Free Video Speed Changer

Worked example

Add a 3-minute 1920×1080 clip called clip.mp4 and press the "8×" button. Under the settings the page says "The result will be 0:22 long. Estimated time on this device: about 4:31." A warning follows: "This will take a while, about 4:31. Choose a smaller maximum size, Fast, or use only part of the video to finish sooner." A "Use 720p" button appears with it. Click it: "Maximum size" moves to "720p" and the estimate drops to about 2:03, which still warns. Now choose "Fast" under "Speed / size". The estimate becomes about 1:43 and the warning goes. Press "Make it 8×". The page shows "Processing… 40%", then "8× video ready" with the file size and a player. The download is named "clip_8x.mp4".

Which settings to use, and the limits

• Pick a speed with a button or the custom box. The buttons are 0.1×, 0.25×, 0.5×, 0.75×, 1.5×, 2×, 3×, 4×, 8× and 16×, and the ones below 1 are marked "slow-mo". "Custom speed" takes any value from 0.05 to 20. If the box is empty or outside that range, the main button is greyed out. The page opens on 2×. The preview player runs at the speed you choose, with pitch kept, so you can check it before encoding. The preview itself is limited to between 0.0625× and 16×.
• Decide whether to keep the sound. "Keep audio (pitch-corrected)" stretches the sound with FFmpeg's atempo filter, so voices keep their pitch. Outside 0.25× to 8× the box starts unticked, and the page says "Audio is hard to understand at this speed, so it starts off. Tick the box to keep it anyway." Once you tick or untick it yourself, changing the speed no longer changes it. Sound is saved as AAC at 160 kbps in an MP4, or as Opus at 128 kbps in a WebM.
• Use only part of the video to cut the wait. Once the preview has loaded, "Only use part of the video" appears. Tick it and set "Start" and "End" with the sliders, which move in 0.05 second steps and stay at least 0.1 seconds apart. The summary shows the range and how many seconds are kept. Only that part is decoded and encoded, so the wait is shorter, and the speed applies to the kept part. A range that covers the whole video counts as no trim.
• Lower Maximum size or choose Fast to finish sooner. "Maximum size" offers "Original", "1080p", "720p" and "480p". It limits the short side of the frame, never enlarges the video, and rounds to even numbers. "Speed / size" offers "Fast", "Balanced" and "Small file", which use the x264 presets ultrafast, veryfast and fast at quality values (CRF) of 28, 26 and 30. "Fast" is the starting choice on a phone or touch screen. "Save as" offers MP4 or WebM. WebM uses VP8, is slower to make, and hides the "Speed / size" buttons.
• Use smooth slow motion and the frame cap on purpose. Below 1×, the box "Smooth slow motion (adds in-between frames, much slower)" appears. Without it, slowed video repeats frames, so a 30 fps clip at 0.25× can look choppy. With it, the tool makes new frames at 30 frames per second, and the time estimate assumes the encoding takes eight times as long. Above 4×, "Limit to 30 frames per second (smaller file, faster)" appears, ticked by default. Untick it to keep the clip's own frame rate.
• Read the estimate and warnings before you start. The time is a rough guide from the frame size, the speed and quality choices and an assumed 30 frames per second, and it is multiplied by 2.5 on a phone or touch screen. A warning appears when the time is 2 minutes or more, when the result would be 10 minutes or more, or when you slow down a file of 40 MB or more. The tool takes one video at a time. A file above about 2 GB is refused (1 GB on a computer reporting 4 GB of memory or less, 400 MB on a phone), and the whole file is copied into memory.

Common problems and how to fix them

It says "Canceled."

You pressed "Cancel" during the encode. Cancel stops the encoder at once and drops the half-finished file. This is a grey notice, not a red error, and nothing is wrong. The Cancel button only appears after the engine has loaded. Press "Make it 8×", or whichever speed you chose, again. The engine reloads from the browser cache, which is quick.

It says "This video has no audio track, so the result is video-only."

With "Keep audio (pitch-corrected)" ticked, the tool first tries to include the sound. If that run fails because the file has no audio stream at all, it runs once more without audio and shows this note. A screen recording or timelapse made with no sound is the usual cause, and nothing is wrong. If you expected sound, play the original: when it is silent there too, the sound was never recorded. A clip that does have sound never gets this note; if its encode fails, you see the reason instead.

It says "This conversion failed — the chosen format or codec may not be supported for this input. Try a different output format, or a different source file."

The encoder could not make the chosen format from this clip, for example an unusual codec it cannot decode or encode. Running out of memory, an AV1 video and a damaged file each have their own message instead. Switch "Save as" between MP4 and WebM and try again. If it still fails, convert the clip to a standard H.264 MP4 first, then change its speed.

It says "This file is 2.6 GB, which is too large to process in your browser on this device (limit ~2 GB). Try a smaller file or trim a shorter clip first."

The numbers are your file and your device's limit: about 2 GB, 1 GB or 400 MB, as listed above. The check runs when you add the file. Cut the video shorter first in the Video Trimmer, then change its speed, or use a computer. This tool shows no caution for large files below the limit, and it copies the whole file into memory, so a big file can still fail later.

The main button is greyed out and shows a dash instead of the speed.

The speed box is empty, or holds a number outside 0.05 to 20, so the tool will not start. Type a number in that range, or press one of the speed buttons. The hint line under the box disappears while the value is not valid.

How it compares

This tool applies one speed to the whole video, or to the one range you keep, and always re-encodes it, so the wait grows with the frame size and the length, unlike a cut that only copies. The preview is instant because it only changes the playback rate. The saved file is H.264 with AAC in an MP4, or VP8 with Opus in a WebM. It cannot ramp the speed part-way, reverse a clip or keep several ranges. A desktop editor can do those things and copes better with very large files. In exchange, nothing is installed, and the video is processed in your browser and not uploaded.