Change Video Speed
Processed locally in your browser — nothing is uploaded.
Quick answer
To change a video’s speed, you rescale its timeline so the picture and the audio play faster or slower together — a 2× speed-up makes a timelapse, a 0.5× slow-down makes slow-motion. This tool does it in your browser with ffmpeg.wasm, adjusting the video with `setpts` and the audio with `atempo` so they stay in sync. Nothing is uploaded; a one-time ~10 MB engine loads on first use.
How to change video speed
Load your video
Drop your clip in. On first use the ~10 MB engine downloads and caches, then runs instantly next time.
Choose a speed
Pick a multiplier — 2× or 4× to speed up, 0.5× or 0.25× to slow down. Both the picture and the sound are rescaled by the same factor.
Export the retimed clip
Encode and download. Changing speed re-encodes the frames, so a longer clip takes a single-threaded while to finish.
How does changing speed work?
A video is a stream of timestamped frames. Speeding it up or slowing it down means rewriting those timestamps. The video filter `setpts` multiplies each frame’s presentation time: setpts=0.5*PTS halves every timestamp so the clip plays in half the time — twice as fast — while setpts=2.0*PTS doubles them for half speed. The audio is handled separately by `atempo`, which stretches or compresses the sound *without* changing its pitch, so sped-up voices do not turn into chipmunks. Applying the matching factor to both keeps sound and picture locked together.
| Speed | setpts factor | Effect | Common use |
|---|---|---|---|
| 4× | 0.25 × PTS | Much faster | Long timelapse, screen recordings |
| 2× | 0.5 × PTS | Faster | Trimming dead time, quick demos |
| 1× | 1.0 × PTS | Unchanged | Reference |
| 0.5× | 2.0 × PTS | Slower | Slow-motion, detail replays |
| 0.25× | 4.0 × PTS | Much slower | Dramatic slow-mo |
Why does extreme slow-motion look choppy?
Slowing footage down spreads its existing frames over more time, but it cannot invent new ones. A clip shot at 30 fps and slowed to 0.25× effectively shows about 7–8 real frames per second, so the motion stutters. True smooth slow-motion has to be *filmed* at a high frame rate (120 or 240 fps) so there are enough frames to stretch. For gentle slow-downs the stutter is minor; for dramatic slow-mo from ordinary footage, expect some choppiness — it is a limit of the source, not the tool.
What are the browser limits?
Retiming re-encodes every frame on the single-threaded WebAssembly engine, so it is slower than a desktop app and bound by the tab’s memory on large inputs. Speeding a clip *up* produces a shorter output that encodes quickly; slowing it *down* produces a longer one that takes more time and memory. Keep the source short — especially before a big slow-down — and the job stays comfortable.
Is my video uploaded to change its speed?
No. The retiming runs in your browser through ffmpeg.wasm — the file is processed in memory and returned as a download, never uploaded. Confirm it in DevTools → Network: no request is made, and it works offline once the page has loaded. Your footage stays private.
Related video and audio tools
Making a timelapse to share? Compress the sped-up clip for a small file, or turn it into a loop with video to GIF. Speeding up a talky clip and want the sound on its own? Extract the audio as MP3 — although if you only need the visuals, mute it instead. Trim to the section you want to retime first with trim video.
Your files never leave your device
Processed locally in your browser — nothing is uploaded. You can verify it yourself: open your browser’s developer tools, watch the Network tab, and use the tool — no upload request is ever made.
Frequently asked questions
How do I make a timelapse from a normal video?
Speed it up with a high multiplier like 4×. The tool rescales the timeline with setpts so a long clip plays back quickly, compressing minutes into seconds.
Does changing speed keep the audio in sync?
Yes. The picture is retimed with setpts and the audio with atempo by the same factor, so they stay locked together. The audio is stretched without changing pitch, so voices sound natural.
Why do sped-up voices not sound like chipmunks?
Because atempo changes the tempo of the audio without shifting its pitch. A naive speed-up would raise the pitch; this keeps voices at their normal pitch while playing faster.
Why does my slow-motion look choppy?
Slowing footage spreads its existing frames over more time but cannot create new ones, so ordinary 30 fps video stutters when slowed heavily. Smooth slow-mo needs footage shot at a high frame rate.
Can I speed a video up by 4× or more?
Yes. The tool chains the audio filter to cover multipliers beyond 2× while keeping pitch natural, so large speed-ups stay in sync and sound right.
Does changing speed reduce quality?
It re-encodes the frames, which adds the usual small encode loss. Use a low CRF if you compress afterwards to keep it minimal; the retiming itself does not blur the picture.
Is speed change done privately?
Yes. It runs entirely in your browser via ffmpeg.wasm with no upload. You can verify there is no network request in your developer tools.