Video Speed Changer

Speed up or slow down video playback with smooth motion interpolation.

Tested tool guide Tested browser tools Checked August 16, 2026

What Video Speed Changer does, with a checked example

A video stores a fixed set of moments, and how fast they play is decided later. This page makes that decision permanent: it takes the file you choose, re-times it to a speed you set, and saves a new video. Everything runs in the browser; nothing is uploaded. The surprise is on the slow-down side. A clip contains only the moments that were actually filmed, so a slow-down needs frames that do not exist. The tool estimates how objects moved between the real frames and manufactures the missing ones, which is why slow motion comes out smooth rather than stuttering, and why extreme slow-downs can look faintly synthetic.

Worked example

A concrete input and expected output from the current implementation.

Input

A 60-second clip, speed set to 1.5x

Expected output

A 40-second video file: 60 / 1.5 = 40 seconds.

Duration divides by the speed factor, so the same frames finish in 40 seconds instead of 60. Setting 2x would give 30 seconds, and 0.5x would give 120 seconds (60 / 0.5 = 120).

How the result is produced

Re-timing the timeline

The speed factor applies to the timeline, not to the pixels. At 2x, the same frames are packed into half the time, and surplus frames are dropped so the output keeps a steady frame rate. At 0.5x, the frames are spread over twice the time, and the gaps between them are filled with frames computed from the motion of the real ones. That interpolation is what keeps slow motion smooth.

What the download is

The result is a new video that keeps the original resolution, with its duration divided by the speed factor: 2x halves the length, 0.5x doubles it. The sound track is compressed or stretched to the new length so picture and audio stay in sync, and the re-timed file is offered for download.

Good uses

Condensing a long recording: a 2x re-timed meeting or lecture downloads as a half-length file, so viewers get the same content in less time without touching the player's speed controls.
Making slow motion from ordinary footage: replaying a highlight at 0.5x or slower turns it into a dramatic replay, and the interpolated frames keep the motion fluid instead of frame-by-frame.
Fitting a clip into a fixed slot: a 60-second intro that must run 45 seconds can be re-timed at about 1.33x, keeping the whole content rather than cutting it.

Limits and checks

The factor divides duration, not file size. A slow-down produces more frames than the source contains, and each interpolated frame is encoded into the output, so the download can be larger than the original even though it shows the same footage.
In-between frames are estimates, not recordings. At very low speeds, moving edges can look soft, wobbly, or ghosted, and fast-moving or occluded objects are where the estimation is weakest. The frames that were actually filmed are not altered.
The change is not losslessly reversible. Frames dropped during a speed-up and frames invented during a slow-down cannot be recovered, so re-timing a processed file back to the original speed will not restore the source footage.

Common questions

If I set 2x on a 60-second video, is the download really 30 seconds long?

Yes. Duration divides by the speed factor, so 60 seconds at 2x becomes 30 seconds, and at 0.5x it becomes 120 seconds. The same relation works backwards: pick the factor from the length you need. The whole clip is re-timed rather than cut, so nothing is removed from the content.

Does slowing down damage the original footage?

No. The frames that were filmed stay intact; only the timing changes, plus the added interpolated frames. Those added frames are computed from motion estimates, so extreme slow motion can look slightly synthetic near fast-moving edges. And like any re-encode, the output is one generation away from the source file's exact data.

References and verification

The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.