Slow a video down without putting it online

Personal footage, work recordings, material covered by an NDA: all of it can be slowed down 8x or 16x without ever handing the file to a third party. The browser decodes the frames, builds the in-between ones and muxes an MP4 on your device.

Pick a file, a factor and a frame mode. Everything is computed in your browser and the file goes nowhere.

Slow down video

Drop a video here

or pick a file from your device

MOV and MP4 are supported, with H.264 or HEVC video.

Settings

Factor
Frame mode
Audio

At x8 and x16 stretched audio is unintelligible: you get a drone, not speech.

Quality
Advanced

Slow down video

Ordinary slow motion services ask you to upload, then keep the file on a server for a while and delete it according to a policy you cannot audit. Here there is nothing to audit: the file physically never leaves the tab, and once the page is closed it is not even in memory.

Speed up video The same thing without a browser

How it works

Not a single byte of your video goes over the network. Everything below happens inside your browser tab.

  1. The file opens locally

    You pick a video and the browser reads it through the File API, like any file on disk. No request carries your file to a server: the page itself is static, and from there only your device does the work.

  2. The container is unpacked

    The MP4 or MOV is split into video packets, and the headers give up the codec, frame rate, rotation and color space. From those we build a plan: how many frames you will get and how long it should take.

  3. Frames are decoded with WebCodecs

    The browser's VideoDecoder turns packets into frames with hardware acceleration. Frames live in the tab's memory and are closed the moment they are used, so memory does not run away.

  4. New frames are built in between

    Duplicate repeats a frame, blend cross-fades neighbours, and motion mode computes motion vectors on the GPU through WebGPU. These new frames are what make the movement smooth.

  5. The result is encoded back to MP4

    VideoEncoder writes H.264 at a constant frame rate with a keyframe every two seconds, it is wrapped into an MP4, and it lands in your downloads. No copy is left anywhere.

Frequently asked questions

Is this free?

Completely. There are no paid tiers, no cap on how many files you run, and no watermarks. Your own computer does the work, so there is nothing for us to charge for.

Is it safe? What happens to my video?

The video is processed inside the tab and is never sent anywhere. We physically cannot receive it: the server only holds HTML, CSS and JavaScript. Close the tab and nothing is left behind.

Is my video really not uploaded?

Really. It is easy to check: open developer tools, go to the Network tab and run a job. No request carries the file. You can even go offline after the page loads and processing still works.

Does it work on iPhone and on a MacBook?

Yes. On a MacBook it works in Safari, Chrome and Edge, HEVC from the camera included. On iPhone it needs Safari 17 or newer, and the tab has to stay in the foreground, because iOS stops the work in the background. Trim long clips first on a phone.

Which browsers are supported?

Chrome, Edge and other Chromium browsers from version 94, Safari 17 and newer, and Firefox with limits. WebCodecs is what matters; without it the ffmpeg.wasm fallback kicks in, which is slower.

Why does HEVC not work in Firefox?

Firefox does not decode HEVC through WebCodecs. That is a browser limitation, not a site one, and iPhone footage is often HEVC. Open the file in Chrome, Edge or Safari, or convert it to H.264.

Why is my portrait video sideways?

MP4 stores rotation as a separate flag and plenty of software ignores it. We read that flag and rotate the actual picture while encoding, so the result opens the right way up everywhere, old players included.

What is the maximum file size?

There is no hard limit, only your device's memory. On a desktop, files up to a couple of gigabytes are comfortable; on a phone, stay within a few hundred megabytes. If memory looks tight, we warn you before the job starts.

Why is the output so long?

Slowing down 8 times turns 10 seconds into 80, and 16 times gives you over two minutes. There are that many more frames, so processing time and file size grow with it. If that is more than you need, trim the source first.

Can I do the same thing without a browser?

You can. The "Without a browser" page has ready ffmpeg commands for macOS, Windows and Linux: the same three modes, plus variable frame rate, HDR and rotation. For batches that is the better route.