Change Video Speed

About Video Speed Changer

This online tool changes the playback speed of video files directly in your browser using FFmpeg.wasm. Create slow motion or timelapse effects, reverse clips, and set a defined output frame rate for clean conform and broadcast/streaming delivery. By default audio pitch is preserved so voices sound natural at any speed, but you can toggle pitch preservation off for a chipmunk or slow-motion audio effect. Output is faststart MP4 (H.264 video + AAC audio, yuv420p) that previews and seeks instantly. Files never leave your device.

Can I change the frame rate (fps) when changing speed?

Yes. Use the Output frame rate selector to keep the original or force 24, 25, 30, 50, or 60 fps. This is important when slowing footage down: with setpts alone the container keeps the original fps and simply holds each frame longer (judder). Setting a defined fps resamples the stream by dropping or duplicating frames (no motion interpolation) so the result conforms cleanly to a timeline and to broadcast/streaming standards.

What output codec, container, and quality are used?

Output is always MP4 with H.264 video (libx264, CRF 23, yuv420p chroma for universal decoder compatibility) and AAC audio at 128 kbps. The file is written with +faststart so the moov atom sits at the front, letting the in-browser preview and any downloaded copy start playing and seeking instantly instead of buffering the whole file first.

Why does my MP4 now seek instantly?

The output is muxed with -movflags +faststart, which relocates the moov atom (the index) to the start of the file. Progressive and streaming players can then begin playback and scrubbing immediately. Without faststart the index lives at the end, so players had to download the entire file before starting.

What video formats are supported?

Input: MP4, WebM, MOV, AVI, MKV, and other common video formats. Output: MP4 (H.264 video + AAC audio, yuv420p) for universal compatibility.

What speed options are available?

Preset speeds: 0.25× (very slow), 0.5× (slow), 0.75×, 1.0× (normal), 1.25×, 1.5× (fast), 2.0× (very fast), 3.0×, 4.0× (ultra fast). You can also enter any custom speed from 0.1× to 10×.

Does it preserve audio pitch, and what is atempo?

By default yes: the Preserve audio pitch toggle is on, so voices and music sound natural at any speed using FFmpeg's atempo filter. Because atempo only supports 0.5×-2.0× per pass, the tool automatically chains several atempo stages for extreme speeds. Toggle pitch preservation off to get the classic chipmunk (fast) or deep slow-motion (slow) audio effect, which shifts the sample rate relative to your source rate so the pitch ratio stays correct even for 48 kHz broadcast audio.

Does the audio stay in sync at extreme speeds?

Yes. Both the video timestamps (setpts) and the audio tempo (chained atempo) are scaled by the same speed factor, and audio is re-encoded to AAC against the muxed timeline, so picture and sound stay aligned even at very slow or very fast settings.

How does speed affect duration?

Speed multiplier directly affects duration. 2× speed makes video twice as fast (half duration). 0.5× speed makes video half as fast (double duration). The tool shows the new duration before processing.

What is the file size limit?

Maximum file size is 100MB. This ensures smooth processing in your browser. For larger videos, consider using desktop software.

Can I preview before downloading?

Yes! After processing, you can preview the speed-changed video to verify the result before downloading.

Is my video file safe?

Absolutely! All processing happens directly in your browser using FFmpeg.wasm (WebAssembly). Your video file is never uploaded to any server. Everything stays private on your device.

How long does processing take?

Processing time depends on video length and speed setting. Typically 30-90 seconds for a 5-minute video. Extreme speed changes (very slow or very fast) may take longer.

What are common use cases?

Slow motion: Sports analysis, dance tutorials, nature videos. Speed up: Long lectures, time-lapse effects, compress lengthy content. Creative: Music videos, artistic effects, presentation timing.