OneTaskPublished
Finding the tempo of a song, and turning BPM into delay times
Tempo is the one number every musician, DJ and editor needs from a track, and it is surprisingly easy to get wrong. A song can feel like 85 BPM and be counted as 170. A live recording can drift by several beats per minute between the verse and the last chorus. And once you have the number, you usually need to turn it into something else: a delay time, a loop length, a video cut.
What BPM measures
Beats per minute counts the pulse you would tap your foot to. At 120 BPM there are two beats every second, and each beat lasts 500 milliseconds.
ms per beat = 60000 ÷ BPM
That one formula is the basis of everything else in this post.
Three ways to find it
Count it. Start a stopwatch, count beats for 15 seconds, multiply by four. It is quick and needs nothing, but a miscount of one beat moves the answer by four BPM. Counting for a full minute is more accurate and more tedious.
Tap it. Tap along to the beat and let a tool average the gaps between taps. After eight or so taps the average settles within a beat or two. Tapping is good for checking a number and for music with a clear pulse, and less good for anything rubato or heavily syncopated.
Detect it. Automatic detection looks for regular peaks in the audio, usually the attack of drums, and finds the repeating interval. It is fast and precise on steady, produced music and on anything with a click track behind it. It struggles with the same things people do: free-time intros, ballads with no percussion, and live recordings that speed up.
The BPM detector does the detection in the browser without uploading the file, reports a confidence score and the position of the first beat, and lets you tap along to check the result.
Half and double time
The most common "wrong" result is out by exactly a factor of two. A drum and bass track at 174 BPM has a half-time feel, so a listener might tap 87. A slow hip-hop beat at 80 BPM might be read as 160 because the hi-hats run at double speed.
Neither number is wrong as such; they describe the same grid at different zoom levels. What matters is which one the rest of your workflow expects. DJ software and genre conventions usually settle it:
| Genre | Usual range |
|---|---|
| Hip-hop | 80–100 |
| House | 118–130 |
| Techno | 125–140 |
| Drum and bass | 160–180 |
| Dubstep | 140 (felt as 70) |
If a detector reports a number outside the range you expect for the genre, try halving or doubling it before you assume it is wrong.
When the tempo is not steady
Most electronic and modern pop music is made to a fixed grid. A lot of other music is not. Bands recorded without a click drift, often speeding up in choruses and slowing for the outro, and classical recordings treat tempo as expressive.
For a drifting recording, a single BPM is an average. If you are syncing anything to it, such as lights, a video edit or a remix, you will need a tempo map with markers at each change, or you will need to warp the audio to a grid. A useful check is to detect the tempo of a short section near the start and another near the end. If they differ by more than a beat or two, plan for drift.
From BPM to delay times
Once you have the tempo, the most common next step is setting a delay or reverb pre-delay that sits in time with the track. The note lengths come straight from the beat length:
| Note | At 120 BPM |
|---|---|
| Whole (1 bar of 4/4) | 2000 ms |
| Half | 1000 ms |
| Quarter | 500 ms |
| Eighth | 250 ms |
| Sixteenth | 125 ms |
| Dotted eighth | 375 ms |
| Eighth triplet | 166.7 ms |
A dotted value is one and a half times the plain one; a triplet is two thirds of it. The dotted-eighth delay is the classic guitar delay sound, because its echoes land between the beats and fill the gaps.
LFOs for tremolo, auto-pan and filter sweeps are usually set in hertz rather than milliseconds. To convert a note length to a rate:
Hz = 1000 ÷ ms
So a quarter-note tremolo at 120 BPM runs at 1000 ÷ 500 = 2 Hz. The BPM to milliseconds calculator produces the full table, including dotted and triplet values, LFO rates and sample counts at common sample rates.
Practising at the right tempo
If you are learning a part rather than editing it, a metronome set to the track's tempo is the next step. Start slower, at 70–80% of the target, and step up by a few BPM once a passage is clean. A metronome that schedules clicks on the audio clock rather than with timers stays steady even when the browser is busy; the metronome works that way and can export a click track as a WAV to drop into a project.
The rest of the audio tools, from trimming to normalising, are in the audio section.