Speed Up or Slow Down a Video

How to change a video's speed

1. Open the video. Tap Open a video or drop an MP4 or MOV on the box. The preview starts playing at the chosen speed.
2. Pick a speed. Slower for slow motion, Faster for a quicker clip, Timelapse for a sunset, a drive or a bake squeezed into seconds, or type any speed from 0.1× to 100×. The new length is shown at once: a 1:30 clip at 2× becomes 45 seconds.
3. Pick a direction. Forwards as filmed, Backwards for the whole clip reversed, or Boomerang: forwards then backwards, 1 to 5 times. Picking Backwards or Boomerang before any speed sets 1×; any speed still combines with it, so 0.5× backwards is reversed slow motion.
4. Choose the sound. Keep the pitch (0.5× to 2×) keeps voices sounding normal; Speed the sound too (0.25× to 4×) moves the pitch with the speed; No sound leaves the track out, the default above 4× and for timelapses.
5. Save. Tap Save MP4. The page decodes every frame, keeps or repeats the ones the speed needs and encodes a new H.264 MP4, with the speed in its name (beach-2x.mp4, sunset-timelapse-60x.mp4).

What happens to the frames

The saved video plays at the source's own frame rate, at most 30 frames a second. Faster keeps every n-th frame: at 2× a 30 fps clip keeps frames 0, 2, 4 …, at 8× every eighth, and a 60× timelapse one frame in sixty, so a 5-minute sunset (9,000 frames) becomes 150 frames, 5 seconds. A 60 fps phone clip at 2× keeps every fourth frame, because the result runs at 30 fps.

Slower can only show the frames the camera recorded. At 0.5× each frame of a 30 fps clip is shown twice, at 0.25× four times, which looks a little steppy on fast motion. Smooth blends each repeated frame with the next one (at 0.5×, the in-between frame is half of each), which softens the steps into a short dissolve; it does not invent the motion in between, as AI slow-motion apps try to. The best slow motion comes from a clip filmed at 60, 120 or 240 fps: a 60 fps clip at 0.5× shows every recorded frame once, with nothing repeated.

What happens to the sound

Playing sound faster raises its pitch by the same factor, as a sped-up tape does: 2× is a full octave up, 1.5× about seven semitones. Keep the pitch avoids that with a time-stretch written for this page: it cuts the sound into overlapping 40 ms slices and lays them down closer together (faster) or further apart (slower), moving each slice by up to 12 ms to where its waveform best lines up with the one before. On the test tone, a 440 Hz sine at 1.5× comes out at 440 Hz and two-thirds the length. Speech and music keep their pitch too, but past about 1.5× a voice can sound a little phasey or echoey, and drums can lose some snap; that is why it stops at 2× (and at 0.5× the other way).

Playing it backwards

A video file cannot simply be read from the end. H.264 stores a full picture (a key frame) only every second or two; the frames in between are stored as changes from the frames before them, so frame 59 of a GOP (a group of pictures, from one key frame to the next) can only be rebuilt by decoding frames 0 to 58 first. Backwards therefore works one GOP at a time, starting with the last: it decodes the GOP from its key frame, holds the pictures it needs, writes them out last-first, and moves to the GOP before. At most 256 MB of pictures are held at once (32 frames at 1920 × 1080, about 870 at 320 × 240); a longer GOP is decoded again from its key frame for the rest, which is slower but never runs out of memory.

The sound is reversed sample by sample, after the speed change, so a 2-second clip comes back as exactly 2 seconds of sound played from the end. A rising tone falls, footsteps walk backwards and a splash sucks itself back in. Boomerang plays the clip forwards, then backwards, without showing the turning frame twice: with 10 frames and 2 repeats the order is 0 … 9, 8 … 0, 1 … 9, 8 … 0, 37 frames, so it loops without a stutter. Its sound plays forwards with the forward part and reversed with the backward part; No sound makes a silent boomerang, the usual choice for a loop.

