Audio Speed Changer (Speed Up / Slow Down Audio)

Slow down a fast passage to learn it, speed up a lecture or podcast, or match a track to another tempo. Set the speed from 0.25× to 4×, or enter the song's BPM and the BPM you want. With "Keep the pitch" the music stays in the same key; switch it off for the classic record-player effect where pitch follows speed. Files are processed in your browser and never uploaded.

How to use this tool

1. Choose one or more audio files.
2. Enter the speed (1.25 = 25% faster, 0.75 = 25% slower), or fill in the song tempo and the target tempo in BPM.
3. Leave "Keep the pitch" on to time-stretch, or switch it off for pitch that follows the speed.
4. Pick the format and press Calculate; the results show the old and new length.

Formula

New length = old length ÷ speed · speed = target BPM ÷ song BPM · pitch change (pitch not kept) = 12 × log2(speed) semitones

With the pitch kept, the ffmpeg engine's atempo filter (WSOLA: overlapping slices of sound, repeated or skipped where they line up) changes the length; changes beyond 0.5–2× are done in equal steps. This needs the engine, a one-time download of about 30 MB. The stretch can soften the last few hundredths of a second (up to about 30 ms) of a file that stops abruptly. Without keeping the pitch, the audio is resampled with a band-limited filter in your browser by an exact ratio, so 1.5× gives exactly two thirds of the samples and raises every frequency by half.

Example

A 4:00 song (240 s) at 1.25× lasts 240 ÷ 1.25 = 192 s (3:12). To take a 120 BPM track to 128 BPM, the speed is 128 ÷ 120 = 1.0667, so it becomes 240 ÷ 1.0667 = 225 s; without keeping the pitch it would also rise by 12 × log2(1.0667) ≈ 1.12 semitones.