Counting the number of an interval is the basic operation of all music theory. The number tells you how many staff positions the interval covers, whatever the accidentals are.
1. The counting method
To find the number of an interval, count the positions (lines and spaces) from the lower note to the upper note, always including both notes.
Example: from F to B
- F (1), G (2), A (3), B (4)
- Result: an interval of a 4th. Its quality is not decided yet: F–B is an augmented 4th, and F–B♭ is a perfect 4th. The quality comes from counting half steps (UK: semitones).
Interval number quiz
2. Build intervals by number
A note appears on the staff together with a number and a direction. Place the second note by counting diatonic steps.
Harmonic and melodic intervals →
To train your ear as well, use the reference tones of the online tuner and compare the sound of each interval with a pure tone. The online metronome helps you build sight-reading speed.
Frequently Asked Questions
What does the number of an interval measure?
The number (or distance) of an interval says how many staff positions there are between two notes, counting both notes. For example, from C to G there are 5 positions (C-D-E-F-G), so it is a 5th. The number ignores accidentals: it only depends on the letter names.
How do you count the number of an interval?
Count every note from the lower one to the upper one, including both. The total gives the name: 2 positions = 2nd, 3 = 3rd, 4 = 4th, 5 = 5th, 6 = 6th, 7 = 7th, 8 = octave.
What is the difference between the number and the quality of an interval?
The number says how many positions separate the notes (2nd, 3rd, 4th…). The quality says the exact size in half steps (major, minor, perfect, augmented, diminished). To describe an interval completely you need both: for example, "major 3rd" or "perfect 5th".
What is a unison?
A unison is the interval of number 1: the same note twice, in the same staff position. A unison can be perfect (exactly the same note) or augmented (one note with an accidental and the other without, both on the same line or space, such as C and C♯).
What is an octave interval?
An octave is the interval of number 8: the same note name one register higher or lower. On the staff, an octave spans 8 positions: one note on a line and the other in a space (or the other way around), such as C4 on the first ledger line below the staff and C5 in the third space. A perfect octave has 12 half steps.