How the frequencies are calculated
The table uses 12-tone equal temperament. MIDI note 69 is A4, and each semitone changes frequency by the twelfth root of two. Changing the A4 field moves every calculated key by the same proportional amount.
frequency = A4 × 2((MIDI note − 69) / 12)
Calculated targets versus an acoustic piano
These values are exact mathematical equal-temperament targets. A piano technician also listens to the instrument's partials and adjusts octave stretch for its string scale. That means the lowest and highest keys on a professionally tuned acoustic piano will not necessarily match this table exactly.
For background, see the University of New South Wales explanation of piano stretch tuning. Use the table for reference, comparison, and education—not as a complete tuning curve for a specific piano.