What is video trimming?

Cutting the dead air off the start and end of a video is the most common edit there is: the five seconds of fumbling before a screen recording gets going, the accidental extra minute after the presentation ends, the shaky walk to the subject in a phone clip. This tool trims a video to the segment between a start and end time you specify, producing a new MP4 file.

The interesting part is how it cuts. The tool uses stream copying — it does not re-encode your video, it just slices the existing data at the boundaries you set. That makes trimming nearly instant even for large files, and it means zero quality loss: the output is bit-for-bit the same video data as the input. Your file is processed locally in your browser by a WebAssembly build of FFmpeg; nothing is uploaded, and a multi-hundred-megabyte clip never has to travel anywhere.

The trade-off of speed and losslessness is cutting precision. Stream copy can only start a cut at a keyframe — the compression points that occur every second or few seconds in most videos — so the actual start of the trimmed clip may land slightly before the timestamp you typed. For cutting out long, boring sections this is invisible. If you need frame-exact cuts, you want a re-encoding editor instead, at the cost of much longer processing.

Practical tips: timestamps are in seconds, so 90 means 1:30. Trim conservatively and check the result, then re-trim if needed. And since the output is MP4, source videos in other containers (like MOV or MKV) are re-wrapped accordingly.