Local Processing · FFmpeg WASM · No Upload
Video Speed Converter
Speed up to 4× or slow down to 0.25× — re-timed with setpts, audio tempo-matched with pitch preserved via atempo. Output is a clean MP4, processed entirely in your browser.
video-speed-converter v2.0
1 Playback speed
2 Audio track
Kept audio is tempo-adjusted with FFmpeg's atempo filter, so 2× does not sound like chipmunks and 0.5× does not sound dragged.
Show conversion log
Re-time any video in four steps
Upload, pick a speed, convert, download. Everything stays on your device.
Upload video
Drop a file up to 10 MB. It is read from a local object URL, never uploaded.
Pick a speed
0.25× to 4× via selector or presets. The estimate updates instantly.
Convert
setpts re-times video frames; atempo re-times audio while preserving pitch.
Download
Preview the new MP4 with full metadata, then save it.
Precision, privacy, and predictability
What the converter does with your video — and what it deliberately does not.
Zero upload
FFmpeg WASM runs in a worker inside your browser. Your uploaded video is processed locally and is not sent to a video-processing server.
Pitch-safe audio
atempo chains keep every speed inside the filter's stable 0.5–2.0 window (4× = atempo=2,atempo=2), so voice stays natural.
Web-ready MP4
H.264 + AAC with +faststart for progressive web playback and broad MP4 compatibility.
Which speed fits your use case?
Common presets and what people actually use them for.
0.25×
Frame-by-frame review of sports moves, dance steps, or incident footage.
0.5×
Step-by-step tutorials and cooking demos where hands move too fast.
1×
Original timing — useful to re-encode with the same speed and clean metadata.
1.5×
Lectures, meetings, and long talks without the chipmunk effect.
2×
Recaps, highlights, and skipping through slow sections quickly.
4×
Quick scans of surveillance-style footage or long recordings.
What it does, and what it does not
Clear boundaries so you know when to reach for another tool.
What it does
- Re-time video from 0.25× to 4× with accurate setpts math.
- Keep audio with tempo matched and pitch preserved.
- Mute audio for silent outputs.
- Show live estimated output duration before you convert.
- Preview source and result with full metadata.
- Export web-ready MP4 (H.264 + AAC + faststart).
What it does not
- Accept files above 10 MB — browser memory is the ceiling.
- Invent frames for buttery slow motion (no interpolation).
- Output formats other than MP4 in this tool.
- Decode codecs your browser/FFmpeg build cannot read (e.g. some HEVC).
Common issues and fixes
Quick answers to the most frequent problems.
Engine won't load
The FFmpeg WASM engine needs WebAssembly and Web Workers, and its files must not be blocked. Exclude ffmpeg from JS minify/combine/delay in optimisation plugins, or self-host the engine files.
Processing fails on long videos
Re-encoding happens in browser memory. Near the 10 MB cap or with long durations, out-of-memory errors can occur. Use a desktop encoder like HandBrake for those.
Output has no audio
Either Mute was selected, or the source video has no audio track. Check the Audio meta box in the result card.
Slow motion looks choppy
Expected behaviour: slowing down spreads the original frames over more time. True smooth slow motion needs frame interpolation, which browser tools cannot do reliably.
Frequently asked questions
Honest answers about the speed converter.
Is my video uploaded to a server?
No. The FFmpeg WASM engine runs entirely in your browser. Your video is read from a local object URL, processed in memory, and never transmitted to AllInOneTools or any third party.
Which speeds are supported?
Any speed from 0.25x to 4x. Quick presets cover 0.5x, 0.75x, 1x, 1.5x and 2x, and the selector adds 0.25x, 1.25x, 3x and 4x.
Why does the audio not sound like chipmunks at 2x?
Audio is processed with FFmpeg's atempo filter, which changes tempo without changing pitch. For speeds outside the 0.5-2.0 range the filter is chained (for example 4x becomes atempo=2,atempo=2) so pitch stays natural.
What is the output format?
MP4 with H.264 video and AAC audio at 128k, plus the faststart flag for web playback. If you choose Mute, the output has no audio track.
Why does slow motion look choppy?
Slowing a video stretches existing frames; it does not create new ones. Without frame interpolation (which browser tools cannot do reliably), 0.25x and 0.5x playback shows the original frame rate spread over a longer time.
Why is the first conversion slow?
The FFmpeg WASM engine is about 30 MB and downloads once on first use, then stays cached. Conversion time after that depends on video length, resolution, and your CPU.