Trim Video

How do I trim a video?

1. Drop a video onto the drop zone, or click to browse. A preview player loads so you can scrub through it.
2. Type a Start time as hh:mm:ss or plain seconds, or play to the right moment and set the start at the current playhead.
3. Type an End time the same way, or leave it blank to keep everything to the end of the video.
4. Choose a Method: Fast cut (nearest keyframe) for speed, or Precise cut (re-encode) for exact edges.
5. Click Trim video and download the clip.

Fast cut runs FFmpeg with -ss, -to and -c copy: it seeks to your start time, copies the compressed video and audio packets until the end time, and writes them to a new MP4. No frames are decoded, so a cut from a 1 GB file takes a second or two and the picture quality is untouched. The catch is that a copied stream can only begin on a keyframe, which cameras and phones place every 1 to 2 seconds, so the actual start may land slightly before the time you typed.

Precise cut decodes the clip and re-encodes it with libx264, which lets it start and stop on the exact frame you asked for. Use it for reaction clips, subtitles that must line up, or anything where a second of slack matters. Because the browser build of FFmpeg is single-threaded, expect roughly real-time to 3x real-time for 720p on a recent laptop, slower for 4K and on phones. The output is visually very close to the source but not bit-identical.

Times accept hh:mm:ss, mm:ss or a number of seconds, with decimals allowed, so 1:30, 90 and 00:01:30.5 all work. Leaving End blank keeps the video to its natural end, which is the quickest way to remove an awkward opening. The preview makes this easier: pause where the clip should begin, set Start at the playhead, pause again for the end, and check the range before you commit.

Nothing is uploaded. The video is read from your device into the browser's memory and the clip is written back to your downloads folder. The FFmpeg engine (about 30 MB) is fetched once and cached, so later trims start instantly and work offline. Files up to about 1 GB are comfortable; the browser's 2 GB WebAssembly memory limit is the practical cap, and the preview player itself needs a format your browser can play, which for most people means MP4, MOV or WebM.

How it works: this tool uses FFmpeg compiled to WebAssembly (ffmpeg.wasm), loaded into your browser. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

Frequently asked questions