Change video speed online

Speed changes are one of the most common needs in short-form video editing. Slow motion isolates a moment that happened too fast to read at normal speed: a golf swing, a pour shot, a product reveal. Time-lapse condenses hours of footage into a watchable clip: a sunset, a construction project, a cooking process. Double speed is the standard for training content and tutorials where viewers prefer watching at 2x.

Most editing software handles speed changes in the timeline and re-renders on export, which means waiting for a full export pass. For a quick speed adjustment on a single clip before sharing, that workflow is more than the task requires.

Video speed is changed using the setpts filter. FFmpeg's setpts=N*PTS multiplies the presentation timestamp of each frame by N, which stretches or compresses the timeline. For slowdown (N greater than 1), the filter produces more time between frames. For speedup (N less than 1), it compresses them. The actual value of N is 1/speed.

Audio speed must be handled separately because audio and video are independent streams. The atempo filter changes audio playback rate but is constrained to values between 0.5 and 2.0. For speeds outside that range (0.25x or 4x), the code chains multiple atempo filters: 0.25x becomes atempo=0.5,atempo=0.5 and 4x becomes atempo=2.0,atempo=2.0. If audio is disabled, -an is passed instead. The output uses CRF 23 with the ultrafast preset.

For slow-motion below 0.5x, the tool automatically disables audio because stretching audio below 50 percent of its original speed produces a sound that is generally unusable. If you need ultra-slow audio (for pitch analysis or phonetics work), re-enable audio manually and expect artifacts.

4x speed on a 10-minute video produces a 2.5-minute output, which is manageable in browser memory. Very long videos at extreme speeds can run out of WebAssembly memory because FFmpeg must decode the full file before applying the filter. Keep inputs under 30 minutes for reliable results in the browser.

Every major video editor handles speed changes, but all of them tie the speed change to a project timeline, meaning you cannot quickly speed-adjust a single clip without creating a project. Command-line FFmpeg supports this in one command but the atempo chain for extreme speeds is non-obvious, and many users hit the 0.5 to 2.0 atempo limit without knowing why. This tool handles the chaining automatically and runs entirely in the browser.