Trim a video without uploading it
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Worked example
Trimming is the one operation where a browser can genuinely beat a desktop app, because copying streams needs no encoder at all.
There are two ways to cut a video, and they differ in a way that matters:
| Approach | Trade-off |
|---|---|
| Stream copy | Instant, lossless, cuts only on keyframes |
| Re-encode | Frame-exact, slower, loses a little quality |
The equivalent command
ffmpeg -ss 30 -i holiday-clip.mp4 -t 15 -c copy -avoid_negative_ts make_zero -movflags +faststart -threads 1 -y out-holiday-clip.mp4This is exactly what runs in the browser. Copy it to run the same job locally on a file too large for a tab.
For a frame-exact cut the video has to be decoded and re-encoded, which is slower and loses a little quality:
The equivalent command
ffmpeg -ss 30 -i holiday-clip.mp4 -t 15 -c:v libx264 -crf 23 -pix_fmt yuv420p -preset veryfast -c:a aac -movflags +faststart -threads 1 -y out-holiday-clip.mp4This is exactly what runs in the browser. Copy it to run the same job locally on a file too large for a tab.
Why it matters
The usual advice is "just use -ss and -t", which hides the decision that actually matters. Copying streams is instant and bit-for-bit lossless, but it has no decoder, so it cannot produce a frame that was never a keyframe — the cut lands on the nearest preceding one, which can be seconds early. Re-encoding is frame-exact and costs time and a little quality. Both are offered here, and the keyframe caveat is stated rather than hidden.
What this needs
A 30.7 MB FFmpeg download, cached by your browser after the first time. Nothing is downloaded until you click.