Video Speed Changer
Speed a video up or slow it down from 0.25× to 4×, with correct audio handling either way.
Convert with this tool
or choose a file from your device — it is processed in this tab and never uploaded
Accepted: MP4, MOV, MKV, WebM, AVI
Files stay on your device. There is no upload, no queue and no watermark, so a conversion works even with the network switched off after the page has loaded.
Supported formats & processing
| Accepted input | MP4, MOV, MKV, WebM, AVI |
|---|---|
| Produced output | MP4 or WebM at the new speed |
| Processing | FFmpeg.wasm (video engine, downloaded on first use) |
| Upload required | No — the file is read locally and never transmitted |
How to use this tool
- Add the videoDuration is read from the container so you can predict the new length.
- Choose the speed0.25× and 0.5× for slow motion, 1.5× and 2× for tightening, 4× for a fast summary.
- Decide how audio should behaveKeep pitch is right for speech; let the pitch change for effects.
- ConvertThe encoder processes the whole timeline at the new rate.
- Download and checkThe result card reports the filters that were applied.
About this tool
Changing playback speed touches two things at once. The video timestamps have to be rewritten — that is the setpts filter — and the audio has to be handled separately, because simply playing audio faster raises its pitch and makes voices sound like chipmunks.
This tool gives you both options. The default uses the atempo filter to keep the pitch natural while the tempo changes, which is what you want for tutorials, interviews and lectures. The alternative lets the pitch follow the speed, which suits creative effects and time-lapse-style edits.
Features
- Speeds from 0.25× to 4× in both directions
- Pitch-preserving audio through the atempo filter
- Optional pitch-following audio for creative effects
- Correct timestamp rewriting via setpts
- Output to MP4 (H.264) or WebM (VP9)
- The exact filters used are reported so nothing is hidden
Frequently asked questions
Does slowing video down create real slow motion?
It creates slow motion from the frames that exist. Going to 0.25× from 30 fps gives an effective 7.5 fps, which looks choppy. True slow motion needs footage shot at a high frame rate — 120 fps or more.
Why does the audio sound strange at 4×?
At extreme speeds the atempo filter is working hard and artefacts appear. Speeds beyond 2× often sound better with the audio removed and a music track added instead.
Can I speed up only part of the video?
Not in this tool, which applies one speed to the whole file. Cut the section first with the video cutter, then process it separately and join the pieces.
Will the file size change?
A faster speed means a shorter file, so the size usually drops proportionally. A slower speed makes the file longer and larger.