How this tool works
Set a start and an end, and everything outside that range is discarded. The preview plays the whole video so you can find the marks by watching rather than guessing, and each mark can be set from wherever the preview is paused. Nothing is uploaded — the file is decoded and re-encoded by your own device.
Trimming in a browser always re-encodes rather than copying the wanted part out. A player can seek anywhere, but a file can only begin at a keyframe, so cutting at an arbitrary second without re-encoding would either move the cut to the nearest keyframe or leave a broken opening. Re-encoding costs a little quality and puts the cut exactly where you asked for it. The quality is otherwise preserved: trimming should not quietly compress, and the compressor exists for anyone who wants a smaller file.
What to expect from the result
- The cut lands where you put it: not at the nearest keyframe, which is what tools that copy rather than re-encode have to do. That is the trade — a small quality cost in exchange for the cut being where you asked.
- Timestamps start again at zero: a trimmed file whose first frame still claims to be four minutes in plays as four minutes of nothing in several players and uploaders. Both the picture and the sound are rebased so the result starts at the beginning.
- Audio needs AAC, which not every browser can write: AAC encoding is unavailable in Firefox on every platform and in every browser on desktop Linux. Where it is missing the sound is dropped and the page says so before you start. Chrome, Edge and Safari keep it.
- A long source costs memory even for a short cut: the audio track has to be decoded in full before the wanted part can be taken from it, because there is no way to decode only a section. Trimming ten seconds out of an hour still reads the hour of sound.