MusicReasonPitch, Tuning, Intonation, and Fret Geometry
How far is this frequency from a note, in cents?
Convert a note to a frequency, find the nearest equal-tempered note to a frequency, or compare two frequencies directly, all against an A4 reference you control.
Answer
Want to learn more? Tuning, Cents, Temperament, and Reference Pitch
Understand
Cents are a logarithmic unit: 1,200 cents make up one octave regardless of the starting frequency, which is why the same interval sounds the same size everywhere on the keyboard even though the frequency gap in Hz grows as you go up. The nearest equal-tempered note label is a convention useful for orientation, not a verdict about musical intonation: real instruments and ensembles routinely play meaningfully away from exact equal temperament on purpose.
Worked examples
A4 measured at exactly 440 Hz against a 440 Hz reference is 0 cents: the equation cents = 1200 · log2(f1/f0) returns zero whenever the two frequencies match. A4 measured at 880 Hz against the same 440 Hz reference is +1200 cents, exactly one octave, since doubling a frequency always adds 1,200 cents regardless of which note you start from.
Method and limitations
Frequency for MIDI note m at a given A4 reference is f = A4 · 2^((m−69)/12). Cents from reference frequency f0 to f1 are 1200 · log2(f1/f0). For a frequency-to-note lookup, the nearest MIDI note is the internal value rounded to the nearest integer; ties round to the nearer note using standard half-up rounding, and the primary cents figure is rounded once to the nearest whole cent for display while the internal calculation keeps full precision. Changing the display's enharmonic spelling never changes the underlying frequency or cents value. This tool does not evaluate whether a result is "in tune" for any musical purpose, and it does not represent any temperament other than 12-tone equal temperament.
Why A4 is editable, not fixed at 440
440 Hz for A4 is a widely used convention, not a physical law: some orchestras tune to 442 or 443 Hz for a brighter ensemble sound, older recordings and some early-music ensembles use lower historical references, and some regional traditions differ further still. Because every note-to-frequency and cents calculation on this page depends entirely on which A4 you declare, changing that one field re-derives every other value consistently, rather than requiring you to re-do the arithmetic by hand for a different tuning standard.