What a time signature tells you
A time signature is two stacked numbers at the start of a piece. The top number is how many beats are in each measure, also called a bar. The bottom number is the note value that counts as one beat: 4 means a quarter note, 8 an eighth note and 2 a half note.
Common time signatures
| Signature | What it means |
|---|---|
| 4/4 | Four quarter-note beats per bar. It is known as common time. |
| 3/4 | Three quarter-note beats per bar, the time of a waltz. |
| 2/4 | Two quarter-note beats per bar, often used for marches. |
| 2/2 | Two half-note beats per bar, also called cut time. |
| 6/8 | Six eighth notes per bar, felt as two groups of three. |
Simple and compound time
In simple time, such as 2/4, 3/4 and 4/4, each beat divides into two. In compound time, such as 6/8, 9/8 and 12/8, each beat divides into three. A 6/8 bar has six eighth notes but usually two main beats, so people count it either as six or as two.
How the time signature relates to BPM
BPM counts beats, and the time signature says which note is the beat and how many make a bar. The BPM number alone does not tell you the time signature. At 120 BPM a beat lasts 0.5 seconds, so a 4/4 bar lasts 2 seconds and a 3/4 bar lasts 1.5 seconds. To get a bar length, multiply the beat length by the beats per bar, which the BPM to ms calculator helps with.
In compound time the BPM is often counted on the dotted quarter note. Pro Tools, for example, lets you choose the note value the tempo refers to and suggests a dotted quarter note for 6/8.
Using time signatures with the metronome
The metronome clicks at the BPM you set and counts equal clicks per bar. For 6/8, set 6 beats per bar and treat each click as an eighth note, or set 2 and treat each click as a dotted quarter note. The two settings need different BPM numbers, because a dotted quarter note lasts three eighth notes, so the dotted quarter BPM is one third of the eighth note BPM.
Tapping a song in 6/8
If you tap once per main beat, the tap tempo counter shows the dotted quarter BPM. If you tap every eighth note, you get a number three times as high. Both describe the same music, which is the same idea as half-time and double-time.
Common questions
What does 4/4 mean?
Four beats in each bar, with a quarter note getting one beat.
What time signature is a waltz?
3/4, with three quarter-note beats per bar.
Does the BPM change with the time signature?
No. The BPM counts beats at whichever note value you choose. The time signature only says how those beats are grouped into bars.