What frequency is middle C?

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).

Why is A4 = 440 Hz the standard?

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.

What is equal temperament?

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.

What is MIDI note number?

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).

AudioCalcs

Note Frequency Calculator — Musical Note to Hz

Find the exact frequency of any musical note in any octave using equal temperament.

Note

Octave

A4 Reference

How We Calculate This

Equal Temperament Formula

f = A4 × 2^(n/12)

Where n = number of semitones from A4. Positive for notes above A4, negative for notes below.

Wavelength

λ = speed of sound / frequency = 343 / f

MIDI Note Number

MIDI = 12 × (octave + 1) + note index

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Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.