Video Trimmer

How to trim a video

1. Drop a video on the box above, or click it to choose a file (up to 500 MB).
2. The video appears with a position slider. Play or drag to the first moment you want to keep and press Set start here. Move to the last moment and press Set end here. You can also type the times as seconds (75.5) or minutes:seconds (1:15.5). The coloured bar under the slider shows the selection.
3. Press Play selection to watch exactly what will be kept.
4. Choose the Cutting mode: - Fast: copy without re-encoding (cuts at the nearest keyframe); - Accurate: re-encode (frame-exact, slower), with an Output format: MP4 (H.264 + AAC) or WebM (VP8 + Opus).
5. Press Trim video. A progress bar appears with a Cancel button. When it is done, check the result in the second player and download it.

Fast mode or accurate mode?

Compressed video doesn’t store every frame as a full picture. Every second or so there is a keyframe, a complete image; the frames in between only store what changed since the previous ones. This is why the two modes behave differently.

Fast mode copies the existing video and audio data into a new file without decoding it (“stream copy”). It is very quick, even for a long file, and loses no quality at all, because nothing is re-compressed. The catch: a clip can only start on a keyframe. The cut therefore starts at the keyframe just before your start time, so the result may begin up to a few seconds earlier than requested (how much depends on how the video was encoded; phone videos usually have a keyframe every 1–2 seconds). The file keeps its original container: MP4 stays MP4, MOV stays MOV, WebM stays WebM, and other formats become MKV, which accepts any codec.

Accurate mode decodes the video and encodes it again, starting exactly on the frame you chose. The cut is frame-exact, but it takes much longer and the picture is compressed a second time (at a high quality setting, so the difference is usually invisible). Choose MP4 (H.264 + AAC) for the widest compatibility, including phones and messaging apps, or WebM (VP8 + Opus) for the web.

A simple rule: use fast mode when a second or two of margin doesn’t matter, and accurate mode when the clip must start on a precise frame.

Speed and limits

Everything runs in your browser with FFmpeg compiled to WebAssembly: the first time, about 30 MB are downloaded and then kept in your browser’s cache. Your video is never uploaded. The file is read directly from your disk in slices rather than copied into memory, which is what makes files of several hundred MB workable.

Be aware that a browser is slower than desktop software. Fast mode takes a few seconds even for large files. Accurate mode re-encodes on a single processor core: on a recent computer, a full HD video may take one to three times its own duration to process, and longer on a phone or an older laptop. For very long or very large videos, desktop tools such as FFmpeg, Shotcut or HandBrake remain the better choice.