Internaltion Music Score Library. Piano key frequencies. The following equation gives the frequency f of the nth key, as shown in the table: (a' is the 49th key on the idealized piano) Alternatively, this can be written as: Conversely, starting from a frequency on the idealized piano tuned to A440, one obtains the key number by: List[edit] A printable version of the chart below. See also[edit] External links[edit] Free Online Piano Lessons. Fantasie Impromptu. Free sheet music for piano.