Video Speed Changer | Browser Kitty

Operation demo See the basic workflow in a short video.

About this tool

Video Speed Changer speeds up or slows down a video from 0.25× to 4.00× and exports the result as MP4. You can set a multiplier directly or calculate the required speed from a target final duration.

Audio can keep its pitch, shift pitch together with speed, or be removed. You can preview the source at the chosen speed before conversion and preview the converted MP4 before saving. Loading, FFmpeg processing, and output generation happen locally without uploading the selected video to Browser Kitty.

Good for

Speed up a long explanation or process video to make it shorter
Slow down sports or motion footage for easier review
Calculate the speed needed to fit a specific target duration
Change video speed without uploading the source to an online conversion service

What it can do

Key capabilities available in this tool.

Change speed from 0.25× to 4.00× and preview the resulting duration
Calculate the needed speed from a target duration
Keep pitch, change pitch with speed, or remove audio
Choose quality and filename, then preview the converted MP4 before saving

How to use

Select a video file or drag and drop it into the tool.
Choose a speed from 0.25× to 4.00×. If useful, enter a target duration to calculate the required speed.
Choose whether to keep pitch, change pitch with the speed, or remove audio.
Adjust quality and the output filename if needed, then start the conversion.
Preview the converted MP4, check its duration and file size, then save it.

Supported

Speed: 0.25× to 4.00× with slider and numeric input.
Target duration: calculate a supported speed from the desired final length.
Audio: keep pitch, change pitch with speed, or remove audio.
Output: MP4 encoded with the bundled FFmpeg/x264 profile, with high / standard / smaller-file quality choices.
Workflow: source speed preview, converted preview, conversion cancellation, and editable output filename.

Limitations and notes

Output is MP4. This is not a general-purpose video editor; trimming, cropping, subtitles, transitions, and speed ramps are out of scope.
Long, high-resolution, or very large videos can take substantial time and may hit browser or device memory limits.
Direct preview support depends on the browser’s native codecs. Some files that cannot be previewed may still be convertible by the bundled FFmpeg runtime.
The browser speed preview and final FFmpeg conversion can differ slightly, including in pitch-preserving audio behavior.

Frequently asked questions

Is my video uploaded to a server?

No. The selected video, speed conversion, and result are handled in the browser and are not uploaded to Browser Kitty.

Can I change speed without changing audio pitch?

Yes. Choose the keep-pitch option. You can also let pitch change with speed or remove audio entirely.

Can I make a video a specific duration such as 30 seconds?

You can calculate the required speed from a target duration. When the result falls within the supported 0.25×–4.00× range, the tool can set that speed for conversion.

Can I create a speed ramp that changes speed partway through?

No. v1.0.0 applies one speed multiplier to the whole video. Variable speed ramps are out of scope.

Offline version and source code

The generated dist/index.html is a single HTML file containing the FFmpeg JS/WASM and Worker runtime. Save it and open it directly in a compatible browser to load, retime, and export video without runtime external connections.

View source on GitHub

Use together

Useful next steps and companion tools, organized around the workflow.