In music theory, a perfect fifth is an interval spanning 7 semitones. Counting up from F#: F# to G (1), G to G# (2), G# to A (3), A to A# (4), A# to B (5), B to C (6), C to C# (7). Therefore, C# is the perfect fifth above F#. C is a diminished fifth, Eb is an augmented fourth, and D is a perfect fourth above F#.