Convert between musical notes and their frequency in Hz — with adjustable A4 tuning.
In equal temperament — the tuning system used by virtually all Western music since the 18th century — each octave is divided into 12 equal semitones, and each semitone is a frequency ratio of 2^(1/12) ≈ 1.05946. The reference pitch is A4, which is 440 Hz by international convention (ISO 16).
The relationship between note and frequency is f = 440 × 2^((n−69)/12), where n is the MIDI note number (A4 = 69). This tool converts in both directions and shows the deviation in cents when the input frequency does not land exactly on a note.
One cent is 1/100 of a semitone. A trained ear can detect a difference of about 5-10 cents; a difference of 50 cents is an obviously out-of-tune note. The cents readout is useful for checking an instrument against a reference or for identifying microtonal intervals.
f = A4 × 2^((semitones from A4) / 12). Cents deviation = 1200 × log₂(f_actual / f_nearest_note).
By convention — ISO 16 adopted it in 1955. Historical pitch was lower (415-430 Hz), and some orchestras today tune to 442-443 Hz. You can change the reference in this tool.
1/100 of a semitone. 100 cents = one semitone, 1200 cents = one octave.
Only for equal temperament. Just intonation, Pythagorean tuning and other systems have different frequency ratios.
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