Trim a video
Keep the section you meant and drop the rest.
How it works
Cutting the tail off a video does not have to touch a single pixel. A compressed clip is a sequence of packets, most of which describe how one frame differs from the one before it, punctuated by keyframes that stand alone. Keep the head of the file and you can copy those packets straight into a new container, rewrite the duration, and stop. The footage that comes out is bit-identical to the footage that went in.
Cutting from the middle cannot work that way, and the reason is the same sentence read backwards. A frame three seconds in is stored as a difference from frames one and two, which the cut is throwing away. Nothing is left to decode it against, so the section has to be decoded while those frames still exist and encoded again on its own.
What that costs, measured
The two paths are far apart and the page tells you which one ran. On a ten-second 640×360 H.264 clip in Chrome: keeping 0:00 to 0:07 copied the packets in 139 milliseconds and produced 1,480,845 bytes. Keeping 0:03 to 0:07 had to re-encode, took 605 milliseconds, and produced 961,429 bytes — which is 240 KB per second of video against the source's 207 KB.
That comparison is the one worth remembering. A re-encoded cut is not smaller because it is shorter; per second it is usually larger, because the encoder is reproducing a picture that has already been through one lossy pass and spends bits describing the previous encoder's artefacts. If the goal is a smaller file rather than a shorter one, trim first and compress second.
The result line says `frames copied, not re-encoded` or names the codec it re-encoded with, so this is never something you have to infer from how long it took.
What happens to the audio
Audio packets are much shorter than video ones, tens of milliseconds rather than seconds, and each is independently decodable. So the audio track cuts cleanly at any mark and the two tracks are aligned to the same output start rather than each beginning wherever its own data allowed.
A clip that begins mid-word will still begin mid-word. Nothing here fades, and a hard cut into speech is audible. Where that matters, move the start back to the nearest silence rather than expecting the tool to hide it.
Containers and what comes out
The output is an MP4 whenever the source video is a codec MP4 carries, which covers H.264, HEVC and AV1 — in other words nearly every camera, phone and screen recorder. VP8 and VP9 sources come out as WebM, because that is the container those codecs are expected in and a VP9 file with an .mp4 extension is the kind of thing that opens in a browser and nowhere else.
Nothing is uploaded at any point. The video is read by the page, cut in a Web Worker on your own machine, and handed back as a download. That is worth more here than on most tools: a screen recording is a picture of whatever else was on the screen, and a clip off a phone carries a room and the people in it.
Questions
Is my video uploaded anywhere?
No. Open developer tools, switch to the Network tab, and run a trim. You will see same-origin requests for the page's own code, and nothing else. The file is read by the page, processed on your machine, and returned as a download. Nothing carrying it leaves the tab.
Why did one trim take a fraction of a second and another take several?
Because only one of them re-encoded. A cut that keeps the start of the clip copies the compressed packets straight across, which is bounded by how fast the file can be read. A cut that begins part-way in has to decode the section and encode it again, because the frames at your mark are stored as differences from frames the cut discards. The result line names which happened.
How long a video can this handle?
The limit is memory rather than duration. The source and the finished cut both live in the tab at once, so the tool projects the total before it starts and refuses with the figure if it exceeds the roughly 250 MB a browser tab has to work in. On a desktop a two-hour recording trimmed to a few minutes is comfortable. On a phone the ceiling arrives much sooner, and the refusal names how much of the clip would fit.
Which formats can I trim?
MP4, MOV, WEBM, MKV, AVI and MPEG transport streams, provided the video inside is a codec the browser can decode. H.264, VP8, VP9 and AV1 are read everywhere. HEVC is read by Safari and by Chrome on hardware that supports it, and by nothing else, so an iPhone recording in HEVC may trim on one machine and not another. The page says which it found rather than failing silently.