Video Speed Changer
Faster, slower or a time-lapse — with the sound kept in tune, pitched up, or gone.
Speed and sound
The preview plays at the speed you choose, with the sound set as below — exactly what the file will do.
What will happen
About the Video Speed Changer
Change how fast a video plays, and keep the new speed in the file itself: from 0.25× (quarter speed, slow motion) to 16× (a time-lapse). A speed of s makes the video ÷ s as long — a 2-minute clip at 2× becomes 1 minute — and every frame keeps its place in the new timeline.
The sound can keep its pitch (time-stretching, so voices still sound like themselves at 1.5×), follow the speed like a tape played faster, or be left out. Speeding up drops frames, slowing down repeats them: no new frames are invented, so slow motion looks stepped rather than smooth.
Everything happens inside your browser with its own video encoder, so the file is never uploaded.
How to use it
- Drop a video on the box or click Choose a video. MP4, MOV, M4V, MKV, WebM, MPEG-TS and 3GP files can be read.
- Pick a speed — a preset such as 0.5×, 2× or 8×, or type any value between 0.25 and 16 — and watch the preview play at that speed.
- Choose what the sound does: Keep the pitch, Pitch follows the speed, or No sound. Optionally set the frame rate of the result and the quality.
- Read What will happen, press Change the speed, then play the new video on this page and Download it.
Examples
2:00 MP4, 30 fps, stereo AAC · speed 1.5× · Keep the pitch
1:20 MP4 at 30 fps — one frame in 1.5 is kept and the sound is stretched, so the speaker still sounds normal
0:10 clip, 30 fps · speed 0.5× · frame rate 30
0:20 at 30 fps: each original frame is shown twice (repeated, not interpolated), and the sound plays an octave lower if its pitch follows the speed
20:00 screen recording, 30 fps · speed 16× · No sound
1:15 at 30 fps — about one frame in 16 is kept
What happens to the frames
A video is a list of pictures with times. At speed s every time becomes t ÷ s, so the file is ÷ s as long.
- Faster than 1×: keeping every frame would give an unplayable frame rate (a 30 fps clip at 4× would be 120 fps), so frames are dropped to reach the frame rate you chose. At 8× or 16× that is a time-lapse: about one frame in 8 or 16 survives.
- Slower than 1×: there are not enough frames to fill the longer result, so the previous frame is repeated. This tool has no AI frame interpolation, so slow motion is stepped, not fluid. Video shot at a high frame rate (120 or 240 fps on a phone) slows down smoothly because the frames are really there — keep its own frame rate for the result.
What happens to the sound
- Keep the pitch stretches the sound in time without moving it in pitch, with Signalsmith Stretch (an MIT-licensed time-stretcher that runs in your browser). Best for speech and tutorials at 0.5×–2×.
- Pitch follows the speed resamples the sound, exactly like playing a tape or record faster: 2× sounds an octave higher, 0.5× an octave lower. The resampler is band-limited, so speeding up does not add the harsh aliasing a naive speed-up would.
- No sound leaves the audio track out, which is usual for a time-lapse.
Above 4× a stretched voice cannot be understood any more, so the stretcher is not offered there; choose Pitch follows the speed or No sound instead.
Quality and format
Changing the speed rewrites the picture, so the video is re-encoded in the same container (MP4, MOV, MKV or WebM) with a codec this browser can encode — H.264 where possible. Choose Very high quality to keep the loss smallest. The sound is re-encoded too, because its samples change.
The tool uses WebCodecs, the browser’s own encoder, through the Mediabunny library: no server, no upload, and no 32 MB WebAssembly download.
Limitations
- Needs WebCodecs (current Chrome, Edge, Safari or desktop Firefox) and a decoder for the video’s codec; HEVC needs a hardware decoder in the browser.
- No AI frame interpolation: slow motion repeats frames instead of inventing new ones.
- Keeping the pitch holds the whole sound in memory, so it is offered for videos of up to about 7 minutes (less for slow motion, where the result is longer). Longer videos can still use “Pitch follows the speed”.
- Only the first sound track follows the new speed; extra audio tracks are left out.
- HDR video is converted to standard dynamic range when re-encoded, and subtitle tracks and chapters are not carried over.
- Variable frame rates are evened out to the constant frame rate you choose.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
Will slowing a video down make it smooth slow motion?
Only if the video was filmed at a high frame rate. This tool repeats frames to fill the extra time, so a 30 fps clip at 0.25× shows each frame four times. Phone video shot at 120 or 240 fps has the frames to be genuinely smooth at quarter speed.
Why does the sound change pitch?
Only in the Pitch follows the speed mode, which is the tape effect: 2× is an octave up. Choose Keep the pitch and the sound is time-stretched instead, so a 1.5× tutorial still sounds like the person speaking.
Does changing the speed lower the quality?
The picture is encoded again, which loses a little detail like any export. Pick Very high quality for the smallest loss. Dropping frames (speeding up) does not blur anything; repeating frames (slowing down) does not add detail.
Can I change the speed of a video on my phone?
Yes, in current mobile browsers. Long videos and 4K take a while on a phone, and the tool shows a progress bar with a Cancel button while it works.
What is the difference between this and a player’s speed setting?
A player only changes how it plays the file. This tool writes a new file at the new speed, so it stays that way wherever you post or send it.
Is my video uploaded?
No. It is decoded, retimed and encoded inside your browser, and the result is saved to your device.