resolutions differ. Re-encoding is required, but it is a highly versatile approach.

Basic Command (Joining 2 Files)

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" output.mp4

Explaining the filter_complex Syntax

[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[outv][outa]

Joining 3 Files

ffmpeg -i input1.mp4 -i input2.mp4 -i input.mkv -filter_complex "[0:v][0:a][1:v][1:a][2:v][2:a]concat=n=3:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" output.mp4

Set n=3 and list all stream pairs.

Joining with a Specified Codec

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" -c:v libx264 -crf 23 -c:a aac output.mp4

Normalizing Resolution and Frame Rate

Even when using the concat filter, mismatched resolutions or frame rates can cause distortion in the joined video. It is recommended to normalize them beforehand using the scale and fps filters.

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex "[0:v]scale=1280:720,fps=30[v0];[1:v]scale=1280:720,fps=30[v1];[v0][0:a][v1][1:a]concat=n=2:v=1:a=1[outv][outa]" -map "[outv]" -map "[outa]" -c:v libx264 -crf 23 -c:a aac output.mp4

Choosing the Right Method

Choose concat demuxer when:

• All files share the same codec and resolution
• You have many files and want fast concatenation without re-encoding
• You are joining homogeneous data such as split recording files

Choose concat filter when:

• Files have different codecs or resolutions
• You want to specify files dynamically in a script (no list file needed)
• You want to directly join trimmed results

Common Errors and Solutions

DTS ... out of order Error

Occurs when joining files with discontinuous timestamps using the concat demuxer. Change -c copy to -c:v libx264 -c:a aac.

Audio Goes Out of Sync

Occurs when frame rates or audio sample rates differ. Use the concat filter and normalize with the fps and aresample filters beforehand.

Specific Files Are Skipped

If file paths in list.txt contain spaces or special characters, verify that quoting is handled correctly.

Related Articles

• Trimming Video — -ss/-to/-t and the Keyframe Problem

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