What this solves
Every DAW and every DJ software has a time-stretch algorithm, but they all degrade past a
certain ratio and they all change the track's duration in a way your ears will catch but
your brain won't name. This page computes the new length before you commit to the stretch,
flags when you've crossed the 20% zone (where most algorithms start smearing transients
or warbling vocals), and gives you a one-line hint about which algorithm to reach for.
It's aimed at bedroom producers prepping a remix master, hip-hop and electronic producers
fitting an acapella to a different beat, and DJs who want to pre-flight a key-lock stretch
before they crossfade a 30-second loop into another track.
How to use
- Type the original BPM shown in your DAW's project tempo field.
- Type the target BPM you want to stretch to.
- Type the original track length in mm:ss (drag-select the readout in your DAW to copy it).
- Read the new duration in the magenta card. Compare to the cyan one — that's the actual time difference you'll lose or gain.
- Check the Quality zone badge. Green = safe, amber = listen closely, red = audible artifacts likely.
- Match the recommended algorithm from the table below to your DAW (Elastique, élastique v3, élastique pro, Rubberband, ZTX, varispeed, slice-and-shift).
How to read the result
The two top cards show the new length and the new bar count at the target BPM. The bar
visualisation underneath draws the original 64-bar blocks next to the stretched blocks so
you can compare widths at a glance — a longer target means slower beats (you're stretching
up in time); a shorter target means faster beats (you're compressing).
The amber per-bar delta is the millisecond change on each bar between the two BPMs — useful
for translating the result into musical terms. A 10 ms per-bar delta in the safe zone is
what pro time-stretch engines hide well; 30 ms per bar is where you'll start hearing
phasey snares, vocals drifting flat, and the low-end losing weight.
Limitations
- The 20% safe-zone threshold is a rule of thumb for modern phase-vocoder / elastic algorithms. Solo piano, classical strings, and human voice without editing can audibly degrade at lower ratios; drum & bass and full-band house hold up at higher ones.
- It computes bar counts assuming 4/4 time. 3/4, 7/8 and other signatures need you to mentally divide by 3 or 7 instead.
- This calculator does not actually stretch audio. It's a planning tool — run your track through your DAW or DJ software's stretch engine for the real result, then come back and verify against the numbers here.
- Tempo maps inside imported MIDI or REX files will skew the math. If your track has built-in accelerando, treat any "target BPM" you see here as a single-section average.
FAQ
What is the ±20% rule of thumb based on?
Empirically, modern phase-vocoder engines (elastique, Rubberband, ZTX) start to lose
transient definition above about 20% on full-mix material. For monophonic instruments or
vocal a cappellas you can push that to ±30% with good results. For acoustic ensembles,
strings and highly reverberant material, stay under 10% unless the track was already
edited to be stretch-friendly.
Why does the stretch direction matter?
Stretching up in time (slower BPM) leaves more room between transients and tends
to preserve percussive clarity — that's the easier direction. Stretching down in
time (faster BPM) compresses transients into each other and is where phase vocoders start
smearing hi-hats and cymbals. The same 15% stretch will sound noticeably different in each
direction.
Why does my DAW show a different duration after stretching?
Probably because your DAW is tracking elapsed bars/beat instead of wall-clock seconds, and
bars don't perfectly divide seconds. If the calculator shows
7:14.8 and your DAW shows 7:15.1, that's a
rounding artefact in the meter, not a time-stretch error. Use a millisecond-accurate
export to verify.
Is this different from key-shifting?
Yes. Time-stretching changes duration without changing pitch. Pitch-shifting changes the
pitch without changing the duration. Formant-correcting pitch-shifting (modern vocal
tuning, vocal synths) does both, but introduces its own artifacts. If you need to change
BPM and key simultaneously, do the time-stretch first (preserve transients), then
pitch-shift.
What about genres with extreme tempo differences?
If you're stretching a 174 BPM DnB track to 87 BPM (half-time DnB played as house), you're
at 100% — far past any algorithm's safe zone. Consider slicing the audio and re-editing,
re-pitching the master, or doubling the rhythm programmatically instead of asking one
algorithm to do all the work.