Six semitones
Three whole tones equal six semitones. From C, move by whole tones through D and E to F♯. This distance divides the twelve-semitone octave in half.
Augmented fourth or diminished fifth
C–F♯ is an augmented fourth (A4), because F is the fourth letter from C. C–G♭ is a diminished fifth (d5), because G is the fifth letter. F♯ and G♭ share a key, so the keyboard and sound are identical; written spelling determines the name.
Practice the relationship
Move six keys from each root to locate the tritone. Read the target spelling before choosing A4 or d5.