OneTaskBy Tim King · Updated
Time Stretch Calculator
Tempo Change Ratio, New Length and Pitch Shift
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How the Time Stretch Calculator works
This time stretch calculator gives the ratio, speed change and playback rate that take a track from one tempo to another, the new track length, and how many semitones the pitch would move if you resampled the audio instead of time-stretching it.
How the time stretch calculator works
The playback rate is target BPM ÷ original BPM. The formula for the time-stretch ratio, which most audio editors ask for, is original BPM ÷ target BPM × 100%, the new length as a share of the old. The new length is the old length ÷ playback rate. Time-stretching keeps the pitch; resampling (as on a turntable or tape) changes it by 12 × log₂(rate) semitones.
First example. Taking a 3:30 track from 120 to 128 BPM is a 1.06667× playback rate, a 6.667% speed-up and a 93.75% stretch ratio. The track becomes 3:16.9 long. Resampled instead, it would go up 1.117 semitones.
Second example. Slowing a 4:00 drum and bass track from 174 to 140 BPM needs a 124.286% stretch ratio, a 19.54% slow-down. The track becomes 4:58.3 long, and resampling would drop it 3.764 semitones.
How many BPM can you stretch before it sounds bad? Changes within about 5–10% are usually clean with a good algorithm. Beyond 20%, transients smear and voices sound artificial, so record or choose material nearer the target tempo.
DJ pitch faders show the same speed change: a +8% fader setting on a 120 BPM record gives 129.6 BPM.
Speed changes with the tempo from 120 BPM, new length of a 3:00 track and the pitch change if resampled
| Speed change | Tempo from 120 BPM | Length ratio | 3:00 track becomes | Semitones if resampled | Cents |
|---|---|---|---|---|---|
| -20% | 96 | 125% | 3:45.0 | -3.86 | -386 |
| -18% | 98.4 | 121.95% | 3:39.5 | -3.44 | -344 |
| -16% | 100.8 | 119.05% | 3:34.3 | -3.02 | -302 |
| -14% | 103.2 | 116.28% | 3:29.3 | -2.61 | -261 |
| -12% | 105.6 | 113.64% | 3:24.5 | -2.21 | -221 |
| -10% | 108 | 111.11% | 3:20.0 | -1.82 | -182 |
| -8% | 110.4 | 108.7% | 3:15.7 | -1.44 | -144 |
| -6% | 112.8 | 106.38% | 3:11.5 | -1.07 | -107 |
| -4% | 115.2 | 104.17% | 3:07.5 | -0.71 | -71 |
| -2% | 117.6 | 102.04% | 3:03.7 | -0.35 | -35 |
| 0% | 120 | 100% | 3:00.0 | +0 | 0 |
| +2% | 122.4 | 98.04% | 2:56.5 | +0.34 | 34 |
| +4% | 124.8 | 96.15% | 2:53.1 | +0.68 | 68 |
| +6% | 127.2 | 94.34% | 2:49.8 | +1.01 | 101 |
| +8% | 129.6 | 92.59% | 2:46.7 | +1.33 | 133 |
| +10% | 132 | 90.91% | 2:43.6 | +1.65 | 165 |
| +12% | 134.4 | 89.29% | 2:40.7 | +1.96 | 196 |
| +14% | 136.8 | 87.72% | 2:37.9 | +2.27 | 227 |
| +16% | 139.2 | 86.21% | 2:35.2 | +2.57 | 257 |
| +18% | 141.6 | 84.75% | 2:32.5 | +2.87 | 287 |
| +20% | 144 | 83.33% | 2:30.0 | +3.16 | 316 |
FAQ
What is the difference between time-stretching and resampling?
Time-stretching changes the length while keeping the pitch, using algorithms that cut and overlap small pieces of audio. Resampling plays the audio faster or slower, like a turntable, so the pitch and length change together.
How do I work out the percentage change between two tempos?
Divide the target tempo by the original tempo, subtract one and multiply by 100. Going from 120 to 126 BPM is 126 ÷ 120 − 1 = 0.05, a 5% increase in speed.
Which ratio do I type into my audio editor?
Most editors that ask for a stretch percentage want the new length as a share of the old, which is the time-stretch ratio. Editors that ask for a tempo change or rate want the speed change or playback rate instead.
Does time-stretching change the key of a song?
No. A good time-stretch keeps the pitch, so the key stays the same. Only resampling or a varispeed change moves the key, by about one semitone for every 6% change in speed.
Why does my new track length not match exactly?
Audio editors round lengths to the nearest sample or frame, and some stretch only between warp markers. Small differences of a few milliseconds are normal and do not affect the tempo match.
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