How to slow down or speed up a video
Changing the speed of a clip is simple, but a few details decide whether the result looks good: the frame rate you started with and what you do with the sound.
Change the speed in four steps
- Open Change video speed and add your clip.
- Choose a speed from 0.25x to 4x.
- Choose to keep the sound (its pitch is preserved) or remove it.
- Press Change the speed and download the result.
Which speed for which job
| Speed | Effect | Good for |
|---|---|---|
| 0.25x to 0.5x | Slow motion | Sport, splashes, a trick or a gesture you want to study |
| 0.75x | Slightly slower | Tutorials where the viewer has to follow along |
| 1.25x to 1.5x | Slightly faster | Lectures and long explanations, with the voice still natural |
| 2x to 4x | Fast forward | Boring stretches, screen recordings, a quick time-lapse look |
Why slow motion can look jerky
A video is a series of still frames, usually 24, 30 or 60 per second. Slowing it down cannot invent frames that were never captured, so at 0.25x each frame stays on screen four times as long. A clip filmed at 30 frames per second then shows only about 7 distinct pictures every second, and it looks stepped. If you plan to slow something down, film it at 60 or 120 frames per second when your phone allows; slowed to half or a quarter, it stays smooth.
What happens to the sound
If you speed up audio the simple way, a voice turns high and squeaky; slowed, it turns low and slurred. This tool stretches the sound in time without changing its pitch, so speech stays recognisable. It works well up to about 2x. At 3x or 4x speech is very fast, and at 0.25x it sounds drawn out. For fast-forward clips of an activity there is often no reason to keep the sound: choose to remove it and add music later.
Practical limits
- Slowing a clip down makes the file larger and the result longer, and the result must stay under the maximum duration.
- To change the speed of only part of a video, trim that part first, change its speed, and join the pieces again with Merge videos.
- If the sound ends up too loud or quiet afterwards, use Change video volume.
What happens to the file size
Speed changes affect duration, and duration is half of the size equation. Slowing a clip to 0.5x doubles its length, so the file roughly doubles unless the encoder lowers the bitrate to compensate. Speeding it up to 2x halves the length and usually cuts the size substantially. The picture is re-encoded in both directions, so a speed change is a generation: start from the best copy you have and do not repeat it.
Extreme speeds and audio
Pitch-preserving audio works well within a moderate range. Beyond about 2x, consonants start to blur together even though the pitch is right, and at 4x speech is usually unintelligible. At 0.25x, the stretched sound has a slight echo, which is normal for time-stretching. For a time-lapse of clouds, traffic or a build, remove the sound entirely: it carries no information and only adds noise. For a lecture at 1.5x, keep it: that is the case this feature is really for.
Mixing speeds in one video
A common edit is a normal intro, a slow-motion middle and a fast-forward ending. There is no single-pass tool for that, and there does not need to be: cut the clip into its three parts with the trimmer, set each part’s speed, then join them in order with Merge videos. Choose the same resolution for every part so the join does not rescale anything, and check the audio at the joins: a speed change also changes the sound’s length, and a cut mid-word becomes obvious.
Speeding up to fit a limit
When a platform refuses a clip that is slightly too long, speeding it up is a tempting fix. It works, but the result sounds hurried and viewers notice. Trimming is usually better: a 62-second clip that becomes 58 seconds by cutting two seconds at each end plays naturally, while the same clip at 1.07x sounds subtly wrong for its whole length.
Several clips at once
When a set of clips needs the same treatment, batch processing saves time: each file is processed with the same speed and sound options, and the results come back in a ZIP. This is common for time-lapse sequences assembled from several recordings, or for a folder of tutorial clips that all need the same slight speed-up. Check one result before using the rest, since a clip filmed at a different frame rate behaves differently when slowed.