How note frequency calculator works
Every note's frequency follows from one anchor: A4 = 440 Hz. Equal temperament divides the octave into 12 exactly equal semitones, so each step multiplies the frequency by the twelfth root of two (about 1.05946). The formula is frequency = 440 × 2^((midi − 69) / 12), where midi 69 is A4 and midi 60 is middle C — so C4 comes out at 261.63 Hz and A5 an octave up at exactly 880 Hz.
Choose a note (sharps and flats both supported) and an octave from −1 to 10, and the tool returns the frequency to two decimal places along with the MIDI note number and the distance from A4 in semitones. If you tune to a different reference — 442 Hz orchestral pitch, 432 Hz, a vintage 435 Hz — set it in the A4 field: the frequency recalculates and the cents column shows how far that reference sits from the 440 Hz standard (+7.85 cents for 442).
The results are exact for the equal-tempered scale, which is the tuning almost every digital instrument, tuner and DAW uses. Real acoustic tuning varies: pianos are stretch-tuned, choirs drift, and baroque ensembles sit near A = 415 Hz. For the numbers behind a synth patch, a tuner calibration or an acoustics calculation, though, these are the values to use.