4th, a 5th or an octave

First, count the lower notes as one and then count up the relevant letters until you reach the upper note. The number you reach is the ‘size’ of the interval

If the interval is a 4^th, a 5^th or an octave, and the upper note belongs to the major scale
(T T S T T T S) starting on the lower note, it is a perfect interval

If the interval is a 2^nd, 3^rd, 6^th or 7^th, and the upper note belongs to the major scale

If the interval is a semitone smaller than a major interval, it is a minor interval

If the interval is a semitone smaller than a minor interval or a perfect interval, it is a diminished interval

If the interval is a semitone larger than a major interval or a perfect interval, it is an augmented interval

Hence:

For intervals of a 2^nd, 3^rd, 6^th and 7^th For intervals of a 4^th, 5^th and Octave

Augmented Augmented

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Major Perfect

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