Almost every video has unwanted footage at the beginning and end. The first few seconds often capture the moment before the action starts: someone adjusting the camera, saying "is it recording?", or waiting for a cue. The last few seconds show someone reaching for the stop button, the camera being set down, or a pause after the action ends.
Trimming these dead zones takes seconds and makes the video feel intentional rather than accidental. Professional content creators trim every video before publishing, and it is usually the single most impactful edit you can make.
The difference between trimming and cutting
Trimming means removing content from the beginning, the end, or both. You are shortening the video by adjusting the start point and end point. The middle of the video remains untouched.
Cutting (or splitting) means removing a section from the middle of the video or dividing it into separate parts. This requires identifying precise in-points and out-points within the footage.
Most quick video edits only need trimming. You recorded a 45-second clip, the first 3 seconds are setup and the last 2 seconds are someone saying "okay, stop." Trimming gives you a clean 40-second video.
How trimming affects file size
Trimming always reduces file size because you are removing data. The reduction is roughly proportional to the duration removed. If you trim 10% of the duration from a 100 MB file, the output will be approximately 90 MB (the exact ratio depends on whether the trimmed sections had above or below average bitrate).
Some tools can trim without re-encoding (called "lossless" or "stream copy" trimming). This is faster and preserves the original quality exactly, but the cut points may not land precisely where you want them because they snap to the nearest keyframe (more on this below).
Why trim points sometimes seem inaccurate
Video compression groups frames into GOP (Group of Pictures) structures. A GOP starts with a keyframe (I-frame) that contains a complete image, followed by predictive frames (P-frames and B-frames) that store only the differences from nearby frames.
When you trim without re-encoding, the cut can only happen cleanly at a keyframe boundary. If you set your trim point at 3.2 seconds but the nearest keyframe is at 3.0 seconds, the tool will start the video at 3.0 seconds. This can mean your trim includes a couple seconds of footage you wanted to remove.
When you trim with re-encoding (which is what most browser tools do), the cut can happen at any frame, giving you precise control. The trade-off is that re-encoding takes longer and introduces a single generation of compression.
Batch trimming multiple clips
If you have multiple clips that all need the same amount trimmed (for example, the first 2 seconds of every file from a batch recording), most command-line tools support scripted trimming. For occasional trimming of individual videos, a visual tool where you can preview the start and end points is faster and less error-prone. The MediaBrew trim tool lets you scrub through the video and drag handles to set the start and end frames, then re-encodes the selection via FFmpeg WebAssembly so the cut lands exactly where you placed it.