Trim Video

Fast versus precise, and why there is a choice

Video is not stored as a sequence of independent pictures. Most frames describe only what changed since the frame before, and only occasional keyframes contain a complete image. Playback can only start cleanly from a keyframe.

The fast method copies the compressed streams straight through without decoding them, which takes seconds regardless of clip length and loses no quality at all. The catch is that it can only cut at keyframes, so your clip may begin up to a few seconds earlier than you asked. Keyframes typically occur every two to ten seconds depending on how the video was encoded.

The precise method decodes and re-encodes, which allows an exact cut at any frame. It takes as long as a full compression pass and slightly reduces quality, since the video is re-compressed.

In practice: use fast for rough cuts and for trimming dead air off the ends, and precise when the cut has to land on a specific word, beat or action.

Finding your cut points

The preview player above accepts scrubbing, and its current time is what you want to note down. Both the start and end fields accept either mm:ss or a plain number of seconds, so 1:23 and 83 are equivalent.

When using the fast method, set the start a little after the moment you actually want, not before. Because the cut snaps backwards to the previous keyframe, aiming slightly late usually lands closer to your intent than aiming early.

For the end point, precision matters less: the encoder simply stops, so the end is accurate in both modes.

Trimming as a way to shrink a file

Removing the parts you do not need is usually the most effective size reduction available, and it costs nothing in quality when using the fast method. A two-minute clip cut from a ten-minute recording is a fifth of the size with no re-encoding at all.

That makes trimming the right first step before compressing. Cut, then compress the shorter clip — compressing first and trimming afterwards wastes time encoding footage you were going to discard.

It is also the practical answer to a video that is too large for a messaging app: often the file only needs to lose its long silent introduction.

Output format

The result is written as MP4 with H.264 video and AAC audio, which is the most broadly compatible combination available — it plays on every phone, browser, television and editing application in common use.

The file is also written with the metadata index at the front, which allows it to start playing before the whole file has downloaded. That matters if the clip is going onto a web page.

In fast mode the audio and video streams are copied unchanged, so the output quality is identical to the source. In precise mode both are re-encoded at a good default quality.

One consequence of always writing MP4 is that the container may change even when the streams do not. A WebM file trimmed in fast mode comes out as MP4 carrying the same video data, which is usually what you want — but if you specifically need the original container preserved, a desktop tool will give you that control and this one will not.