What a tempo changer does, and what it does not
Tempo is how fast the music moves, measured in beats per minute. A tempo changer makes the same recording play faster or slower. The hard part is doing it without the side effect everyone remembers from vinyl and cassette: spin a record faster and the singer climbs into chipmunk territory, spin it slower and everyone sounds sleepy and low. That happens because speed and pitch are tied together in a simple replay.
A real tempo changer pulls them apart. TuneSplit uses SoundTouch, an open time-stretching library that works on tiny overlapping slices of the waveform (the method is called WSOLA). To slow a song down it repeats and blends slices of the existing sound; to speed it up it drops and blends them. Because the slices keep their original frequency, the notes stay where they were. The song takes more or less time, and the key does not move.
That also tells you what the tool is not. It does not detect the beat and re-quantize your music, it does not follow tempo changes inside a song, and it does not stretch one section differently from another. The whole file moves by one fixed percentage. If your track drifts in tempo, as many live recordings do, it will drift by the same proportion afterward.
The same panel also has a Pitch slider from -12 to +12 semitones. This page is about tempo, so we leave it at zero in most examples, but it is there when you want both. If key is your main goal, the pitch changer page covers that side in detail.
Percentage and BPM: the conversion table
The slider speaks in percentages, but musicians think in BPM. The link between them is one multiplication: new BPM equals original BPM times the percentage. The table shows common source tempos against every chip and some round slider positions. Find your row, then read across to the speed you want.
| Original BPM | 50% | 75% | 90% | 110% | 125% | 150% |
|---|---|---|---|---|---|---|
| 80 | 40 | 60 | 72 | 88 | 100 | 120 |
| 100 | 50 | 75 | 90 | 110 | 125 | 150 |
| 120 | 60 | 90 | 108 | 132 | 150 | 180 |
| 128 | 64 | 96 | 115 | 141 | 160 | 192 |
| 140 | 70 | 105 | 126 | 154 | 175 | 210 |
| 174 | 87 | 130 | 157 | 191 | 218 | 261 |
Numbers with decimals are rounded to the nearest whole BPM: 128 at 90% is exactly 115.2, and 128 at 110% is 140.8. Rounding only matters if you are lining the result up against a metronome for a very long stretch.
To go the other way and find the percentage for a target tempo, divide the target by the original. Slowing a 150 BPM song to 120 BPM needs 120 divided by 150, which is 0.80, or 80%. Moving 124 BPM to 128 BPM needs 1.032, and the nearest slider step is 105%, which lands at about 130 BPM. If that is too far off, see the limits section on step size.
How long the file gets: duration at every speed
Speed also changes how long the song is. The tool shows both numbers above the sliders: the original length, an arrow, and the new length. The new length is the original divided by the speed. A three-minute song, 180 seconds, behaves like this.
| Speed | New length of a 3:00 song | What it feels like |
|---|---|---|
| 50% | 6:00 | Half speed, every note audible, good for transcribing |
| 75% | 4:00 | Classic practice speed for a hard passage |
| 90% | 3:20 | Slightly relaxed, nearly natural |
| 100% | 3:00 | No change |
| 110% | 2:44 | Tighter, more energy |
| 125% | 2:24 | Clearly faster, a workout feel |
| 150% | 2:00 | Very fast; consonants start to blur |
The 10-minute limit applies to the file you add, not to the result. A 10-minute file at 50% produces a 20-minute WAV, and WAV is uncompressed, so that file is big. It is worth knowing before you slow a long recording down: the output grows with the stretch.
Tempo settings for real jobs
These are starting points, not rules. Each is a setting you can try in under a minute.
Practice at a slower speed
Slow practice works because it gives your hands time to learn the pattern before they have to play it fast. A common method is to start at 50% or 75%, play the passage cleanly several times, then step up by one chip. The tool offers 75% and then 90% as the natural next rungs, and you can also drag the slider to 55%, 60%, 65%, 70%, 80% or 85% for a finer ladder. Because pitch stays fixed, you can play along on your real instrument without retuning.
