# How to trim and join video clips with FFmpeg

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Turn long footage into trimmed clips and join them into one MP4 with FFmpeg: the exact commands for frame-accurate cuts and lossless joins. Free and local. Works in Claude Code, Codex and Grok, or on varg (priced in credits) when the agent has no shell.

```bash
ffmpeg -ss 00:00:05 -to 00:00:12 -i input.mp4 -c:v libx264 -crf 18 -c:a aac clip.mp4
```

## Step by step

1. **Probe the clips.** `ffprobe -v error -show_entries stream=codec_name,width,height,r_frame_rate -of compact=p=0 input.mp4`. Joining without re-encoding needs the same codec, size and frame rate in every clip.
2. **Trim to the frame** with the command above. Re-encoding makes the cut land exactly on `-ss` and `-to`.
3. **Or trim fast without re-encoding.** `ffmpeg -ss 00:00:05 -to 00:00:12 -i input.mp4 -c copy clip.mp4` is instant and lossless, but it cuts on keyframes, so the clip can start up to a few seconds off.
4. **Join clips that match** with the concat demuxer, losslessly. Write a list, then join:

   ```bash
   printf "file 'clip1.mp4'\nfile 'clip2.mp4'\n" > list.txt
   ffmpeg -f concat -safe 0 -i list.txt -c copy joined.mp4
   ```

5. **Join clips that differ** (sizes, frame rates) with the concat filter. It scales and pads each clip to one size, then re-encodes:

   ```bash
   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
   ```

6. **Check the result.** The duration in `ffprobe -v error -show_entries format=duration -of csv=p=0 joined.mp4` should equal the sum of the clips.

## Gotchas

- Every clip in the concat filter needs an audio track. Give a silent clip one with `-f lavfi -i anullsrc=r=48000:cl=stereo -shortest`.
- `-ss` before `-i` seeks fast; with re-encoding it is still frame-accurate.
- For cuts chosen from a transcript (removing silences and filler words), get word timestamps first, then generate the trim list. The editing guide below compares the tools.

## No shell? Run it on varg

The agent calls the varg `trim`, `slice` or `ffmpeg` tools (MCP `call_tool`, or `POST https://api.varg.ai/v2/ffmpeg`) with URLs in and out. It runs in the cloud, priced in credits, with an estimate before it runs.

## Related

- [How to edit a video with AI and FFmpeg](https://varg.ai/how-to/edit-a-video.md)
- [How to burn captions into a video with FFmpeg](https://varg.ai/how-to/burn-captions-with-ffmpeg.md)
- [How to crop a video to 9:16 vertical with FFmpeg](https://varg.ai/how-to/crop-a-video-to-vertical-with-ffmpeg.md)
- [How to add music to a video with FFmpeg](https://varg.ai/how-to/add-music-to-a-video-with-ffmpeg.md)
- [Best AI video editing tools for agents in 2026: 35 compared](https://varg.ai/how-to/best-ai-video-editing-tools.md)
- [Best AI video clipping tools in 2026: 3 compared](https://varg.ai/how-to/best-ai-video-clipping-tools.md)
- [Best AI video caption tools in 2026: 4 compared](https://varg.ai/how-to/best-ai-video-caption-tools.md)
- [Best AI transcript video editors in 2026: 4 compared](https://varg.ai/how-to/best-ai-transcript-video-editors.md)
- [Best AI video editing models in 2026: 4 compared](https://varg.ai/how-to/best-ai-video-editing-models.md)
- [Best AI video dubbing tools in 2026: 3 compared](https://varg.ai/how-to/best-ai-video-dubbing-tools.md)
- [Best AI video upscaling tools in 2026: 3 compared](https://varg.ai/how-to/best-ai-video-upscaling-tools.md)
- [Best video editing APIs for AI agents in 2026: 4 compared](https://varg.ai/how-to/best-video-editing-apis-for-ai-agents.md)