How to crop a video to 9:16 vertical with FFmpeg

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Turn a 16:9 video into a 1080×1920 vertical 9:16 MP4 for TikTok, Reels and Shorts, with one FFmpeg command. Free and local. Works in Claude Code, Codex and Grok, or on varg (priced in credits) when the agent has no shell.

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

Step by step

  1. Check the input size. ffprobe -v error -show_entries stream=width,height -of csv=p=0 input.mp4.

  2. Center crop with the command above. crop=ih*9/16:ih keeps the full height and the middle 9:16 slice. scale=1080:1920 sets the output size.

  3. Move the crop when the subject is off-center. Add the x offset: crop=ih*9/16:ih:200:0 starts 200 px from the left. For a speaker who moves, find their position per scene (a face tracker, or ask a video model for timecodes), then cut and crop each scene on its own.

  4. Keep the whole frame instead of cropping: blur a copy behind it.

    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

  5. Check the result. ffprobe should say 1080x1920. Look at a frame from the busiest scene to see that nothing important was cut off.

Gotchas

No shell? Run it on varg

The agent sends the same command to the varg ffmpeg tool, or calls the resize tool (MCP call_tool, or POST https://api.varg.ai/v2/ffmpeg). It runs in the cloud, priced in credits, with an estimate before it runs.

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