Trim and cut video

Why there are two ways to cut

A video does not store every frame as a full picture. Every few seconds it stores a complete one, called a keyframe, and in between only what changed. A frame between two keyframes cannot be shown without the frames before it, so a cut that starts there needs work. There are two ways to do it:

• Exact cut decodes the part you chose and encodes it again with your device's video encoder, so the result starts and ends on the exact frames you picked.
• Fast copy copies the compressed video untouched. It is instant and loses nothing, but it has to start at the keyframe before your start point, so it carries a little extra at the beginning.

What the difference looks like

We cut 4 seconds, from 3.3 to 7.3, out of two 10 second full HD clips with a keyframe every 2 seconds:

Fast copy keeps the original quality and the original size per second, and here it also carried the 1.3 seconds between the keyframe at 2 seconds and the chosen start. Those extra frames are marked to be skipped, and in Chrome the copy started playing on exactly the chosen frame and lasted 4 seconds, but some players and editors show them. The exact cut was a fraction of the size, because the encoder used a far lower bitrate than the phone did.

Which one to choose

• Fast copy for long videos, for keeping the original quality untouched, and whenever starting a moment early does not matter. Keep the container on Same as the original, since copying needs a format that can hold the video as it is.
• Exact cut for clips that must start on a precise frame, for a smaller file to send, or for a different format such as WebM.

For a strict size limit, trim first, then use compress video: the shorter the video, the more data each second can have.

Choosing the part

Drag the two handles on the timeline, or play the video and press Start here and End here at the right moments. You can also type the times. Loop selection plays the chosen part over and over so you can check both ends before cutting. Each video in the list keeps its own selection.

If something goes wrong

• The preview does not play. Some browsers cannot show every format, but trimming can still work. Chrome and Edge read the most formats.
• It says the browser cannot decode the video. Videos from recent iPhones are often HEVC, which not every browser can decode. Try Chrome or Edge, or Safari on a Mac.
• It says the video cannot be written to the chosen format. Choose MP4 or WebM instead.
• A fast copy starts too early. Use Exact cut for that video.

How we tested. Two 10 second clips drawn in Chrome at 1920 by 1080 and 30 fps and saved as H.264 at 40 Mbit/s with a keyframe every 2 seconds, without sound. Cut with the trimmer's own code in Chrome on a desktop computer; playback of the fast copy checked in a video element.