How to speed up or slow down audio
- Drop an MP3, M4A, WAV, FLAC, OGG or Opus file. Phone voice memos work, including iPhone memos recorded with the Lossless setting.
- Pick a speed (0.5× to 2×), drag the slider anywhere from 0.25× to 3×, or type the length you need, such as
3:00. - Press play: the player already runs at that speed, so you hear the result before anything is saved.
- Choose MP3, M4A or WAV and press Change speed. The new file is checked and ready to download.
Keep the pitch, or let it change
Playing a recording faster the simple way raises every voice and note with it, as a record does on the wrong speed: 1.25× is almost four semitones higher, 2× a full octave. Keep the pitch (the default) avoids that. The sound is cut into short overlapping slices of 40 milliseconds; to slow it down, slices are repeated a little, to speed it up, some are left out, and each slice is joined where its waveform lines up with the one before (the method is called WSOLA), so there is no click at the seams. The speed changes, the pitch does not.
Change with speed does the record version on purpose: the audio is resampled, so the length and the pitch move together. That is the sound of sped-up “nightcore” songs and of slowed tracks. And with Keep the pitch selected, the Shift pitch slider moves the key by semitones without touching the speed, for singing along in a lower key or matching a recording to an instrument.
| Speed | New length of a 3:00 song | Pitch change, if the pitch is not kept |
|---|---|---|
| 0.5× | 6:00 | −12 semitones (one octave down) |
| 0.75× | 4:00 | −5.0 semitones |
| 0.9× | 3:20 | −1.8 semitones |
| 1.25× | 2:24 | +3.9 semitones |
| 1.5× | 2:00 | +7.0 semitones |
| 2× | 1:30 | +12 semitones (one octave up) |
What we measured
We tested the page with files whose right answer is known: a stereo test tone (440 Hz left, 660 Hz right), a melody of 32 short notes with sharp attacks, a 98-second speech recording, a 96 kHz FLAC and an iPhone Lossless memo. Every result was decoded again with FFmpeg, separately from the page.
| Test | Result |
|---|---|
| Tone, 1.5×, pitch kept | 440.00 and 659.99 Hz, level within 0.01 dB, exactly 294,000 samples (10 s ÷ 1.5) |
| Tone, 1.5×, pitch with speed | 660.00 and 990.00 Hz, as a record at 1.5× |
| Tone, +3 semitones, same speed | 523.26 and 784.90 Hz (expected 523.25 and 784.88), same length to the sample |
| MP3 192 kbps, 0.75× | MP3 at 192 kbps, exactly the expected length after the encoder delay is removed |
| Melody, 0.5× and 2× | all 32 notes start where they should; no attack is doubled |
| 98-second speech, 1.5× to MP3 | 65.28 s, about 3 s to process on a laptop |
We also put the melody through FFmpeg’s atempo filter, the method some online speed changers say they run.
The timing of the notes came out in the same range as this page (on average 13 ms early at half speed, against 15 ms here,
and no doubled attacks in either), but its files were not the exact length: 27 ms short at 0.5× and 5 ms long at 2×.
Here the new file is cut or filled to the exact sample count, which matters when the audio has to line up with a video or a
slide deck. One thing we tried along the way: 90 ms slices, a common setting for slowing down, doubled the attack of 26 of the 32 notes
at half speed. That is why the slices here are 40 ms, long enough to keep bass notes down to 50 Hz smooth.
Good uses, and the limits
- Learning a song or a solo: 0.75× or 0.5× with the pitch kept, so you can play along in the same key.
- Lectures, audiobooks and voice messages: 1.25× to 1.5× keeps speech clear; above 2× most voices start to blur.
- Fitting a time slot: type the length (a 32-second voice-over for a 30-second ad) and the page sets the speed to match.
- Transcribing: slow an interview to 0.75× to type along.
Large changes are audible in any stretch method: at 0.5× with the pitch kept, held notes can take on a slightly metallic edge, and at 3× fast speech loses syllables. Changing the speed means the audio is encoded again; MP3 keeps its original bit rate and WAV is lossless. To shorten a file without changing its speed, use Trim Audio or Split Audio; to make a quiet recording louder, Increase Audio Volume; for a video, Change Video Speed. Nothing here is uploaded: the file is processed in your browser.