Make one file per rung. The download name carries the percentage, so you end up with a tidy set like song-0st-60pct.wav and song-0st-80pct.wav, and you can see where each rung sits at a glance.
Transcribing a solo or a vocal line
For working out notes by ear, half speed is the most useful setting. At 50% each note lasts twice as long and fast runs open up into separate pitches. Because the key does not shift, what you hear is what you write down. If the part is buried in a full mix, split the song into stems first with the stem splitter, then slow just the stem you care about.
Dance, fitness and cadence
Running and cycling cadences are expressed in beats per minute, and so are most choreography counts. If your track is 118 BPM and your class needs about 130, 110% gives roughly 130. For a warm-up from a 128 BPM track, 90% gives about 115. Check the original tempo with the BPM finder first, because many songs are felt at half time and a displayed 70 can really mean 140.
DJ beatmatching and mashups
To mix two songs in your own software, the tracks need a shared tempo. Find both BPMs, pick the target, and work out each percentage. A 124 BPM track and a 128 BPM track are 3.2% apart, and a step of 5% would overshoot, so the better move may be to leave the first alone and stretch only the second. Large stretches are more audible, so shift the track that needs the smaller change. If the keys also clash, use the Pitch slider as a second pass.
Slowed edits and sped-up edits
The popular slowed-down and nightcore styles come from tape-style speed changes, where pitch moves with the tempo. This tool works differently: the Speed slider keeps the pitch, and the Pitch slider is separate. To recreate the old-school effect on purpose, set both. The table converts a speed ratio into the matching number of semitones using 12 times log base 2 of the ratio.
| Speed | Tape-style pitch change | Pitch slider setting |
|---|---|---|
| 50% | -12.0 semitones | -12 |
| 75% | -5.0 semitones | -5 |
| 90% | -1.8 semitones | -2 |
| 110% | +1.7 semitones | +2 |
| 125% | +3.9 semitones | +4 |
| 150% | +7.0 semitones | +7 |
The slider takes whole semitones, so the match is close rather than exact. Reverb is not part of this tool; add it elsewhere if your edit calls for it. Only publish edits of music you have the rights to use.
Hear it on real audio
Here are two songs we generated with TuneSplit while building the site. The lo-fi one is a steady instrumental with a clear pulse, which makes tempo changes easy to judge by ear. The indie pop one has a lead vocal, so it shows how a voice sounds when the song is stretched.
Honest limits: quality, size and memory
Time-stretching is a reconstruction, so there is a quality cost, and it grows the further you stretch. As a rule of thumb, moves of 10% or less are hard to hear on most music, and moves up to about 25% are usually fine. At the ends of the range, 50% and 150%, expect artifacts.
- Slow stretches (50% to 75%). Sustained notes can develop a faint warble or flutter, and drum hits can lose their sharp attack because they are repeated and blended. Vocals may sound slightly underwater. Solo instruments and quiet material hold up best.
- Fast stretches (125% to 150%). Material gets squeezed, so consonants and transients can smear, and dense mixes sound busier. Speech-like vocals suffer first.
- Steady versus loose tempo. A track with a clear beat stretches more cleanly than a rubato piano piece or a live band recording.
- Step size. The slider moves in 5% steps, so you cannot dial in 103% or 87%.
- Length. The input must be 10 minutes or less. Longer files are refused with a message asking you to trim first.
- Memory and speed. The whole song is decoded and rendered in your browser, so a long file on an older phone can be slow or fail. If a file does not work, try a shorter clip or a laptop. A progress percentage shows while it runs.
- Output format. The download is a stereo WAV for stereo sources and mono for mono sources. WAV is large; convert to MP3 in any editor if you need a small file. The tool does not export MP3 itself.
Only use audio you own or have permission to edit. A tempo-changed copy is still a copy of the original recording.
