Note Frequency Calculator

Enter a note name and octave in scientific pitch notation — middle C is C4 — and get its frequency in hertz. Switch to reverse mode to type a frequency and see the nearest note, how many cents sharp or flat it lands, and its MIDI note number. The A4 reference defaults to 440 Hz and can be changed for the alternative tunings orchestras and individual players use.

The arithmetic is equal temperament: f = A4 × 2^((n − 69) / 12), where n is the MIDI number and 69 is A4. Twelve equal steps to the octave means every semitone is the twelfth root of two, an irrational ratio, which is worth saying plainly because it is the whole compromise of the system. Middle C works out to 261.6256 Hz — a number that is not round and never will be, because rounding it would put something else out of tune.

Equal temperament is a deliberate approximation, and knowing by how much makes tuning by ear easier to reason about. The equal-tempered perfect fifth is 700 cents against the pure 3:2 ratio's 701.955, so it is flat by about two cents, which almost nobody hears. The major third is the one that suffers: 400 cents against the pure 5:4 ratio's 386.31, sharp by nearly fourteen cents, which is audible as a slight beating in a sustained chord and is why barbershop singers and brass sections drift away from the piano when they can. A trained ear resolves somewhere around five cents, so the fifth is inside the noise and the third is comfortably outside it.

The reference pitch is a convention rather than a constant. A440 is the ISO standard, many European orchestras tune to 442 or 443 for a brighter sound, and 432 Hz circulates with claims of historical or physiological significance that do not survive checking — it is simply 31.8 cents, about a third of a semitone, below the standard. Historical performance goes considerably lower, around 415 Hz for baroque pitch, which is nearly a full semitone down. None of these are more correct than the others; they are agreements about where to start, and this calculator will follow whichever one you set.