Note Frequency Calculator — Musical Note to Hz

Middle C (C4) in standard A440 tuning is approximately 261.63 Hz. This is the C in the middle of a piano keyboard, and is the reference point for the scientific pitch notation system (C4).

A4 = 440 Hz was adopted as the international standard in 1955 by the ISO. Before this, tuning varied widely — Baroque tuning was often around 415 Hz, and some orchestras tune to 442-443 Hz. 432 Hz tuning has a following among some musicians who prefer its sound.

Equal temperament divides the octave into 12 equal semitones, each with a frequency ratio of 2^(1/12) ≈ 1.05946. This allows music to be played in any key with equal intonation, though intervals are slightly out of tune compared to just intonation. It has been the dominant tuning system since the 18th century.

MIDI note numbers run from 0 to 127, where middle C (C4) = 60, A4 = 69. Each semitone is one MIDI number. MIDI note 0 = C-1 (approximately 8.18 Hz). MIDI note 127 = G9 (approximately 12,544 Hz).