What is the exact frequency of musical note C#4?

In 12-Tone Equal Temperament (12-TET) with concert pitch A4 = 440 Hz (ISO 16:1975), the exact acoustic frequency of C#4 is 277.18 Hz.

What is the acoustic wavelength of C#4 in air?

At standard room temperature (20°C / 68°F) where sound travels at 343.21 m/s, the acoustic wavelength of C#4 is approximately 123.82 cm (1.24 meters).

What is the piano key and MIDI number for C#4?

C#4 corresponds to piano key number 41 on a standard 88-key keyboard, and MIDI note number 61.

How is the frequency of C#4 calculated?

It is calculated using the formula f = 440 · 2^((k - 49)/12), where k is the piano key number (41) and 49 represents A4 (440 Hz).

Acoustics & Musical Physics Labf = 440 · 2^((k-49)/12) • λ = v / f

C#4 (277.18 Hz) Note to Frequency Calculator

Discover the acoustic frequency, wavelength in air, MIDI number, and physical tuning mathematics for musical note C#4.

12-Tone Equal Temperament Pitch

C#4(Key #41, MIDI 61)

Frequency

277.18 Hz

Wavelength (Air @ 20°C)

1.24 m

Period (T = 1/f)

3.61 ms

Interactive 88-Key Piano KeyboardScroll or click keys to navigate

A0 #

C1 #

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C4 #

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C5 #

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A5 #

C6 #

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A7 #

The Physics of Note C#4 (277.18 Hz)

When C#4 is played on a piano or acoustic instrument, vibrating strings or air columns compress adjacent atmospheric molecules at a fundamental frequency of 277.18 cycles per second (Hz). In standard air at 20°C, each compression wavefront is separated by 123.82 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 C#4, open cylindrical organ pipes require an acoustic length of λ / 2 ≈ 61.9 cm, while closed resonators resonate at a quarter-wavelength λ / 4 ≈ 31.0 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