What is the exact frequency of musical note A4?

A4 is 440 Hz at standard concert pitch: 440 cycles per second, MIDI note 69 and key 49 on an 88-key piano. It is the A above middle C. The tone button plays a synthesized sine wave for comparing pitch; it does not listen to your instrument.

Start at a comfortable volume, play the reference, then play the same A on your instrument. Close frequencies produce audible beats; slower beating means the two pitches are closer. Speakers, headphones and the instrument's harmonics can make the timbre sound different even when the fundamental matches.

One octave below: A3 = 220 Hz
Reference pitch: A4 = 440 Hz
One octave above: A5 = 880 Hz

The label A4 alone does not force every performance to use 440 Hz: an ensemble may choose another reference, such as 442 Hz. These calculator values use A4 = 440 Hz. For the history and standard, read why A4 was standardized at 440 Hz, or see the ISO 16 standard.

The Physics of Note A4 (440 Hz)

When A4 is played on a piano or acoustic instrument, vibrating strings or air columns compress adjacent atmospheric molecules at a fundamental frequency of 440 cycles per second (Hz). In standard air at 20°C, each compression wavefront is separated by 78 cm.

12-Tone Equal Temperament (12-TET)

In modern Western music, the octave is divided into 12 equal logarithmic intervals. The frequency ratio between every adjacent semitone is the twelfth root of two (¹²√2 = 2^1/12 ≈ 1.059463).

f=440·2^{(k − 49) / 12}

Acoustic Standing Waves & Resonators

To produce A4, open cylindrical organ pipes require an acoustic length of λ / 2 ≈ 39.0 cm, while closed resonators resonate at a quarter-wavelength λ / 4 ≈ 19.5 cm.

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Traian AnghelAuthor & Reviewer

Physics Teacher & Educational Technologist • Brăila, Romania

Last Reviewed: August 2026

Wave Lab calculators are constructed using rigorous peer-reviewed physics formulas from verified metrological repositories. Every equation, spectral conversion, and acoustic property is tested against NIST, CIE, and ISO datasets.

Verified Sources:NIST CODATACIE 1931 2° ObserverNASA Glenn Research