Interval Numbers
Before you can talk about a "major 3rd" or a "perfect 5th," you need to know where the number comes from. It's simpler than it sounds — and it's not about counting semitones.
What is an interval number?
An interval's number — 2nd, 3rd, 4th, and so on — counts the letter names from the bottom note to the top note, inclusive of both. It has nothing to do with how many semitones apart the notes are; that's the job of the interval's quality (major, minor, perfect, augmented, diminished), covered in the Major & Minor Intervals lesson.
How to count an interval number
Just count letters, ignoring any sharps or flats along the way:
- C to D: C, D — two letters, so it's a 2nd.
- C to E: C, D, E — three letters, so it's a 3rd.
- C to G: C, D, E, F, G — five letters, so it's a 5th.
- C to the next C: C, D, E, F, G, A, B, C — eight letters, so it's an octave (8th).
The eight interval numbers
| Number | Letters spanned | Example from C |
|---|---|---|
| Unison (1st) | C | C |
| 2nd | C-D | D |
| 3rd | C-D-E | E |
| 4th | C-D-E-F | F |
| 5th | C-D-E-F-G | G |
| 6th | C-D-E-F-G-A | A |
| 7th | C-D-E-F-G-A-B | B |
| 8th (octave) | C-D-E-F-G-A-B-C | C |
Common questions
How do you count an interval number?
Count every letter name from the bottom note to the top note, inclusive. C to E is C-D-E, three letters, so it's a 3rd.
Does the interval number tell you the exact sound?
No — the number only counts letters. The quality (major, minor, perfect, augmented, diminished) pins down the exact semitone distance and sound.
Why is C to D♯ a 2nd and not a 3rd, even though it sounds like E♭?
Interval numbers follow letter-name spelling, not just pitch. C to D is a 2nd regardless of accidentals, so C to D♯ stays a 2nd (an augmented 2nd) despite sounding identical to C-E♭ on a piano.
What is the largest interval within one octave?
The octave itself, counted as an 8th — C up to the next C spans eight letter names.
Train your ear for real
Melody Monkey's Ear Training tool drills intervals and chords — and rewards correct answers with credits.
Open the ear trainer →Still stuck? Ask Your Personal Music Teacher about Interval Numbers →