Two different questions
BPM (beats per minute) answers “how fast?” It counts evenly spaced pulses in one minute. The time signature answers “how are the pulses grouped?” 4/4 groups them in fours, 3/4 in threes, 6/8 in two groups of three eighth notes. One is a rate, the other a pattern. A song can be 120 BPM in 4/4 or 120 BPM in 3/4; the speed of the beat is identical, the bars are different lengths.
The bottom number names the beat — usually
In a simple meter, the lower number of the time signature is the note value that gets one beat: 4/4 and 3/4 count quarter notes, 2/2 (“cut time”) counts half notes, 3/8 counts eighths. A metronome marking such as ♩ = 120 confirms which note the BPM refers to. In pop, rock and electronic music, BPM almost always means quarter notes per minute, whatever the meter.
Compound meters bend that rule. 6/8, 9/8 and 12/8 have six, nine or twelve eighth notes per bar, but they are felt in groups of three, so the beat is a dotted quarter. A 6/8 song with 180 eighth notes per minute is felt at 60 dotted-quarter beats per minute — two big beats per bar. Different sources may call it 60, 90 (quarter-note equivalent) or 180, and all three describe the same music. The table converts between beat units:
| Felt beat | Eighth notes/min | Quarter-note BPM | Dotted-quarter BPM |
|---|---|---|---|
| 40 dotted quarters | 120 | 60 | 40 |
| 50 dotted quarters | 150 | 75 | 50 |
| 60 dotted quarters | 180 | 90 | 60 |
| 70 dotted quarters | 210 | 105 | 70 |
| 80 dotted quarters | 240 | 120 | 80 |
Bar length depends on both
Because the time signature sets the number of beats per bar, bar and phrase lengths need both numbers: bar seconds = 60 ÷ BPM × beats per bar. At 120 BPM, a 4/4 bar lasts 2 seconds and a 3/4 bar 1.5 seconds. A 6/8 bar at 120 BPM counted in eighths lasts 3 seconds, but if the 120 refers to dotted quarters it is 1 second. Always check what the BPM is counting before calculating loop lengths.
| BPM (quarter note) | 4/4 bar (s) | 3/4 bar (s) | 6/8 bar (s) | 16 bars of 4/4 |
|---|---|---|---|---|
| 80 | 3.00 | 2.25 | 2.25 | 48.0 s |
| 100 | 2.40 | 1.80 | 1.80 | 38.4 s |
| 120 | 2.00 | 1.50 | 1.50 | 32.0 s |
| 128 | 1.88 | 1.41 | 1.41 | 30.0 s |
| 140 | 1.71 | 1.29 | 1.29 | 27.4 s |
What a BPM detector can and can’t tell you
An automatic detector measures periodicity in the onsets, so it finds a beat rate — but it has no idea how the composer counted. Two consequences follow:
- Compound meters produce 3:2 alternatives. In 6/8 the strongest pulses can be the dotted quarter, the eighth note or the bar. The detector will report one and list the others as ⅔× or 1½× candidates. Pick the one that matches how you count.
- The beat grid is marked in fours. When you zoom into the waveform, beats are numbered 1–4 and every fourth line is darker. That is a display convenience for 4/4 music; for a waltz, count the lines in threes yourself. The beat positions are still correct.
The detector’s tempo score uses the correlation at one, two and four beats. In 3/4, the four-beat term lines up only every third bar, so waltzes rely more on the one- and two-beat terms. In practice this still finds the quarter-note pulse on most waltzes, but if the result looks like 1½× what you hear, use the candidates.
Odd meters
5/4, 7/8 and other odd meters still have a steady pulse — the eighth or quarter note — so tempo detection works on them. What changes is the grouping: 7/8 is often 2+2+3 eighths, which creates uneven accents. The detector may lock on to the eighth-note pulse (high BPM) or the quarter-note level with an awkward fit. Halving an eighth-note reading gives a quarter-note equivalent that is useful for setting a DAW tempo; the DAW’s time signature then handles the grouping.
Tempo markings and metronome numbers
Classical scores often use Italian terms instead of numbers: Adagio for slow, Allegro for fast. These describe a character and a rough tempo range, not an exact BPM, and they don’t change with the time signature. A metronome marking pins it down. When you measure the BPM of a classical recording, expect more variation than in pop — performers shape the tempo deliberately — and look at the tempo-over-time chart to see how much.
Practical checklist
- Measure the BPM of the recording.
- Count along: how many beats until the pattern repeats? That is the top number of the time signature (or a multiple of it).
- Decide what the BPM refers to — quarter notes, dotted quarters or eighths — and convert if needed.
- Use both numbers for bar lengths, loop points and delay times. For delays, our BPM to ms calculator lists dotted and triplet values for every tempo.