From 128 to 140 — what does it cost?

The 6%-per-semitone rule of thumb

Tempo and pitch are the same knob on any keylock-off system: a 6% speed change is almost exactly one semitone. That single number turns transition planning into arithmetic — 128 to 135 is +5.5%, so a pitch-ride costs just under a semitone; 128 to 150 is +17%, which no keylock rescues gracefully, so you look for the halftime door or a hard cut instead.

The table's verdicts encode the working thresholds: under 4% is invisible, under 8% is club-normal, past 12% plan something cleverer than a long blend.

128 → 124: the small move, worked

Start with the from field at 128 and the to field at 124, then read the direct row. It reads -3.1%, because 124 ÷ 128 - 1 is -0.03125, and -0.55 semitones, because 12 × log₂(0.96875) is -0.5496. That is inside the 4% line, so the verdict says invisible — long-blend it.

Runway is where even a small move bites. A 4/4 bar is 1.875 s at 128 BPM and 1.935 s at 124 BPM, so thirty-two bars of one against thirty-two bars of the other do not finish together.

That is about 1.9 s of drift over half a minute of playing. The Runway column states it the other way round: at 3.125% apart the two grids slip a full bar every 31.0 bars, which at 124 BPM is 60.0 s of playing time. A four-bar blend hides that; a thirty-two-bar blend does not, so sync the two tracks or keep the blend short.

The 128 → 140 default, worked

The page loads with 128 and 140 already in the fields, and that pair is worth working through in full. The direct row reads +9.4%, because 140 ÷ 128 - 1 is +0.09375, and +1.55 semitones, because 12 × log₂(1.09375) is +1.5507.

At 9.4% you are past the 8% line, so the verdict reads hear the stretch — keep the blend short. It stays inside the 12% line, which is why the move still works as a short blend instead of a cut.

A bar at 140 BPM is 1.714 s, so an eight-bar blend spans 13.7 s. That is about all the room this move deserves before the stretch becomes the thing people hear, and the table puts a number on it: the grids slip a full bar every 11.7 bars, which is 20.0 s at 140 BPM, and eight bars of the blend costs 1286 ms of drift.

What the five rows mean

The table always shows five paths, and each one answers a different question about the same pair of tracks. Every row carries the same six figures: the target tempo it implies, the stretch percentage, the semitones a pitch-ride would drift, how many bars of runway you have before the two grids slip a full bar (and how many seconds that is), the drift over the blend length you picked, and a verdict.

The four blend-length chips above the table set how long the move lasts, and the drift column follows them: 4, 8, 16 or 32 bars. Runway is independent of that choice, because it measures the pair of tempos rather than the length of the blend. When two grids already match, runway reads as infinite and the drift as zero — that is a free ride, not a gap in the arithmetic.

The case that makes those rows worth reading is drum and bass against hip-hop. Enter 87 as the from track and 174 as the to track, and the direct row reads +100.0% and +12.00 st, which the verdict calls too far.

The Halftime target row reads 0.0% and 0.00 st, because half of 174 is 87. That is a free ride: the two tracks already share a pulse, and the work left is phrasing rather than tempo. The Doubletime source row reads the same way there, since doubling the outgoing 87 lands on 174.

One row is unusable there, because doubling 174 gives 348, outside the 40 to 240 input range. The Doubletime target row names a target you cannot type back in, and it is marked as outside 40–240 rather than quietly clamped into the range.

How the semitone number is calculated

Semitones are logarithmic, because pitch itself is logarithmic. The tool computes 12 × log₂(to ÷ from), where 12 is the number of semitones in an octave and log₂ turns a frequency ratio into octaves.

The familiar 6%-per-semitone rule is a small-ratio shortcut, not the formula. Going from 128 to 150 is +17.2% and +2.75 semitones, while dividing 17.2 by 6 suggests 2.9, so the shortcut overshoots as the ratio grows.

Use the 6% rule for a fast sanity check on a move of a few percent, and trust the table for anything bigger. The printed semitone value is exact, rounded to two decimals.

What this calculator cannot tell you

It measures tempo and pitch drift, and it stops there. Three things are outside its reach.

• Key compatibility. Two tracks can sit a few percent apart and still clash, and that question belongs to the Camelot wheel.
• Phrasing. The table says nothing about whether the incoming intro gives you the bars you planned to use.
• A steady tempo. Both numbers are treated as constants, so a live or unquantised recording is only an average here.

Treat the output as the arithmetic half of the decision. The other half happens in the room, with the fader under your hand and the next phrase coming.

How to plan a tempo change between two tracks

1. Enter both temposType the BPM you are mixing from and the BPM you are mixing to. Use Swap to reverse them.
2. Read the pathsCheck the stretch percentage, the pitch drift without keylock, and the halftime or doubletime options.
3. Share itPress Copy shareable link to keep the calculation.

Frequently asked questions