What it cannot do

• It does not invent frames. Slow motion repeats or blends the frames the camera took; it never makes new in-between frames (no AI interpolation).
• Keep the pitch works only from 0.5× to 2×, and stretched speech can sound phasey past about 1.5×. Speed the sound too works from 0.25× to 4×. Above 4× the MP4 is silent.
• It needs WebCodecs, in Chrome, Edge or Samsung Internet 94 or later, Safari 16.4 or later, or Firefox 130 or later on a computer, and it opens only MP4 and MOV with a picture format this browser can decode. HEVC (H.265, the iPhone default) opens in Safari and on some other devices only; WebM, MKV and AVI are refused.
• Sound is kept only when it is AAC, the format almost every phone records. A clip with other sound (PCM, Opus, AC-3) is saved silent, and the page says so before you save.
• Length caps: the clip may be at most 3 minutes and 2 GB, and the saved video at most 12 minutes, so 0.25× takes clips up to 3 minutes and 0.1× up to 1 minute 12 seconds. The saved MP4 is at most 1,920 px on its long side; a 4K clip is scaled down.
• It takes as long as decoding every frame. Measured on the build server (4 cores, headless Chrome, H.264 in software): a 30-second 1920 × 1080 30 fps clip with stereo sound took 17 s at 2× with the pitch kept, 55 s at 0.5×, 95 s at 0.25× and 6 s as a 30× timelapse; a 3-minute 1080p clip took 93 s at 2×, and at 0.25× (the 12-minute maximum, 21,600 frames, a 460 MB file) 11 minutes. A phone is slower, and the tab must stay open until it finishes. The new MP4 is built in memory before it is saved, so a long 1080p slow motion can be too big for a phone with little free memory; a faster speed or a shorter clip makes a smaller file.
• Backwards and Boomerang take clips of up to 1 minute, because the clip is decoded GOP by GOP rather than in one run, and a boomerang is several times as long as the clip and is built in memory: a 1-minute 1080p clip as a 5× boomerang is 10 minutes of video, about 1.3 GB. Measured on the build server (4 cores, headless Chrome, H.264 in software, a key frame every second): a 1-minute 1920 × 1080 30 fps clip with stereo sound took 73 s backwards at 1×, against 64 s forwards at 1×, and a 10-second 1080p clip as a 2× boomerang (40 s of video) took 49 s. Reversed speech is not words: it comes out as sounds in a made-up language, and music runs backwards, its notes swelling instead of fading.
• It does not trim or cut a clip, and it changes the whole clip's speed, not one part of it. Trim the clip first in your phone's gallery if you only want a part.

Questions

Can I speed up a video and keep the sound?

Yes, from 0.5× to 2× with the voice at its normal pitch: choose Keep the pitch. The page time-stretches the sound (it cuts it into 40 ms pieces and overlaps them more or less tightly, lining each piece up with the one before), so speech at 1.5× sounds like a fast talker, not a chipmunk. From 0.25× to 4× you can also choose Speed the sound too, which plays the sound faster or slower like a tape, so the pitch moves: 2× is one octave higher. Above 4× and for timelapses the default is No sound.

How do I make a timelapse from a video?

Open the video, tap Timelapse 30×, 60× or 100× and tap Save MP4. At 60× a 3-minute sunset becomes 3 seconds; at 30× a 2-minute drive becomes 4 seconds. The page keeps one frame in every 30, 60 or 100 of a 30 fps clip and writes them at 30 fps, without sound. For a smoother timelapse, record longer: 100× needs 100 seconds of video for each second of timelapse.

Can I reverse a video with its sound?

Yes. Open the video, tap Backwards and Save MP4: the picture and the sound are both reversed, the sound sample by sample, so they stay in step and the saved MP4 is as long as the clip (a 2-second clip stays 2 seconds). Keep the pitch or Speed the sound too still apply if you pick a speed as well. For a clip that goes there and back, tap Boomerang: the sound plays forwards, then reversed, or choose No sound. Reversed speech does not turn into words; it sounds like a made-up language.

Is there a watermark or a size limit?

No watermark is added and there is no sign-up. Because nothing is uploaded there is no upload limit either; the limits are the 3 minutes and 2 GB above, which keep the job within a phone's memory and patience.

Why is my slow-motion video choppy?

Because a 30 fps clip at 0.25× has only 7.5 real frames for every second you watch. Try Smooth, or film at 60 fps (most phones offer it in the camera settings): a 60 fps clip at 0.5× uses every recorded frame once, so nothing is repeated.