Speed videos up to 4x or slow them down to 0.25x with pitch-preserved audio, processed entirely in your browser, files never uploaded.
Your video's speed is changed locally with FFmpeg running in your browser. Nothing is uploaded; the file never leaves your device. First use downloads the ~31 MB engine once. Files up to 200 MB.
Values above 1x make the video faster; values below 1x slow it down. Audio pitch is preserved; voices stay natural at any speed.
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Changing playback speed is the cheapest edit there is: condense a 40-minute screen recording into a 10-minute walkthrough, tighten a tutorial's dead air, or drop a sports or craft clip to quarter speed to study the motion. Speeding a clip up shortens it ( which platforms and viewers both reward), while slowing it down stretches it for slow-motion moments or reviewing your own footage.
Speed is applied in two places. Video frames get new timestamps through FFmpeg's setpts filter (timestamps divided by the factor to speed up, multiplied to slow down). Audio is time-stretched with the atempo filter, which changes tempo without shifting pitch; voices stay natural at 2x instead of turning into chipmunks. Because atempo only accepts factors between 0.5 and 2.0, extreme speeds chain two passes: 4x runs atempo twice at 2.0, and 0.25x runs it twice at 0.5.
Two honest limitations: slowing down does not create new frames (each source frame is simply held longer, so deep slow motion can look choppy unless the footage was shot at a high frame rate), and the output is always re-encoded as MP4 (H.264 + AAC). Files without an audio track are handled automatically: if the audio filter has nothing to grab, the tool retries with the audio stage skipped. To combine a speed change with cuts, trim the result with the Video Trimmer; to strip the audio entirely, use the Video Muter.
Both chain two atempo passes (4x is 2.0 × 2.0, and 0.25x is 0.5 × 0.5), so the math is symmetric. The failures almost always come from files with no audio track at all, where any audio filter makes FFmpeg abort. This tool detects that failure and automatically retries with the audio stage skipped, delivering video-only slow motion for silent sources.
No. The atempo filter is a time-stretcher, not a resampler: it changes tempo while preserving pitch, so speech at 2x sounds like the same person talking faster. The same applies in reverse: 0.5x does not produce deep, dragging voices.
Slowing down re-times the existing frames; it does not synthesize new ones. At quarter speed each frame is held four times as long, so 30fps footage plays at an effective 7.5fps. For smooth deep slow motion, record at 60 or 120fps and slow that clip down instead.
Roughly, yes: the frame rate in the file and the encoding quality stay the same but the duration drops, so a 2x output is typically close to half the size. The reverse holds for slow motion: a 0.25x output plays four times as long and is usually several times larger than the source.
Any FFmpeg-readable input works: MP4, WebM, MOV, MKV, AVI, and more, up to 200 MB. The output is always MP4 with H.264 video and AAC audio at 128 kbps, which plays everywhere and uploads directly to social platforms without re-encoding warnings.