Related free tools on TuneSplit
- BPM finder at /bpm-finder/. Detect the original tempo, key and Camelot code before you change anything, and read it again afterward to confirm where you landed.
- Pitch changer at /pitch-changer/. The same panel with a focus on key changes in semitones, for singers fitting a track to their range.
- Song key finder. Find the musical key if you plan to combine a tempo change with a pitch move.
- Vocal remover and stem splitter. Separate a song first, then slow down only the part you want to study.
- Karaoke maker. Turn a song into a backing track, and change its speed here if the original is too fast to sing.
Frequently asked questions
Is the tempo changer free?
Yes. You can load a file, change the speed, preview the result and download a WAV without paying or registering. The work happens in your browser, so there is no upload queue. Other TuneSplit tools, such as the song generator, have their own limits, but this one is open to use.
How much can I speed up or slow down a song?
The Speed slider runs from 50% to 150% of the original tempo in 5% steps. At 50% a song plays at half speed and takes twice as long; at 150% it plays one and a half times faster. Chips jump straight to 75%, 90%, 110% and 125%. For anything beyond that range, apply the tool twice.
Does changing the tempo change the pitch?
No, not on the Speed slider. The tool time-stretches the audio with SoundTouch, so a track at 75% is slower but stays in the same key. Pitch is a separate slider from -12 to +12 semitones. If you want the old tape effect where both move together, set the two sliders yourself using the table on this page.
How do I change a song to an exact BPM?
Find the original BPM with the BPM finder, then divide the target by it. Going from 100 BPM to 90 BPM means a speed of 90%. The slider moves in 5% steps, so your target needs to be a multiple of 5% from the original; otherwise choose the nearest step and check the result with the BPM finder afterward.
What does the tempo percentage mean?
It is the new speed relative to the original. 100% is unchanged, 75% is three quarters of the speed, and 125% is a quarter faster. Multiply the original BPM by the percentage to get the new BPM. A 120 BPM song at 75% plays at 90 BPM and at 125% plays at 150 BPM.
Will slowing a song down hurt the sound quality?
A little, and more the further you go. Moves under 10% are hard to hear on most music. At 50% to 75% you may hear a faint warble on sustained notes and softer drum attacks. The WAV export adds no extra compression, so any loss comes from the time-stretch. Preview before you download.
Is my song uploaded to a server?
No. The tempo changer decodes and renders the audio inside your browser, and the tool footer says it is processed on your device. You only save the result you choose to download. Still use audio you own or have permission to edit, and note that other TuneSplit tools, like the stem splitter, work differently.
What file types and lengths does it accept?
Files your browser can decode, such as MP3 and WAV, up to 10 minutes long. A longer file is refused with a message asking you to trim it. The result downloads as a WAV file. Very long files may be slow on a phone because the rendering happens on the device.
Can I slow down a song for practice and loop a section?
You can slow the whole song to a practice speed such as 75% or 50% and download it. The tool has no loop or metronome, so for looping, trim the section you want in an editor first, or make the slowed file and loop it in your usual player. Pitch stays put, so you can play along in tune.
Can I make a sped-up or slowed version of a song?
You can build the tempo and pitch parts. Raise the Speed and the Pitch together for a sped-up feel, or lower both for a slowed one, then download the WAV. Reverb is not part of this tool. Only publish edits of music you have the rights to use.
Does it work on iPhone and Android?
It is a web tool, so it runs in a modern mobile browser with nothing to install. Add your file, set the slider and press Apply and preview. Rendering uses your phone's processor and memory, so a long song can take longer than on a laptop, and a very long one may fail.
How do I find the original BPM of my song?
Use the BPM finder at /bpm-finder/. It analyses your file in the browser and reports the tempo with half-time and double-time alternatives, plus a confidence score and the musical key. Check the double or half reading if the number feels off. Then use the table on this page to plan your change.