Piano key frequencies
This is a list of the fundamental frequencies in hertz of the keys of a modern 88-key standard or 108-key extended piano in twelve-tone equal temperament, with the 49th key, the fifth A, tuned to 440 Hz. Every octave is made of twelve steps called semitones. A jump from the lowest semitone to the highest semitone in one octave doubles the frequency. The frequency of a pitch is derived by multiplying or dividing the frequency of the previous pitch by the twelfth root of two. For example, to get the frequency one semitone up from A4, multiply 440 Hz by the twelfth root of two. To go from A4 up two semitones to B4, multiply 440 twice by the twelfth root of two. To go from A4 up three semitones to C5, multiply 440 Hz three times by the twelfth root of two. For other tuning schemes, refer to musical tuning.
This list of frequencies is for a theoretically ideal piano. On an actual piano, the ratio between semitones is slightly larger, especially at the high and low ends, where string stiffness causes inharmonicity, i.e., the tendency for the harmonic makeup of each note to run sharp. To compensate for this, octaves are tuned slightly wide, stretched according to the inharmonic characteristics of each instrument. This deviation from equal temperament is called the Railsback curve.
The following equation gives the frequency of the th key on the idealized standard piano with the 49th key tuned to A4 at 440 Hz:
where is shown in the table below.
Conversely, the key number of a pitch with a frequency on the idealized standard piano is: