Video speed changer

The new length is the old length divided by the speed. These are the speeds that suit screen recordings most often.

Speed up a part when viewers should see that it happened: a long install, a form being filled in, a page building. Fast footage says that time passed and shows the path from start to finish.

Cut a part when it adds nothing: a false start, a wrong turn, a long pause while you think. The video trimmer removes the ends of a clip, and the silence remover takes out quiet stretches throughout. This tool changes the whole video at one speed, so to speed up just one part, split that part out first.

Speech stays easy to follow up to around 1.5×. By 2× it needs close attention, and past that it is mostly useful as a sign that something is happening, so consider removing the sound and adding a caption instead.

Playing sound faster normally raises its pitch, which is the chipmunk effect. To avoid that, the tool rebuilds the sound from short overlapping pieces, 120 milliseconds long below 2× and 50 milliseconds at 2× and above. Pieces are repeated to slow the sound down or skipped to speed it up, and each is nudged by up to 20 milliseconds so its waves line up with the last one.

The picture and sound share one rule: a moment in the original lands at its time divided by the speed. Each piece of sound is tied to that moment, so the two cannot drift apart, however long the video is.

Going faster shows more frames each second, so the tool drops frames to stay at 60 per second or fewer. Going slower holds each frame on screen for longer. It does not invent new frames, so a 30 frames per second video at 0.25× plays at 7.5 frames per second, which looks like a series of steps on fast movement.

• iMovie on a Mac. Select the clip, press the Speed button and choose Slow, Fast or Custom. Slowing a clip lowers its pitch and speeding it up raises it, unless you tick Preserve Pitch. Apple’s guide has the steps.
• Clipchamp on Windows. Select the video on the timeline, open the speed tab and drag the slider or type a value from 0.1× to 16×. Microsoft’s guide has the steps.
• FFmpeg. For double speed from the command line, run ffmpeg -i in.mp4 -vf "setpts=PTS/2" -af "atempo=2" out.mp4. Its atempo filter takes values from 0.5 to 100, so 0.25× needs two filters in a row.
• A browser extension. Extensions that control video speed change playback for you alone. They do not make a new file, so viewers will not see the change.

• One speed applies to the whole video. To change only part of it, trim that part out first with the video trimmer.
• Speech at 0.25× or 4× is stretched a long way. The pitch stays, but slowed speech can sound echoey or robotic, very fast speech loses syllables, and at 0.25× a sharp sound such as a click spreads out and starts a little before its frame. The preview uses your browser’s own processing, so check the sound in the finished video.
• The result is always a new H.264 and AAC MP4 at the same picture size. Re-encoding can soften small text a little. To shrink it afterwards, use the video compressor.
• Surround sound comes out as stereo. Only the first video and sound tracks are kept, and subtitles, chapters and metadata are left out. An HDR video comes out in standard colour.
• In Chrome and Edge on a computer, a big result is written straight to a file you choose. In other browsers files go up to 2.0 GB and the result has to fit in memory.
• Every frame is re-encoded, so a long video takes a while. Keep the tab open until it finishes.
• It needs WebCodecs, and the page says so before you start if your browser does not have it. We have tested it in Chrome and in Safari’s engine on a Mac, but not yet in Firefox. It is meant for a computer; on a phone, long videos may run out of memory.