Video Speed Changer

Changing the speed of a video is about choosing which frames to show and for how long. Play a clip at double speed and you show every second frame; at quarter speed you show each frame four times as long. The sound has to follow, or be left out. This page does both in your browser, with the choice of keeping the pitch of the sound, letting it rise and fall as on tape, or removing it, and it writes a new MP4 or WebM.

How the frames are chosen

The new video runs at a steady frame rate, the source rate but no higher than 30 frames per second. For output frame k the page seeks the original to the time start + k ÷ rate × speed, draws that frame and encodes it. At speed 2 the source time advances two seconds for every output second, so every other source frame is used. At speed 0.5 it advances half a second, so each source frame appears about twice. This is simple and exact, but it does not blend neighbouring frames.

A worked example

Press Sample to record a three-second test clip and keep the default of 2×. The page shows the new length as 0:01.5. Press Change the speed and the result is a 320 × 180 MP4 of about 1.46 seconds, as the browser test measures from the player; the exact figure depends on the length of the recorded clip. Pick 0.5× instead and the same clip lasts about six seconds. The picture’s colour, which changes steadily over the clip, now changes half as fast.

What happens to the sound

Keep the pitch uses the WSOLA time-stretch of the audio speed changer, so voices keep their natural pitch while they speed up or slow down; it works best for moderate changes and sounds increasingly artificial as the speed moves further from 1×. Let the pitch change resamples the sound like a tape, so a double-speed clip sounds an octave higher. Remove the sound drops the audio track altogether, which is the best choice for time-lapse and for speeds of 4× or more.

Using presets

The chips set common speeds. 0.25× and 0.5× are for slow motion, 2× and 4× for skipping through a screen recording or tutorial, 8× for a time-lapse of a long process. The slider goes in steps of 0.05, so you can pick 1.25× to shorten a lecture slightly. The line under the controls always shows the original length and the new one.

Edge cases

Speeds under 1× on a 24 or 30 frames-per-second source look stuttery because there are not enough source frames; high-frame-rate footage works much better. Very high speeds produce very short videos with few frames. The frame rate of the source is estimated by playing the first moments, and a variable-frame-rate recording is sampled at constant steps. The video length is capped only by your device’s memory and patience.

Limits

There are no speed ramps, curves or reverse playback, only one constant speed for the whole clip. Optical-flow smoothing is not available. The video is re-encoded, with the quality and size options of the other video tools, and the work is done on your device.

Related tools

• Audio Speed & Pitch Changer uses the same time stretch for sound-only files.
• Video Cutter & Trimmer selects the stretch of video to change.
• Compress Video shrinks the sped-up clip.
• Video to GIF turns a short clip into an animation.