Change Video Speed

How do I change the speed of a video?

1. Drop a video onto the drop zone, or click to browse.
2. Choose a Speed. Values below 1x slow the video down (0.25x is very slow); 1.25x to 4x speed it up. 2x is the default.
3. Leave Keep audio (pitch corrected) ticked to retime the soundtrack with the video, or untick it to drop the audio entirely.
4. Click Change speed and wait while the video is re-encoded.
5. Download the new MP4.

Changing speed means re-timing every frame, so this tool always re-encodes. For video, FFmpeg applies the setpts filter, which rewrites each frame's presentation timestamp: setpts=0.5*PTS makes the clip play at 2x, setpts=4*PTS at 0.25x. The result is encoded with libx264 at a quality setting close to the source. For the audio, it uses the atempo filter, a time-stretch that changes tempo without changing pitch, so a person talking at 1.5x sounds quicker but not like a cartoon.

atempo works between 0.5x and 2x per pass, so larger changes are built as a chain: 4x is atempo=2.0,atempo=2.0 and 0.25x is atempo=0.5,atempo=0.5. Quality stays good through the chain for speech and music at 0.5x to 2x; at 4x speech becomes hard to follow and at 0.25x audio takes on a smeared quality, which is why many people untick Keep audio for extreme slow motion and add music later.

Slowing down does not create new frames. A 30 fps clip at 0.25x shows each frame for four times as long, which looks stepped on fast action; it is fine for a gentle slow-down of a landscape or a smile, but true slow motion needs footage shot at 120 or 240 fps. Speeding up drops frames, which looks perfectly smooth, and 2x to 4x is the usual range for walkthroughs, tutorials and time-lapse-style clips.

Because the browser build of FFmpeg is single-threaded, expect about real-time to 3x real-time of the output length for 720p on a recent laptop, slower for 1080p and 4K and on phones. A 2 minute clip at 2x produces 1 minute of video and typically takes one to three minutes. Files up to about 1 GB are fine; the roughly 2 GB WebAssembly memory limit is the ceiling. Nothing is uploaded, and FFmpeg (about 30 MB) is fetched once and cached.

How it works: this tool uses FFmpeg compiled to WebAssembly (ffmpeg.wasm), loaded into your browser. No server is involved, so Kilnfile never sees your file. Last reviewed 20 September 2026.

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