Change Video Speed
Pick a video and a speed factor — it's sped up or slowed down in your browser (audio stays in sync), nothing is uploaded.
Change video speed
Pick a video and a speed factor — values above 1 speed it up (2 = twice as fast), values below 1 slow it down (0.5 = half speed). The audio is kept in sync. Everything runs in your browser with ffmpeg; nothing is uploaded.
How it works
- The video timestamps are scaled (
setpts) and the audio tempo is matched (atempo), so picture and sound stay aligned. - Output keeps the original container (mp4, webm, …), re-encoded locally.
- Supported range is 0.25x to 4x.
Notes
- Re-encoding a long video takes time (it all runs on your machine).
- Slowing a video down does not add new frames — it stretches the existing ones.
FAQ
Is my video uploaded?
No — ffmpeg runs in your browser tab; the file never leaves your device.
Will the audio drift?
No — the audio tempo is scaled by the same factor as the video, so they stay in sync.
What speed factors are allowed, and why does 4x still sound normal?
The factor must be between 0.25 and 4. ffmpeg's atempo filter only
accepts 0.5–2 per pass, so factors outside that band are applied as a chain
(4x runs atempo=2 twice, 0.25x runs atempo=0.5 twice) — the pitch is
preserved instead of chipmunking.
Is there a file size limit?
Yes — the input video and the re-encoded output are each capped at 25 MB. For a bigger file, trim or compress it first, then change the speed.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool change-speed 'url=https://example.com/input' 'factor=2'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/change-speed/?url=https://example.com/input&factor=2Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.