Convert a note to its frequency in Hz, or a frequency to the nearest note with its cents offset. Adjustable A4 reference (440, 432, anything).
Equal temperament: frequency = A4 × 2^((MIDI − 69) ÷ 12), where A4 = MIDI 69. Middle C (C4, MIDI 60) is 261.63 Hz at the standard A4 = 440 Hz. Going the other way, the nearest note and the offset in cents are computed from the same relation.
In equal temperament, frequency = A4 reference × 2 to the power of (semitones from A4 ÷ 12). Middle C (C4) is nine semitones below A4, giving 261.63 Hz at A4 = 440.
A cent is a hundredth of a semitone. When you convert a frequency, the tool names the nearest note and how many cents sharp or flat the frequency sits — 445 Hz is A4 plus about 19.6 cents.
Change the reference field and every note shifts proportionally: C4 becomes 256.87 Hz at A4 = 432. Orchestras also tune to 441-443; the maths is the same.