Short-form FFmpeg cookbook: copyable commands for Reels

Input: a video (any size), optionally an SRT file and a music track. Output: a 1080×1920 9:16 MP4 ready for Reels, TikTok and Shorts. Probe first, run one recipe at a time, check the result with ffprobe and a frame grab before the next step.

ffprobe -v error -show_entries stream=codec_type,codec_name,width,height,r_frame_rate -show_entries format=duration -of compact=p=0 input.mp4

1. Crop to 9:16 vertical

# center crop, full height
ffmpeg -i input.mp4 -vf "crop=ih*9/16:ih,scale=1080:1920,setsar=1" -c:v libx264 -crf 20 -c:a copy vertical.mp4
# subject off-center: add the x offset in px
ffmpeg -i input.mp4 -vf "crop=ih*9/16:ih:200:0,scale=1080:1920,setsar=1" -c:v libx264 -crf 20 -c:a copy vertical.mp4
# keep the whole frame over a blurred copy
ffmpeg -i input.mp4 -filter_complex "[0:v]scale=1080:1920:force_original_aspect_ratio=increase,crop=1080:1920,boxblur=20:2[bg];[0:v]scale=1080:-2[fg];[bg][fg]overlay=(W-w)/2:(H-h)/2,setsar=1" -c:v libx264 -crf 20 -c:a copy vertical-blur.mp4

2. Burn in captions

ffmpeg -i vertical.mp4 -vf "subtitles=subs.srt:force_style='FontName=Arial,FontSize=14,Bold=1,Outline=2,Alignment=2,MarginV=60'" -c:v libx264 -crf 20 -c:a copy captioned.mp4
# styled or karaoke captions: convert, edit the .ass, burn
ffmpeg -i subs.srt subs.ass
ffmpeg -i vertical.mp4 -vf "ass=subs.ass" -c:v libx264 -crf 20 -c:a copy captioned.mp4
# soft subtitles (switchable, no re-encode)
ffmpeg -i input.mp4 -i subs.srt -map 0 -map 1 -c copy -c:s mov_text -metadata:s:s:0 language=eng soft.mp4

Keep captions out of the bottom 14% and the right edge of a 9:16 frame, where the platform buttons sit. Check a frame: ffmpeg -ss 2 -i captioned.mp4 -frames:v 1 frame.png, then read it.

3. Trim and join

# frame-accurate trim (re-encodes)
ffmpeg -ss 00:00:05 -to 00:00:12 -i input.mp4 -c:v libx264 -crf 18 -c:a aac clip.mp4
# instant lossless trim (cuts on keyframes, may start a few seconds off)
ffmpeg -ss 00:00:05 -to 00:00:12 -i input.mp4 -c copy clip.mp4
# join clips with the same codec, size and frame rate
printf "file 'clip1.mp4'\nfile 'clip2.mp4'\n" > list.txt
ffmpeg -f concat -safe 0 -i list.txt -c copy joined.mp4
# join clips that differ: scale and pad each to 1080x1920, then concat
ffmpeg -i a.mp4 -i b.mp4 -filter_complex "[0:v]scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2,setsar=1,fps=30[v0];[1:v]scale=1080:1920:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2,setsar=1,fps=30[v1];[v0][0:a][v1][1:a]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" -c:v libx264 -crf 20 -c:a aac joined.mp4

Every clip in the concat filter needs an audio track; give a silent one -f lavfi -i anullsrc=r=48000:cl=stereo -shortest.

4. Music under the voice

# duck the music while people talk
ffmpeg -i input.mp4 -i music.mp3 -filter_complex "[0:a]asplit=2[voice][key];[1:a]volume=0.4[m];[m][key]sidechaincompress=threshold=0.03:ratio=8:attack=20:release=400[ducked];[voice][ducked]amix=inputs=2:duration=first:normalize=0[a]" -map 0:v -map "[a]" -c:v copy -c:a aac output.mp4
# flat music bed
ffmpeg -i input.mp4 -i music.mp3 -filter_complex "[1:a]volume=0.25[m];[0:a][m]amix=inputs=2:duration=first:normalize=0[a]" -map 0:v -map "[a]" -c:v copy -c:a aac output.mp4
# replace the sound, loop the track, fade out (20 s video)
ffmpeg -i input.mp4 -stream_loop -1 -i music.mp3 -map 0:v -map 1:a -c:v copy -c:a aac -shortest -af "afade=t=out:st=18:d=2" output.mp4
# loudness check (platforms normalize to about -14 LUFS)
ffmpeg -i output.mp4 -af ebur128 -f null -

Use music you made or licensed for the platform. normalize=0 needs FFmpeg 4.4 or later.

No shell

Send the same command to the varg ffmpeg tool (MCP call_tool, or POST https://api.varg.ai/v2/ffmpeg), or use its resize, trim and slice tools, with URLs in and out. Priced in credits, with an estimate before it runs. Tutorials: https://skills.varg.ai/burn-captions-with-ffmpeg/, https://skills.varg.ai/crop-a-video-to-vertical-with-ffmpeg/, https://skills.varg.ai/trim-and-join-video-clips-with-ffmpeg/, https://skills.varg.ai/add-music-to-a-video-with-ffmpeg/.