Slow down a video online

How to make a slow-motion clip in your browser

1. 01

   Open the video

   Drag an MP4, MOV, MKV, WebM or AVI onto the drop zone above. Nothing is uploaded; the browser reads the file directly from your disk.

2. 02

   Find the moment

   Scrub the preview to the instant you want to stretch. Clicking the timeline seeks the player, so you can land on the right frame before you add anything.

3. 03

   Add a slow-down edit

   Right-click the timeline and choose Slow down, or press "+ Add edit" with the playhead in position. A purple band marks the range.

4. 04

   Pick a factor

   Choose 0.75x, 0.5x or 0.25x, or type your own anywhere down to 0.1x. Then drag the handles so the band covers exactly the action you want stretched — usually a second or two is plenty.

5. 05

   Apply and download

   Press Apply All Edits. An estimated output duration is shown before you commit; slow motion makes the video longer, so expect the file to grow too.

Why some slow motion looks smooth and some stutters

Slowing footage down does not create frames — it spreads the frames you already have across more time. A clip shot at 30 fps and played at 0.5x shows each frame for two frame-times, so the effective motion sampling drops to 15 fps. At 0.25x it drops to about 7.5 fps, and the eye reads that as stepping.

The practical rule: the higher the source frame rate, the further you can stretch it. Phone footage recorded in a dedicated slow-motion mode (120 or 240 fps) takes 0.25x comfortably. Ordinary 30 fps video usually looks right at 0.75x, acceptable at 0.5x, and choppy below that. Shortening the slowed range helps too — a half-second of stutter reads as an intentional stylistic beat, four seconds of it reads as a broken file.

This editor deliberately does not fake the missing frames. Optical-flow interpolation is expensive, is unreliable on anything with fast or occluded motion, and would smear more often than it would help inside a WebAssembly sandbox.

What FFmpeg does to slow a section down

Mechanically it is the speed-up in reverse. The clip is split into before, selection and after; the selected piece has its presentation timestamps multiplied out with setpts — a factor of 0.5 divides each timestamp by 0.5, which is to say it doubles them — and the three pieces are concatenated back together.

The audio branch uses atempo with the same factor. atempo clamps at 0.5 per instance, so 0.25x is emitted as atempo=0.5,atempo=0.5. Because atempo is a time-stretch rather than a resample, the pitch stays put: slowed speech sounds slow, not deep.

The output is then re-encoded with libx264, because a filtered stream cannot be stream-copied. The step-by-step pipeline is written up here.

Slow motion makes files bigger, not smaller

Worth planning for: stretching a section to 0.25x makes that section four times as long, and the encoded file grows accordingly. If the result is bigger than you want, keep the slowed range short, cap the output at 720p with the max-resolution control, or mute the audio track entirely with the "Mute audio" toggle.

Going the other way — if a clip is too long rather than too slow — you probably want to speed a section up or cut it out entirely.

Questions about slowing down video