How Musical Intervals Are Measured: Semitones, Names and Sizes
Learn how musical intervals are counted, named and measured in semitones, plus which intervals sound consonant and why their frequency ratios matter.
An interval is the distance between two pitches. Every scale, chord and melody is built from intervals, so measuring them is the first practical skill in music theory. This guide explains how intervals are counted, how their names are assigned, and how their sizes connect to the sound you hear.
What an interval measures
An interval has two parts: a number and a quality. The number tells you how many letter names the interval spans, counting both ends. The quality tells you whether the interval is perfect, major, minor, augmented or diminished. Together they give a precise name such as major third or perfect fifth.
The unit of size is the semitone. On a piano keyboard, a semitone is the distance from any key to the very next key, black or white. Two semitones make a whole step, and one semitone is a half step. The whole scale of Western tuning is built from these small units, which is why a keyboard makes intervals easy to see.
Counting by letter names
To find the number of an interval, count the letter names from the lower note to the upper note, including both. C to E is a third because the letters C, D and E are three names. C to G is a fifth because C, D, E, F and G are five names. This works the same on the staff, where a line or space counts as one step, and on a keyboard.
Sharps and flats do not change the number. C to F sharp is still a fourth, because the letters C, D, E, F are four names. Only the quality changes, and the quality depends on the exact semitone count.
Semitones on the keyboard
The table below lists the basic intervals counted up from the note C. Each row gives the semitone count and one example so you can check it against a real keyboard.
| Interval | Semitones | Example from C |
|---|---|---|
| Minor second | 1 | C to D flat |
| Major second | 2 | C to D |
| Minor third | 3 | C to E flat |
| Major third | 4 | C to E |
| Perfect fourth | 5 | C to F |
| Tritone | 6 | C to F sharp |
| Perfect fifth | 7 | C to G |
| Minor sixth | 8 | C to A flat |
| Major sixth | 9 | C to A |
| Minor seventh | 10 | C to B flat |
| Major seventh | 11 | C to B |
| Octave | 12 | C to C |
Learn these twelve sizes first. Almost every other interval you meet is one of them, possibly with an accidental added to one or both notes.
Perfect, major and minor qualities
The unison, fourth, fifth and octave are called perfect intervals. They have only one natural size. If you raise a perfect interval by one semitone it becomes augmented, and if you lower it by one semitone it becomes diminished. The second, third, sixth and seventh have two natural sizes, major and minor. A major interval lowered by one semitone becomes minor, and a major or minor interval raised by one semitone becomes augmented. Lowering a minor interval by one more semitone gives diminished.
Why the two families differ
The split is not arbitrary. The perfect intervals come out of the harmonic series, the pattern of overtones that a vibrating string or air column produces, early and clearly. Their frequency ratios are simple. The major and minor intervals have ratios that are a little more complex, and that is why they sit in a different family.
Frequency ratios and equal temperament
Pure intervals are described by whole-number frequency ratios. An octave is 2 to 1, a perfect fifth is 3 to 2, a perfect fourth is 4 to 3, and a major third is 5 to 4. Those ratios come from just intonation.
Modern pianos use equal temperament, where every semitone is the same size. Each semitone is a frequency factor of the twelfth root of two, about 1.0595. This keeps every key usable, but it slightly alters the pure ratios. The table below compares the two systems for a few common intervals.
| Interval | Semitones | Just ratio | Equal temperament (cents) |
|---|---|---|---|
| Minor third | 3 | 6 to 5 | 300 |
| Major third | 4 | 5 to 4 | 400 |
| Perfect fourth | 5 | 4 to 3 | 500 |
| Perfect fifth | 7 | 3 to 2 | 700 |
| Major sixth | 9 | 5 to 3 | 900 |
| Octave | 12 | 2 to 1 | 1200 |
A cent is one hundredth of a semitone. The major third in equal temperament is about 14 cents wider than the pure ratio, which is why a just major third sounds noticeably sweeter when a singer or string section plays it alone.
Consonance and dissonance
Interval families are often grouped by how stable they sound. Unisons, octaves, fifths and fourths are traditionally called perfect consonances. Thirds and sixths are imperfect consonances and sound sweet and blended. Seconds, sevenths and tritones are dissonances and create tension that usually wants to resolve.
The grouping depends on context. In common practice harmony a fourth above the bass counts as consonant, but in strict species counterpoint it is treated as dissonant against the lowest voice. Learn the rule for the style you study rather than memorizing a single list.
Inverting intervals
An interval can be inverted by moving the lower note up an octave or the upper note down an octave. The major third C to E becomes the minor sixth E to C. Inversion follows three simple rules. The two numbers always add up to nine, so a third becomes a sixth. The two semitone counts always add up to twelve, so a major third of 4 becomes a minor sixth of 8. Major turns into minor, minor turns into major, perfect stays perfect, and augmented turns into diminished.
Inversion explains why there are only a few interval types to master. Once you know thirds and seconds, sixths and sevenths follow automatically.
Ear training with familiar melodies
Your ear learns intervals faster when it has a reference. The opening leap of Twinkle Twinkle Little Star is a perfect fifth from C up to G. The first two notes of When the Saints Go Marching In form a major third, C to E.
Pick one reference per interval, sing it daily, and test yourself by playing a random interval and naming it before you look.
Key takeaways
An interval is a distance between two pitches, named by a number and a quality. The number counts letter names, and the quality is set by the semitone count.
Perfect intervals are unison, fourth, fifth and octave. Major and minor intervals are the second, third, sixth and seventh. Changing the size by one semitone moves an interval between minor, major, augmented and diminished.
Simple frequency ratios explain why some intervals sound stable, while equal temperament slightly alters those ratios for practical keyboard use. Learn the semitone table, use familiar melodies as references, and practice inversions so that the whole system becomes automatic.