{{ summaryTitle }}

{{ summaryValue }}

{{ summaryLine }}

Speed{{ formattedSpeed }}x Audio{{ audioPolicy === 'keep' ? 'Keep' : 'Mute' }} PrivacyLocal only
{{ selectedFile ? selectedFile.name : 'Drop a video here or browse' }}
{{ selectedFile ? sourceMetaLine : 'MP4, MOV, WebM, or MKV up to 80 MB, 5 minutes, and 1080p' }}
{{ fileChecking ? 'Reading video metadata...' : fileStatusLine }}
Sample clip
Generate a tiny browser-local clip to try the retiming workflow.
{{ formattedSpeed }}x
x
Estimated output: {{ estimatedDurationLabel }}. Exact values from 0.25x through 4x are supported.
{{ speedError }}
Choose mute when pitch-shifted source audio would be distracting.
{{ processingProgress }}%

Processing runs on this device and may take a moment for longer clips.

{{ workflowMessage }}

Default: retimed. Letters, numbers, spaces, underscores, and hyphens are converted to a safe filename segment.

Retimed video

Filename
{{ outputFilename }}
Duration
{{ formatDuration(outputDuration) }}
Frame size
{{ outputWidth }} x {{ outputHeight }}
File size
{{ formatBytes(outputBytes) }}

The chart renderer is unavailable. The timing values remain available in the output ledger.

SignalSourceOutputMeaningCopy
{{ row.label }}{{ row.source }}{{ row.output }}{{ row.detail }}
Audio Speed Changer Change audio speed from 0.50× to 2.00× in your browser with pitch-preserved or tape-style retiming, then preview the result and save a WAV.
Timecode Duration Calculator Calculate frame-accurate timecode spans with drop-frame validation and endpoint rules, then add 24-hour wrap or pull handles when needed.
Video Bitrate Storage Calculator Plan video storage from bitrate and recording duration, including multiple cameras and copies plus headroom and whole-drive capacity checks.
Time-lapse Plan Calculator Plan a time-lapse shoot from interval or playback goals with whole-frame timing and practical storage, battery and exposure-gap checks.

Changing video speed alters the timeline of the media. At 2× speed, each second of output represents two seconds of the source, so the result lasts half as long. At 0.5× speed, the source unfolds over twice as much time. The frame dimensions can stay unchanged even though every frame receives a new playback time.

Speed changes are useful for condensed demonstrations, slow-motion review, time-lapse-like summaries, rehearsal clips, and matching a video to a target duration. The best multiplier depends on the material. A lecture may become hard to follow at a high speed, while a repetitive setup sequence may remain clear after a much stronger acceleration.

Audio needs a deliberate choice because changing its timing can change its sound. Simple sample retiming produces a tape-style effect: faster playback raises pitch and slower playback lowers it. Muting avoids that effect when speech, music, or ambient sound would become distracting. Pitch-preserving time stretching is a different audio process and is not implied by an ordinary speed multiplier.

Speed multiplier
The ratio between source time and output time, such as 2× or 0.5×.
Retiming
Changing frame, sample, and track timing without changing the source's pixel dimensions.
Transcoding
Decoding and encoding media into the output video and audio codecs.

Video conversion is limited by the source container, codecs, browser decoding support, memory, and device performance. A file extension alone does not guarantee that the browser can read every track. Long, high-resolution, or heavily compressed videos also need more time and memory to decode and encode.

How to Use This Tool:

Start with a browser-playable source and choose the speed and audio treatment before beginning conversion.

  1. Upload a Source video no larger than 80 MB, 5 minutes, or 1920 × 1080 pixels. Correct any file, duration, or resolution warning before continuing.
  2. Set Playback speed from 0.25× through 4×. Check the estimated output duration; values above 1× shorten the video and values below 1× lengthen it.
  3. Choose whether to keep or mute audio. Keep it only when a tape-style pitch change is acceptable, then edit the filename suffix if needed.
  4. Start the retime and wait for the output to be verified. Preview the result and check duration, motion, and audio before downloading the MP4.

Interpreting Results:

Retimed duration is the expected source duration divided by the speed multiplier. A small difference between the expected and decoded output duration can arise from frame and sample boundaries. The conversion is rejected when the verified duration differs by more than the accepted encoding tolerance.

The output preserves source width and height, but that does not guarantee identical visual quality or file size. Re-encoding can change compression artifacts, bitrate, keyframe placement, color handling, and compatibility. File size is not proportional to duration alone.

  • At 1×, timing is unchanged, though the video is still transcoded.
  • Kept audio is retimed with pitch change; muting writes no audio track.
  • Watch several points across the downloaded file, especially cuts, fast motion, and the ending, before replacing an original.

Technical Details:

Retiming scales media timestamps and sample durations by the reciprocal of the chosen speed. Faster output packs the source into a shorter timeline; slower output spreads it across a longer timeline. The output container is MP4 with AVC video. Kept audio is encoded as AAC, while muted output contains no audio track.

Formula Core:

For source duration T and speed multiplier s, output duration and timestamp scale are:

Toutput=Tsources Timestamp scale=1s

A 12-second source at 2× produces an expected 6-second output and a timestamp scale of 0.5. An 8-second source at 0.5× produces a 16-second output and a timestamp scale of 2.

Transformation Core:

StageTransformationResult
InspectRead duration, frame size, and audio availability.Reject unsupported or over-limit input before encoding.
Video retimeDivide frame timestamps and durations by the speed multiplier.Motion plays at the selected rate with original pixel dimensions.
Audio keepResample source audio positions at the same timing ratio.AAC audio remains synchronized with tape-style pitch change.
Audio muteDiscard the audio track.The MP4 contains AVC video without audio.
VerifyDecode the written media duration and compare it with the formula.Accept when the difference is no more than the larger of 0.12 seconds or 8% of expected duration.

Audio resampling uses linear interpolation between neighboring source samples. At 2×, output audio contains about half as many sample frames; at 0.5×, it contains about twice as many. This keeps audio duration aligned with video timing but does not preserve pitch.

Supported speed is inclusive from 0.25× through 4×. Source duration must be at least 0.05 seconds and no more than 300 seconds. Width and height must not exceed 1920 × 1080. Duration is shown to the nearest tenth of a second where needed, while the timing calculation retains full numeric precision.

Privacy and Limitations:

The source video is read and converted on the current device; it is not uploaded for media processing. Browser capability and available memory still determine which files can be opened and completed.

  • Keep the original file. Transcoding is not lossless and should not replace an archival master without review.
  • Variable-frame-rate media, unusual codecs, damaged files, transparency, metadata, or color features may not survive conversion exactly.
  • The output uses MP4, AVC video, and optional AAC audio. It does not offer codec, bitrate, frame-rate, or pitch-preservation controls.
  • Processing time and memory use rise with duration, resolution, frame rate, and source complexity.