Change Video Speed Online (Free, No Upload)

Adjust video playback speed right in your browser, no file upload needed. Your video stays on your device the whole time. Pick a speed from 0.25x slow motion to 4x time-lapse, and the audio pitch stays correct automatically.

Try it in your browser

Loading FFmpeg engine... (~31 MB)

About MP4 format

Video speed adjustment is one of the most common edits in short-form content. Speed up a tutorial to cut dead air. Slow down a highlight to show detail. Create a time-lapse from a long recording. The FFmpeg setpts filter handles the video side by multiplying each frame's timestamp, and the atempo filter handles audio so pitch stays correct. Most browser-based tools that change video speed upload your file to a server, process it there, then send it back. This tool does all of that locally, using FFmpeg compiled to run directly in your browser. There's nothing to install and nothing leaves your device.

Use the API

curl -X POST https://renderio.dev/api/v1/run-ffmpeg-command \
  -H "X-API-KEY: ffsk_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
  "ffmpeg_command": "-i {{in_video}} -filter_complex \"[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]\" -map \"[v]\" -map \"[a]\" {{out_video}}",
  "input_files": {
    "in_video": "https://example.com/input.mp4"
  },
  "output_files": {
    "out_video": "output.mp4"
  }
}'

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Common Variations

1.5x speed (audio stays in sync)

-i input.mp4 -filter_complex "[0:v]setpts=0.667*PTS[v];[0:a]atempo=1.5[a]" -map "[v]" -map "[a]" output.mp4

2x speed (audio stays in sync)

-i input.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4

0.5x slow motion (audio stays in sync)

-i input.mp4 -filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" -map "[v]" -map "[a]" output.mp4

0.25x slow motion (very slow, audio only)

-i input.mp4 -filter_complex "[0:v]setpts=4.0*PTS[v];[0:a]atempo=0.5,atempo=0.5[a]" -map "[v]" -map "[a]" output.mp4

4x time-lapse (chain atempo for audio sync)

-i input.mp4 -filter_complex "[0:v]setpts=0.25*PTS[v];[0:a]atempo=2.0,atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4

Speed up video only (drop audio)

-i input.mp4 -vf "setpts=0.5*PTS" -an output.mp4

Frequently Asked Questions

Does changing video speed affect audio pitch?+

Yes, if you just speed up the video without adjusting audio, voices and music shift in pitch (the chipmunk effect). This tool uses FFmpeg's atempo filter to correct pitch automatically as it adjusts speed. At 2x speed, audio plays at the right pitch, just faster. Chained atempo filters handle speeds beyond 2x or below 0.5x, since a single atempo filter only works within that range.

What's the maximum speed multiplier I can use?+

There's no hard limit, but practical quality degrades past 4x for most video content. At 4x, FFmpeg uses two chained atempo=2.0 filters to keep audio in sync. Beyond 4x, you get into true time-lapse territory where it usually makes more sense to drop audio entirely and use setpts on the video stream only.

What's the slowest I can go?+

0.25x slow motion is a common target and works well. Going slower than 0.25x means chaining atempo=0.5 twice on the audio side. At very low speeds, the video may look choppy unless your source was shot at a high frame rate (60fps or higher). 24fps source material slowed to 0.1x will show obvious frame stepping.

Does speeding up video reduce file size?+

Usually yes, since there's less content per second. But the relationship isn't linear. A 2x speed-up doesn't give you a 50% smaller file because FFmpeg still re-encodes the output. Actual file size depends on the codec settings you use. If you want to reduce file size specifically, use a higher CRF value or a more efficient codec like H.265.

Can I change speed without re-encoding the video?+

No. Speed changes require re-encoding because every frame's timestamp needs to be recalculated. There's no container-level trick to change playback speed without touching the video data. The tool always re-encodes, which takes a bit longer but gives you a properly formatted output file.

What happens to embedded subtitles when I change speed?+

Embedded subtitle tracks (SRT, ASS, etc.) are not automatically adjusted for speed changes. Their timestamps will be out of sync with the new video speed. If your video has subtitles you need to keep, extract and retime them separately, then burn them in after the speed change.

Will my file stay private when I use this tool?+

Yes. This tool runs entirely in your browser using FFmpeg compiled for the web. Your video never leaves your device. Nothing is uploaded to any server. The processing happens locally, using your computer's resources.

What video formats does this support?+

The browser-based tool works with MP4, MOV, WebM, and most common video formats. For less common formats or large files, the RenderIO API accepts anything FFmpeg supports, which covers hundreds of container and codec combinations.

Privacy

Your video is processed entirely in your browser using FFmpeg. No file is uploaded to any server at any point during processing. Nothing is stored, logged, or shared with third parties.

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